A Speaker Cabinet Project, in Words and Pictures
NOTE: The author is a fast-aging, 'boomer' drummer in a rock/pop group out of Grapevine, Texas, who has done the sound reinforcement for his band and several others for a number of years. The opinions below are his own and do not necessarily represent anything 'official'. You can email him with questions, criticism, or saucy photos of Sigourney Weaver at email@example.com.
NOTE: This article is focused on building speaker cabinets for Pro-Audio use, and not for Home, Home-Theater or Car Audio. While many of the concepts are the same and this informative article may help you in your construction efforts, the end goal of a Pro-Audio speaker system is not the same as Home, Home-Theater or Car Audio. Experiance has shown that high end audiophile speaker systems make lousy PA Speakers and visa-versa. Quality Pro-Audio speaker systems tend to take a lot of abuse and are built to deal with being dropped, pushed thru walls and occasionally falling down a few stairs - in all cases, the quality Pro-Audio component should inflict far more damage than it receives. If you have questions relating to Pro-Audio applications, please feel free to ask. If your questions relate specifically to Home, Home-Theater or Car Audio applications, you will get more appropriate help from people who work in that area.
A while ago, I wrote an article about amplifying electronic drums, which ended with a long exposition of my prejudices about speaker drivers to use with them, and another article about selecting speaker drivers for heavy-duty applications. I invited feedback and got something like, "OK, I bought these fabulous speakers and they're sitting here in their cardboard boxes; what do you recommend I put them in?" I directed these folks to a number of articles (many of the better ones are here on this site) about building live-sound speaker boxes. It was suggested to me that next time I build something, I photograph and describe the process in case that would help someone taking on that task for the first time. So here ya go. This is a PA-cabinet project, but much of the information here is transferable to other types of road-cabinet construction.
One word of warning: this is a lot of work and you can buy inexpensive factory-made cabinets of the same size for probably less money and far less effort. But inexpensive factory cabs almost inevitably have mediocre speakers, substandard cabinet materials, cheesy covering, and so forth. Really good cabinets with really good drivers can be really expensive, with very isolated exceptions. So as with most things, some hard work on your part will yield you a better cabinet at a lower price, then you can find down at Jim and Ed's Discount Music and Office Supply. You can also build exactly what you want, with the size and appearance you want or for any specialized application.
There are also sources of prebuilt, empty cabinets for mounting your own drivers. The best I've found is http://www.avatarspeakers.com, where Dave will sell you most of his cab designs "speakerless." If what you want meshes with one of those designs, this is a way to get an extremely well-built cab at a lower price than you will probably spend making your own, if you value your labor at more than say, a nickel an hour - plus, Avatar uses really good drivers in its cabinets in the first place, if you are starting from ground zero; this is one of the "isolated exceptions" I referred to above. But if you have a specialized need or desire, or want to house a particular array of drivers you have, there is no one I know who will build you a truly "custom" cabinet at a reasonable price. Plus, you like that pride of authorship and being able to say you designed and built it yourself, right?
Please remember that what you see below is just how one guy in his garage in Texas does these things. And, there are arguably some extra steps I take for purposes of appearance or longetivity, which may be unnecessary for your project; I will try to label these as "optional." Whatever you might do differently, if it works, is correct. I'd recommend you read this entire article before buying supplies or sawing anything, in case I've forgotten to mention something in the proper order.
Our band has what I'd call a "flexible" PA system depending on the size of the gig, and whether it's indoors or outdoors. But for most jobs where our smallest (single-12"-driver, single-tweeter) speakers won't do, we used to haul along some cabs I built a couple years ago that are floorstanding, with horns on poles, and roughly the size of Nebraska. They are immensely powerful and built very well, thank you, but partly because of that, they weigh as much as most office buildings. My objective was to build a pair of smaller but relatively powerful cabs that can be used by themselves for those "tweener" gigs where we're inside, but in a larger room. In fact, that's my objective with a lot of equipment design these days - to pack as much power-handling as possible into the smallest, lightest space. Unless you are rich and/or famous enough to have your own roadies, you will be loading and carrying your equipment from place to place for a long time to come. And if you were rich and/or famous, you wouldn't be building your own speakers, now would you?
If you use conventional speaker-design software for powerful bass drivers, and ask that software to suggest a design that will support very deep bass, it will inevitably tell you that your cabinet should have the internal volume of a supertanker. For critical bass applications, that's OK. But for main-PA-speaker use, I will sacrifice some lower end for easier transport and storage. In fact, very few "store-bought" speaker cabs have the optimum internal volume for their drivers, because the manufacturers have to consider how they will be shipped, how music stores will store them in inventory, how they can use every scrap of standard-dimension lumber, and so forth. There is a lot to be said for sacrificing perfect frequency response for portability. Also for this reason, I rarely (OK, never) build fancy horn-loaded cabs which are nineteen times the size of the actual speaker driver in them. They may be very efficient and sweet-sounding, but they tend to be huge. The speakers I'll describe in this article are conventional, ported rectangular boxes whose faces are about as full of speaker cone as it's practical to make them.
Yes, I am a speaker-design Neanderthal. Most books and computer programs for ported-box speaker design depend on what are called "Thiele-Small" parameters which describe how a given driver, in a given box design and maybe with a given port diameter and length, will have a projected resultant frequency response. If you are building a hi-fi speaker to reproduce music in a fixed space like your living room, you need to know this stuff cold, so that the CD or tape you are playing sounds like it's supposed to. DJ speaker designs should pay some attention to this too, although the vagaries of the rooms the DJ will set up in will affect the sound so much that exact, flat frequency response is sort of pointless. But the job of live-band cabinets is not to reproduce recorded music, but rather to produce it, so if "coloration" in your speaker cabinet affects your sound, hey, that's your sound, and who's to say it's right or wrong? Really powerful drivers usually have some vagaries in their response curves because their primary design criterion is to take punishment and pump out lots of power. Plus, if you had perfectly flat frequency response in a speaker cab the day you finished it in your garage, it would sound different the next day in a club, and different again the day after that at an outdoor gig. And for PA cabs, you are going to be good boys and girls and drive them with a setup that has a multi-band EQ that is fine-tuned for each venue and that is set to suppress microphone feedback at whatever frequencies the room you're in introduces, so you can "flatten" the frequency response of your speakers on the fly. So unless you're building cabinets where bass response is critical, like subwoofers that have to reproduce the sound of a bat hitting a sack of flour at 130db, don't be overly concerned with the "perfect" enclosure design for your drivers. Be more concerned with their ruggedness, power-handling, reliability, and portability.
Before you build a box, of course, you need to decide what's going into it. For PA applications, the small-box standard is a two-way woofer-and-horn combination for the simple reason that it works, and works well. Three-way systems require more drivers and more complex crossovers, so if you can get a high-frequency driver and a low-frequency driver to work well together, your life just got easier and you can use a smaller cabinet. That means having a low-frequency driver that extends up a little into the midrange, and/or a high-frequency driver that extends down a little into the midrange, so you don't need a separate midrange driver.
A speaker is basically just a big, round piston that moves back and forth and pushes air around. Since the surface area of a larger-diameter speaker is greater, larger speakers push more air for a given "piston stroke." So why not just use a single speaker the diameter of a trampoline, and be done with it? Unfortunately, speakers are subject to the same laws of physics that apply to the rest of the world. Very few powerful 15" speakers have much midrange, and most heavy-duty 18" speakers don't have any at all. This is particularly true of subwoofer-type speakers whose thick, heavy cones simply can't be made to vibrate at the frequencies necessary to reproduce even midrange sounds. And for my purposes, speakers of that size would require cabinets bigger than I wanted to lift onto speaker stands. On the other hand, I don't know of an 8" speaker I would trust to handle really high power, except in arrays of multiple speakers. So for small 2-way PA systems, I like 10's and 12's for the main speaker(s). When you are rich and/or famous, you can always add separate monster subwoofers, another amp and an electronic crossover and "bi-amp" your system if the smaller drivers don't have enough low-end power.
For reasons having to do with size, power-handling and mostly what drivers were sitting unused in my garage, my project envisioned one 10" and one 12" woofer in each cabinet. Each is an 8-ohm driver so the resulting impedance of the woofers, if wired in parallel (see article on speaker wiringon this site), would be 4 ohms - well within the design parameters of the power amp I intended to use. Select the impedance, number, and wiring scheme of your speakers with an eye to the lowest recommended load your amp is rated for - if your speaker impedance is too high, you'll just have a lower output level. But if your speaker impedance is too low, your power amp could overheat and turn very expensive internal organs into charred junk. With 8-ohm cabinets, you can "daisy-chain" a pair of them and still be within the capabilities of an amp that will accept 4-ohm loads. But I'm only intending to use these speakers by themselves (one connected to each channel of a large, fan-cooled stereo power amp) so 4 ohms is OK.
Because I wanted serious power-handling capability in a relatively small box, I started with serious drivers - Eminence Kappa 12's and Kappa Pro 10's. These speakers are rated to handle hundreds of watts each, are relatively efficient with sensitivity figures in the mid-to-upper 90's, and have big voice coils and giant magnets. We have a couple of their big brothers, the 15" size, in bass-guitar cabs made by Avatar, and I can tell you those guys are tough. When the situation calls for it, I don't want to have to worry that my speakers can't take abuse, and drivers operated well shy of their maximum output capabilities can last a long time. If you can't afford really good drivers, though, by all means build your cabinet around what you can afford. There is always an upgrade path later, since all speakers of a given nominal dimension will pretty much fit in the same holes. My drivers also are not designed as "subwoofer" speakers and thus have useful output into the midrange - 3 or 4 thousand "hertz" or cycles per second, abbreviated in the specs as "hz." Eminence is one of the good guys which posts frequency-response graphs for each of its speakers on its website, http://www.eminence.com. If you don't have a pair of speakers lying around for your project already, my article about speaker specifications and selection has a few nuggets of wisdom woven into all the errors and omissions; it's posted here on this site.
I need something to extend high-frequency response beyond the limits of the Kappas. I will use Motorola KSN1142A piezo drivers for all the reasons I described in my article "Playing out with electronic drums", soon to be a major motion picture. Some people think piezo drivers sound "unnatural"; I use them because they are inexpensive and don't weld themselves into junk under power, the way affordable compression drivers do, and they don't require a complex crossover. For a little more than ten bucks each, the 1142A has usable response from about 2,000 hz on up. Yes, audio purists, I know that by using low-frequency drivers with significant midrange, a high-frequency piezo driver that reaches into the midrange, and no crossover, I am likely to have a broad midrange response peak. I like that in a PA cabinet, because that's where vocals get their intelligibility; in fact the most popular stage vocal mikes, the SM58 and its legions of clones, tout their vocals-enhancing midrange response curves. I want these cabs to reproduce absolutely any note sent to them, at high volumes, but I especially want the vocal ranges to slice through the din. And, I will equalize them electronically, on the fly, for each venue where they are used. But if the result of my design is overwhelming, honky midrange, there is another trick I can use.
Piezoelectric drivers, from the viewpoint of your amplifier, look like a big capacitor, just like the ones used in passive crossovers with conventional woofer-tweeter combinations. So in a way, the crossover is built into the design of a piezo driver, because its impedance starts out ridiculously high at low frequencies (500 ohms or more) and thus rejects low-frequency sounds. This resistance to current drops as the frequency increases, until by the time sparking high frequencies come down the speaker wire, the piezo's impedance has dropped to double digits and those high frequencies come out of the driver at high sound-pressure levels because most piezos are very efficient. Different piezo designs have different response curves, naturally, and the 1142A reaches a bit into the midrange where my Kappas are still delivering powerful response. If I want it to cut in later (i.e. at higher frequencies), I just add more capacitance externally, with a capacitor in series with the circuit, and enhance that low-frequency rejection. Since this is a ridiculously easy "retrofit," I will build my speakers first and then see if they need some fine-tuning.
Since piezos have such high impedance at low frequencies where your amp is working hardest, adding one in parallel with your woofer is hardly noticed by your amp. Full-range music sounds have most of their energy focused in the lower frequencies, so by the time the piezo's resistance drops in the higher frequencies, it's not receiving the current levels your woofer is - actually this is why tweeters of all sorts can be smaller, and have lower power ratings, than the woofers they're paired with, although the lower the frequency point at which horns or tweeters are called upon to operate, the tougher they have to be. This is another good reason for selecting woofers (for PA main speakers, anyhow) that have decent midrange response; horn drivers that are rated to reach very far into the midrange, to compensate for woofers with limited midrange capabilities, can get very, very expensive.
One collateral problem with piezos is that their impedance keeps dropping, and they keep trying to emit sounds, waaaay beyond the range of human hearing. If you have an amp with response that extends into the stratosphere (uncommon except in pricey hi-fi amps), that low impedance at ultrasonic frequencies can cause problems which by definition are not audible to you. So piezo manufacturers recommend wiring a small resistor in series with the piezo driver, so that the amp never sees excessively low impedances; piezos are so efficient that this hardly impacts their response to signals at audible frequencies at all.
CTS, the only piezo manufacturer I know which publishes useful information about its products, recommends a 50-ohm resistor for use with its smaller, very-high-frequency models, a 20-ohm resistor for its larger models, and none at all for the 2 khz horns in its "Powerline" series, which have little protection circuits built in and compress (turn down, on the fly) destructive power levels or peaks by filtering the signal through a fast-acting variable resistor and a little light bulb. The 1142A is one of these. For pro sound applications, I would recommend you use only the Powerline drivers which have relatively high power ratings (100W continuous, 400 peak) because of this protection circuit. These cost more, but only by a few dollars, and give you the security of knowing that your high-frequency driver has a little extra protection against being overdriven. For really high-power applications you may need to use multiple high-frequency horns in arrays.
Now I do a rough comparison of the sensitivity, or "SPL" ratings of my speaker combination, because usually I will have to find some way to attenuate, or cut down, the power of the usually-more-efficient tweeter to match the output of my woofer(s). This is usually done by adding some sort of resistance to the tweeter circuit in the form of fixed resistors or an L-pad circuit. But the 1142A is rated for an SPL of 92db, actually a bit less than my woofers! Thus I will plan my initial design with the tweeter hooked in parallel with the woofers, and no resistance circuit. If I later add a capacitor to smooth the overall midrange as discussed above, that will cut the overall output of the tweeter just a bit at all frequencies, so why start with a design that chokes it back right from the start? But if you use just about any other woofer-tweeter combination, you will probably have to do some resistive matching of SPL levels because most woofers are not as sensitive as my Kappas and most tweeters are far more sensitive than the 1142A. And if you use conventional compression drivers rather than piezos, you will need a crossover! There is copious crossover-design information here and on other sites, or you can buy them premade, but for applications such as this they should have very heavy-duty components because crossovers can burn out just like speakers do.
Your box will have to be large enough so that the largest driver has clearance on all sides to bolt to the baffle board (the front piece with the big holes the speakers mount to), and so that the baffle board can be screwed or bolted to the framing inside the box. Cutting it close, I went with 14" interior (baffle) width, giving myself only an inch on either side of a 12" driver. I bought 14" wide horns that would fit in that same cabinet width (these come in all sizes and the cheap plastic ones work just great). For height, I had to accommodate a 12" driver, a 10" driver, and a 7" high horn, so that's 29" right there, plus at least 1" between each cabinet edge and each driver so there's some baffle board to screw to (4" or 5" more). And I wanted room to insert a port or two, so I rounded the height up to an even 36". I cut a sheet of newspaper (available for free in your neighbor's yard if you get up early) to 14" by 36" and laid the drivers on it to make sure my math squared with reality. Here's the driver setup laid on the newspaper:
Remember that I also wanted to pole- or stand-mount these cabinets, as is typical of small PA speakers that have to project over an audience when you're not on a tall stage. This is done by inserting a "top hat," essentially a polemount sleeve, into the bottom of the cabinet - it's called that because unmounted, it looks like one of those silly things Abe Lincoln used to wear. It projects into the inside of the cabinet about 3 inches, and obviously shouldn't project into the lowest-mounted speaker, so make sure you have enough interior clearance for it (it probably won't be mounted in the center of your cab either; see "balancing act" below). A little of the extra height I obtained by "rounding up" the baffle dimension will give me some margin for error here. If you are building subwoofers to go under your main speakers, top hats in the top of those cabinets will allow you to stack your speakers with a cheap steel pole instead of an expensive tripod stand. If you use top hats, you will find you can buy steel ones or plastic ones, and that the steel ones are only about $1.00 more. Do not trust plastic brackets to support heavy speakers positioned up in the air above a crowd unless you have really, really good insurance and don't care if someone is cold-cocked by a falling speaker. Here's an unmounted pair of top hats:
Steel top hats in their natural state. They will project into the cabinet, but that projection will be made "shorter" by the thickness of the bottom plate of the cabinet.
You may also notice some weird little triangular things in the first photo. These are port tubes, which you usually see as round holes in pro speaker cabs, and whose purpose is to allow low-frequency sound waves from the back of your speaker, to come out of the box and reinforce the low frequencies from the front, which is why this sort of design is called a "bass-reflex" cabinet. If I were designing for optimum reproduction accuracy, instead of just a wad of power in a small box, I would use a computer program to tell me the diameter and length of any port(s) for my box volume and driver parameters. My port choice is much less scientific than that; I want those bouncing bass notes to come out the front of a crowded baffle board, no matter what combination of frequencies are reinforced, and more importantly, I don't want those big woofers with their long excursions trying to move back and forth with the rear of their cones working against a small volume of sealed air in the box. At the same time, I don't have room on the front of the baffle for a conventional round port. I'm using triangular ports, which look like big eyebrows and will fit in the triangular "corner" space around my largest speaker.
Actually, I'm not a total Neanderthal and did check my port design with a computer program, estimating the area of my weird-shaped ports and trying to adjust for the two different sizes of speakers in one box, and it told me that (1) my box is uncomfortably small and for optimum bass extension, should be the size of a gymnasium - I knew that; and (2) my intended port setup is not optimum but "ballpark" close for my purposes and maybe should be shortened a little bit with a hacksaw, an easy chore with plastic ports and it doesn't have to be a neat one since no one will see the rear of the port tubes with the back installed. Again, port/box/speaker tuning is most critical in the low bass ranges, which weren't a critical issue for these cabinets. If I ever run bass guitar or kick drum directly through my PA, I will use biamped subwoofers anyhow, and my PA mains will never be asked to reproduce the lowest notes. Generally, ports too large and/or short can defeat the whole point of putting speakers in a box in the first place, and ports too small and/or long can raise the tuning frequency of the box/speaker combination, which for today's purposes and within reason, is fine with me. There are a number of shareware/freeware box-design programs like WinISD that remove all the algebra and calculus from box design, but if you are going to try them, know all your speaker parameters from the manufacturer's specs and if you are used to inches and feet, have a metric-conversion utility open in another screen window since you will be converting cubic inches to liters and furlongs to hectares and stuff like that. More on this in an upcoming subwoofer article, where these things are far more critical. For today, just remember three gross oversimplifications: (a) a sealed, portless box lowers efficiency and should only be used with a floppy-coned (or more properly, "compliant") speaker designed for that; (b) ports that are too small in diameter (little 1"-2" things designed for hi-fi) can "whistle" if large volumes of air move through them at high speed, like blowing air over the rim of a Coke bottle; and (c) most voices and non-bass instruments emit tones above the range where all this is critical for our purposes. This is an article about construction, after all, not electroacoustic design.
Figure out all the fittings and accessories you will need and order them now. Unless you have an unusually well-stocked music or electronics store nearby, you will need to mail-order things and it will be frustrating if you have the time and inclination to work on your cabinet project but have to wait a few more days for the mailman to bring the necessary stuff. Here's a checklist I use, in addition to the drivers and cabinet wood:
Okay, now we have to build a box, whose inside face dimensions accommodate a baffle board the size of the piece of newspaper template. The articles you just read tell you about leaving a little extra space between the edge of the baffle and the inside of the box for the thickness of carpet coverings; I'm going to figure about 1/4" extra in both width and height because I am going to carpet-cover both the sides of the box, and the baffle board too because I think that looks cool. So I subtract that number from the baffle dimensions, or add it to the height and width of the cabinet, to create a gap between the baffle and cabinet while they're raw wood that will be filled with carpet later.
I think you read this in one of the other articles already, but it bears mentioning again. Do not build a speaker cabinet you wish to move around, out of "particle board." It is somewhat water-soluble, crumbly and inflexible, and wood or drywall screws will not hold well. When exposed to rain, beer, or even long spells of humidity, it has the structural integrity of a marshmallow and breaks down into its component parts of sawdust and cheap glue. Its only resemblance to real wood is its color. MDF is a good material structurally and sonically, but my problem with all composite "woods" is their density - a large cabinet will weigh as much as a rhinoceros. For small home speakers, use MDF. For large, movable cabs use plywood (and if you already have a "particle board" cabinet you need or expect to last very long, stop reading now, remove the back or largest drivers, and coat any exposed interior board with some sort of sealant - paint, varnish, diluted Elmer's glue, whatever. Let it dry thoroughly before reinstalling the back or driver. It won't fix the basic problem but it will forestall failure a little longer. And if your bass speaker is mounted in a particle board baffle with wood or drywall screws, replace those with bolts and T-nuts at your earliest convenience or someday that speaker could just leap out of its cabinet and bounce off the stage).
OK, plywood. Thicker is better, sonically and structurally, but thicker is also heavier. I have built heavily reinforced cabinets with ¾" plywood and 2" square interior framing (the ones with their own Zip codes I referenced at the start of the article) and I'm sure they will outlive me and survive being dropped from planes. I have built guitar cabs entirely out of ½" plywood, and with enough internal bracing they work fine and can be lifted by a healthy fourth-grader. 5/8" stock is a good compromise for medium-duty cabs. For this project I'm going to use ¾" plywood for the bottoms and baffles, which take the most abuse and have the most heavy and/or stressed things attached to them, and ½" plywood for the sides, top and back, which don't have to support much (these are PA cabs, so no heavy amp head sitting on top), to save weight. My calculations will be made more complex by mixing materials thicknesses, but I'll save a couple pounds over the alternative of making everything of ¾" plywood and I will reinforce the thin ½" panels with copious interior bracing. If you are using this article as a guide to build a subwoofer or bass guitar enclosure, by all means use ¾" plywood all around.
I will be using butt joints and internal bracing strips as described in the other articles. The weight of the cabinet and speakers will be borne by the top hat, which is supported by the bottom plate of the box, so I will do my calculations so that the bottom plate is full-width and the side plates rest on it - this is not critical if the cab is properly braced but it makes me feel better. So the bottom plate needs to be the width of the baffle board, plus the 1/4" I am allowing for the carpet thickness, plus 1" representing two thicknesses of ½" sides - a total of 15-1/4" in width. The top plate will be the same width, obviously. So the sides that are "sandwiched" between the top and bottom plates need to be only the height of the baffle board plus that ¼" for the carpet, or 36-1/4", and the resultant width and height will be 15-1/4" and 37-1/2" respectively. The "perimeter" distance (all the way around the cabinet and back to the starting point) will be about 105-1/2", or nearly three yards. I will need at least three yards of covering material, twice as wide as the depth of the cabinets, just to cover the four sides of the boxes, and another square yard each to cover the fronts and backs because there will be two of each.
How deep should each cabinet be? Well, at a minimum, deep enough that none of the drivers have to stick out a hole in the back! My deepest driver, the horn, is about 11" from its mounting face to the back of the driver. Add to that at least a ¾"-1" baffle-board recess from the front cabinet edge (more if you will use a mesh grille over it), the ½" thickness of the back, and some space to run a bracing strip down or across that thin back or to line the back with eggcrate foam, and I need an overall cabinet depth greater than 13 inches. Depth is a cheap way to get cabinet internal volume, but remember that I'm aiming for a small package, so I will cut it close and make the overall cabinet 14 inches deep.
Many lumberyards or home centers have giant saws with long, accurate saw guides and will cut plywood to your dimensions for a nominal charge. In my experience, this is well worth it, especially if you don't have a vehicle that can carry 4'X8' sheets of plywood or the ability to make very straight saw cuts. Otherwise you can draw your dimensions on a sheet of plywood and cut it yourself (measure twice, cut once). And get enough small-dimension lumber for internal bracing, to fit around the interior of each cab twice - in my case, twice that "perimeter" number of 3 yards, or 18 feet per cab, plus enough to brace the corners between the two perimeter "frames". I'll use six eight-foot 1" X 2" precut strips, ripped to length with my circular saw as I go. Because I'm using relatively thin plywood, I will use the bracing strips in their full dimension, which is actually about ¾" by 1-1/2". This is larger bracing than many people use, but it allows me to secure it with longer screws (1-5/8") from the cabinet exterior, and results in extremely strong boxes with a negligible penalty in weight. Here's the pile of lumber:
I build my boxes unbraced, and then add the bracing. Although that makes them a little delicate early on, it works if you're careful and allows you to cut the bracing to fit exactly. A helpful item if you do this is a 90-degree clamp, which looks like this:
One of these, on one corner, will help keep the whole thing square while the glue dries and until the bracing makes everything solid. Your precut back board, laid on the floor while you build the box around it, will also help (it will be a little loose because of the ¼" carpet allowance). Use newspapers because you will drip a lot of glue. Put a line of wood glue along each joined edge, and then carefully run a small drywall screw or two about 3" from each edge to hold it together while the glue sets. Place the back board inside the box to help square it, and/or use the 90-degree clamp. Then you can start cutting and installing internal bracing.
If one of your screws breaks through the side of the bracing, it was either not centered, or screwed in crooked. Reverse your power drill and back it out, then go a little ways down your measured line and put it in right. An extra small screw hole or two won't hurt, and if you're anal you can always plug them up with putty, or even bubble gum.
I want the back removable but flush, so the bracing recess there is the ½" thickness of the back plus about 1/4" for the carpet which will wrap around the back. I then measure and cut 4 corner-brace pieces to run from the front bracing to the back bracing, and glue and screw them in place. Plywood is soft enough that I don't countersink the screw holes; drywall screws can sink themselves, pretty much. Since I am using relatively thin, ½" plywood over a fairly long, 36" span, I will put an extra brace or two midway up the sides of the cabinet (but not where my handle will go, see below) and another on the back to help dampen resonances that might develop in a large area of unbraced wood. You can now remove the original drywall screws at the edges of the plywood, or if they're not sticking out to snag the carpet covering and you don't intend to round the cabinet corners, you can leave them there. I usually leave them. Conventional wisdom will tell you to file the outside corners smooth and sort of rounded where the carpet will bend over them. I find it just as effective and far quicker to mash the corners down gently with a series of hammer taps, and then rough-sand off any splinters I've caused. You can even run a piece of black duct tape front-to-back at each corner to ensure smoothness here. You now have a pretty darn strong box, fully braced:
This next step is completely optional: I paint the entire completed cab with a water-based latex (cleans up easily) flat black paint. This helps seal the wood, although plywood doesn't really need it that much; it prevents a possible tear or gap in the carpet later on from exposing shiny white wood (look at any tear in a Tolex-covered cabinet; don't you wish the manufacturer had painted it before covering?); it keeps shiny white wood inside the cab from being visible through a vent port; and most of all, it looks really cool if someone else takes off the cabinet back sometime in the future. Later I will do the same with the baffle and back. If I need to mark dimensions on the cabinet after it's painted, I use an orange "colored pencil" I stole from one of the kids; it shows up great on a black surface.
NOTE:If you plan to use white wood glue (or yellow carpenters glue) to hold your Tolex or carpeting on to the cabinet, don't paint the cabinet anywhere you plan to glue to - this type of glue won't stick to painted surfaces.
Now your baffle needs mounting holes for the speakers. Place the speakers on the baffle board as you have planned to mount them. Draw a circle around each speaker with a pencil, and stick the pencil in each little speaker-screw hole far enough to make a mark. Remove the speaker and draw a line from each mounting-hole mark to its opposite mark. Where all those lines intersect is, duh, the center point. Now determine the diameter of the hole you need based on your speaker - this should approximate the diameter between the inside edges of the front speaker gasket - and put two holes in a piece of sturdy cardboard exactly half as far apart as the hole diameter. Put a small nail in one hole and tack it into your center point. Put the pencil lead through the other hole and draw your circle like a grade-school compass. For the horn, measure the necessary hole by calculating the width of the flange and reducing your tracing around the horn by that amount. Get ready to fire up that saber saw.
The best platform for holding the baffle board while you saw, is the back of the cabinet you just built with its negligible bracing recess, because the baffle board will fit right in there almost flush. Drill a pilot hole with a large bit right at the edge of the speaker hole, insert the saber saw blade, don those protective goggles and saw away. Do the same for any jack-mounting plate on the cabinet back. Measure the (sometimes odd-shaped) holes your cabinet handles will need in the sides of the cabinet, but for balance, offset these just a little (maybe ½" to 1") toward the front of the cabinet. Here's why. All the heavy speakers and stuff are mounted to the front baffle board, so the weight of the cabinet is very much positioned toward the front. If the handles are in the exact front-to-rear centerline of the cabinet, and the handles are mounted too low, its top will tilt forward when lifted by the handles. This is OK most of the time, but on really heavy cabs can cause the handles to be uncomfortable or even unusable. Mounting the handles further toward the top of the cab minimizes this effect, but for really tall cabinets this may result in the handles being unusably high and hard to lift off the ground without bending your arms at an odd angle. You want people of normal height to be able to hold the cab at least a few inches off the ground with their arms extended down. My own personal handle-grabbing hand is a little more than two feet from the floor, so I don't want the horizontal centerline of the handle more than about 20 inches from the bottom of the cab (or less, if I were going to put casters on it later and make it effectively taller). Again, this is only a concern with large/tall cabs, but worth considering in the planning stage before you cut big, gaping holes in the side of your beautiful new boxes. I don't want the handles mounted too high on my cabs for two other reasons: I will be lifting them onto stands, so lower handles are more convenient; and I just may use the top sides for something else described later.
A quick exposition about jack plates: their purpose is to hold the connector jack firmly, and to seal the hole they make in the cabinet back. They come in a bewildering array of types and sizes, but for pro cabinets you don't want the home-use kind with spring clips. The common speaker connector for most folks is the ¼" "phone" plug, just like the one on your guitar cord, but for high-power applications the little, tiny contact point between the tip of the phone plug and the jack can pose a problem. The accepted solution is either "banana plugs" or Neutrik "Speakon" connectors, and if you have a big, strong power amp, its speaker outputs are probably one of these. Banana plugs pull out easily and Speakons don't, so I like the latter even though the plugs and jacks are sort of pricey. At the same time, you never know when you will want to use your speaker with an amp with only a phone-plug output, when you're in the wilds of some remote gig and don't have a cord that adapts between the two, so I use a jack plate that mounts both Speakon jacks and regular phone jacks for convenience. And I mount two of each, all wired in parallel, so the signal can pass through from your power input to the speaker, to whatever other speaker you might want to add in parallel. Since these speakers will have 4 ohms impedance, I probably won't daisy-chain anything else to the extra jacks for fear of lowering the effective impedance too far for my amp, but I think it's a good practice anyhow.
Test-fitting my baffle board, I found I had to "notch" the front-side bracing a little (hacksaw and chisel) to clear the ports and horn because I chose such a narrow cabinet width; in a more generously-sized design this wouldn't have been necessary, and it's still better than using smaller bracing in my opinion. So here are my cabs with all the holes cut for handles, speakers, etc., and with the baffle board just sitting loosely in the front:
Before you cover the boxes and make them pretty, I'd advise using them to support the baffle again and drilling the speaker-bolt mounting holes (any drilling or sawing you do after carpet covering is applied will result in sawdust sticking to that pretty new carpet). I put the baffle in the cabinet back, drop the speakers into it just as they will go when mounted, make sure the baffle hole is not too small to allow the speaker to mount flat (more saber sawing if it is), and select a drill bit slightly smaller than the mounting holes in the speaker so I'm only drilling through wood and not my speaker frame. Then I make sure I'm awake and sober and careful, because few things are more depressing than mistakenly drilling a hole through your expensive speaker cone, and drill the eight holes I'll need. Then I blow the sawdust off the speaker and put it away.
Some people mount light plastic horns with drywall screws, some use bolts; the former works if your horn/driver combination isn't too heavy (plastic horns with piezo drivers weigh about as much as a sparrow, but good compression drivers have heavy magnets hung way out on the back). For the bass speaker(s), however, always use bolts and T-nuts, because its job is to jump around like it's being stung by killer bees, and drywall screws can loosen and pull out under that sort of stress. The holes you just drilled are probably big enough for your bolts but not for the T-nuts. With your speaker safely stashed away, drill the baffle bolt holes larger so that the T-nuts will fit in from the back of the baffle. You can then put the baffle on the floor face down and hammer the T-nuts in from the back, but I also run a "test bolt" with a washer into each one from the front (i.e. the same spare bolt, over and over) to make sure they're aligned properly and won't give me any problems during assembly.
You can also use T-nuts on the front of the baffle face so it can be bolted onto the cabinet bracing strips from the back with bolts of carefully-chosen length, but with a good press- fit, drywall screws can be enough (you can also use some glue, if you don't care about the future removability of the baffle, but that complicates future repairs or changes). Don't mount it in the box yet, I'm just talking about the future here. Handles must also be mounted with T-nuts, or at least a few of them in the holes surrounding the actual handhold. If you use casters on heavy cabs, my recommendation is T-nuts there also. And definitely, T-nuts (or bolts and nuts with big "fender" washers inside, no relation to the guitar company) for the top hat if you use one. Wood screws can strip, pull out and loosen with time; any part attached to the wood and which will be stressed needs to be attached with bolts and T-nuts, or washers and locking or "aircraft" nuts). Pretty much all such bolts will go in from the outside, so with T-nuts you can check and tighten them from time to time if necessary without disassembly of the cabinet.
One more word about bolts and screws. I buy black-finished ones for any use on the cabinet face or exterior (speaker, top hat, handles) because I want them to be semi-invisible when installed. I sometimes even buy the socket-cap kind that require an allen-wrench driver, just for looks; this is my own peculiarity I guess, and adds complexity and expense you don't really have to undertake. My point is, black bolts and most drywall screws are black because they're coated with "black oxide," which is only sufficient to keep them from rusting long enough for you to get them home from the hardware store. In one or two years, exposed to humidity, they will oxidize to a particularly-offensive dark orangish color, while all your other black stuff will still be, well, black. I gather all the bolts and screws that will be visible from the outside of the cabinet, poke a bunch of holes in an old cardboard box, insert all the screws so just their heads are showing, and give them a light coat of black epoxy spray paint. They will look shiny and new for years. I have done the same thing with conventional, bright-zinc screws; regular spray enamel doesn't stick to shiny, smooth things very well but spray epoxy seems to. One can of black epoxy spray paint, used only for blackening bolt heads, will last you about a thousand years. Of course this step, like painting the cabinets before covering, is entirely optional.
If you will not polemount your speaker, skip this section. You have not cut the hole in the cabinet bottom for the top hat yet, and there is a reason for this. Since this speaker will be hoisted on a relatively thin pole, you want the top hat at the optimum front-to-rear balance point. Stick your baffle in the front of the box and secure it with a couple drywall screws (carpet will cover the holes you're making, later). Mount your drivers in the baffle holes with 2 or 3 bolts, whatever it takes to hold them briefly but securely. Put the back on the cab; if it's loose try a little duct tape or masking tape to hold it. You now have all the heavy items in the box. Sit the box on a broomstick, piece of dowel, or any similar cylindrical object so that the rolling surface is parallel to the cabinet front, and determine the front-to-rear balance point. Mark it. Your top hat will go at this point, centered side to side of course, and the pole-mounted weight of your cabinet will be focused straight down rather than trying to topple itself.
Now cut the holes for the top hat and its associated bolts and nuts. Some people put a second layer of plywood on the inside bottom of the cabinet where the top hat will go; that may be overkill except on very heavy cabs or where you are using thin cabinet materials. These cabinets will be relatively heavy, and I had leftover scrap plywood of the right dimension, so I secured a second "floor" of ½" plywood inside each cab, just for insurance. A second layer is probably a necessity if you are attaching casters or top hats with wood screws, but again my advice is, don't use wood screws for these critical attachment points. You can also get inside-mounting steel plates for your top hat which are like giant reinforcing washers; Parts Express carries these but I don't know that they're worth the cost and they do not allow use of T-nuts on the interior. Take your speakers out of the baffle, clean your work area well, and find that roll of carpet.
Some people use thin vinyl or "Tolex" to cover cabs. To me, that requires too much smoothness and precision in the underlying cab surface, and it will rip and tear under even moderate use. Sleeve-type vinyl or fabric covers help with that problem, but they are another expense and/or fabrication challenge, and have to be stashed somewhere during a gig. Some people now use a textured cabinet coating that goes on as a thick liquid and is used mostly for spraying pickup truck bedliners. It's tough but a mess to apply. Carpet is the way to go - thin synthetic carpet, almost like thick felt. It will take abuse, will hide minor surface imperfections in your cabinets, and will have the added advantage of serving as a cushioning and sealing "gasket" for speakers, handles and so forth. Amazingly, I have not found this stuff locally, and order it from Parts Express, which has two varieties, backed and unbacked. The unbacked is OK for most uses and easier to work with because it is thinner and stretchy; the latex-backed stuff is stiffer and more difficult to bend and maneuver but a bit more durable. I'd recommend the unbacked carpet for your first-ever project for its ease of use, but for these cabinets I will spring for the toughness (and slight extra cost) of the backed material. Either way, cut it to the rough dimensions you will need, leaving several extra inches all around. Again, clean the sawdust from your floor before you lay it out; if the floor surface is "moppable," do that too. If there are sharp folds or creases in your carpet from shipping, lay it out flat the night before using it and let it "relax" a bit.
You can use wood glue to glue the carpet to the cabinet. I don't like that approach because I have had wood glue soak through the carpet and appear on the outside of the cabinet, which is only partially fixable with water and paper towels and then only before the glue sets into a shiny acrylic spot on the outside. I open the garage door for ventilation and use 3M "77" spray adhesive, which goes on uniformly, gets tacky quickly, but doesn't set so quickly or hold so strongly that you can't peel off the carpet and try again if it's misaligned or has unwanted folds or bumps.
NOTE: you could also use 'Contact Cement' - this is painted on and allowed to dry until its 'tacky', however its a one shot thing - once you put the 2 'tacky' pieces together, they are not going to move ever again, so you have to make sure everything is aligned right. If this is the first time building and covering cabinets, you may want to use the spray on 3M "77" spray because it allows you better control during this process - any mistake here can make a great set of cabinets look poor.
You will have to have a seam somewhere, and that somewhere is where you start affixing carpet, usually at the center of the bottom of the cab. I put a little glue on half the bottom, take the end of the carpet, overlap the centerline a little and put in a couple staples to hold the carpet. Then I go around the cabinet, gluing and stretching, until I'm back at the starting point. Use especially generous amounts of glue at the edges of the handle holes, to ease trimming. Make sure there is enough material overlapping the fronts and backs of the cab that you can do the wrap-around later (i.e. more on the front, where there is a greater recess). When I've overlapped my starting point and everything is glued down well except the area right around the bottom seam, I remove the original hold-down staples and take a very, very sharp razor knife (buy a new blade for this, they're cheap) and cut the seam through both thicknesses of overlapped carpet so they will butt together perfectly, then peel them back and glue them securely. For overkill, I sometimes staple along both edges of the seam and put a little flat black paint on the shiny staples, which is a good reason to use black carpet. It can show dust and wear, but it's easy to match with paint if and where necessary.
Then go around the cabinet edges, cut the corners diagonally where the carpet will fold over the corner edge and then straight where it will fold over into the inside of the cabinet:
Wrap the cut edges around to the inside of the cabinet and glue and staple it down as you go. Staples placed back by the braces (i.e. where the edge of the baffle board or back board will go) will not be visible after assembly. Small overlaps or gaps at the very corners will be covered by the corners you screw on later. Aren't you glad you painted this area flat black, so gaps and goofs don't show much? With your razor knife, trim off excess carpet where the sides meet the bracing strips. Carefully trim out the holes for handles, top hat, etc. If you mount the handles now, it will be easier to work with the cabinet from now on. Stand back and admire your work, you're in the home stretch.
If the corners you're using aren't the "wraparound" kind with a "finger" that hooks the interior of the corner and will interfere with baffle and back fitting, you can install them now and they will hold the carpeted surfaces just a bit above the floor and keep them from picking up so much dust and dirt. If you use top hats, install them now too.
Now try to place your uncovered-wood back and baffle in the cabinet. If they are snug, that's good; after the carpet is wrapped around them they will be even more snug. But if they will not fit in the cab without a lot of force, then after they are wrapped they probably won't fit in at all, so put all the carpeted stuff out of the way and trim them slightly and evenly with your circular saw. Paint them black if you want to. Or don't; your call. They will suffer less abuse than the sides of the cab. If you are using T-nuts that will be installed under the carpet to secure the baffle, make sure they're on there before you carpet it.
Baffles and backs can be covered so the carpet wraps around the back, or so that it's trimmed flush right after it covers the edge. In either case, use especially generous amounts of glue where the carpet spans the edges of the speaker holes and wraps around the edges of the baffle, and hold the carpet in place with staples on the edge or back. When it's dry, trim the excess "wraparound" carpet, and trim out the speaker holes in the baffle and jack-mount hole in the back. Mount the jacks (unless you want to wire them first, but soldering is easier when they're mounted and can't move around). If you can't find holes you predrilled under carpet, an icepick(or a long nail) inserted from the back , through the T-nut hole, will help. Thrust through from the front, it will probably clear an adequate hole for your bolt or screw. For holes near a mounting edge you can cut a small "V" from the bolt hole to the inside edge of the speaker or handle hole with your razor knife, because the speaker, handle or whatever will cover up your cut when mounted.
Before you finally mount the baffle, it's a good time to put eggcrate foam on the inner sides of the cab while they're still easily accessible. It purpose is to dampen "standing waves" from the back of the speaker bouncing from side to side in your cabinet and causing an unpleasant resonance at certain frequencies. Some people use carpet for this, but it's dense and takes up valuable interior volume, whereas open-cell foam sold for this purpose is mostly air and works better, which is why you see it on the walls of recording studios. It is easily cut to size with scissors. Some people use fiberglass insulation, I guess because they like getting fiberglass impregnated in their skin and having the paper backing buzz against the cabinet and like people to see that bright pink stuff inside the ports (ugh). In a pinch I have also used that rubbery stuff they sell in rolls as shelf liner, although it's not as absorptive as other materials. It's hard to beat eggcrate foam. Cut it to size, glue it in (3M 77 or even silicone glue) and staple it in place, on all four interior sides of the enclosure. Later you will put it on the back also; the baffle doesn't need it because it's full of speakers and doesn't have much reflective area to speak of. Test-fit the back, stick your head in the cabinet and shout. It will sound "dead." That's good.
OK, now we put the carpeted baffle in and this all starts to look like a speaker box. It should fit snugly, but be careful because with all those big holes, it's not as strong as it will be when all those big round metal things are bolted into it. If you haven't preinstalled T-nuts, screw the baffle to the bracing with drywall screws. From the back is OK if you don't want the screws to show, but use enough that the baffle won't fall out absent a thermonuclear explosion. You can glue it too, if you don't want it to be removable; I don't recommend that. Go get your speakers from their safe hiding place, and bolt them into your T-nuts. If one of the t-nuts falls out the back, just stick it on the bolt from behind and snug it up. Work around the speaker, tightening each bolt equally and snugly. Your ports should be a press-fit if you cut the holes for them correctly, and they weigh next to nothing so they will probably stay put. Even so, I run a bead of silicone adhesive/sealant where the port tubes enter the inside of the baffle (that is, on the interior of the cabinet) for a little extra insurance.
Before you proceed to wire the speakers or install the back, you need to find a way to protect those delicate speaker cones - both from your further construction activities, and from the knocks and abuse they will take over the next many years. The simplest kind of grille is a round metal-mesh affair the size of each speaker, used both for pro sound and auto-sound cabinets so they are easy to find. They mount with four little lever-dealies and screws (t-nuts if you're anal, but all you need to do is hold them in place so wood screws are probably OK). Remember, though, these are mounted to the front of your front-mounted speaker, which probably has a paper gasket on the front, and the grilles themselves can add about ½" of depth when mounted. You don't want them projecting past the front edges of your cabinet, so make sure your baffle board is recessed enough.
An alternative is grille cloth, like so many guitar amp manufacturers use. They are into high-speed production, so typically they will mount the speaker to the back of the baffle board, cover the baffle itself with grille cloth, and slap the whole thing in a cabinet. Your speakers sound better front-mounted, grille cloth may or may not be sonically "transparent," and no matter how tough it is, it doesn't have the protective qualities of metal mesh. If you do use grille cloth for looks, front-mount your speakers and suspend the grille cloth on a separate "frame" with the outside dimensions of your baffle board, and holes or entire giant areas cut out for the sound to come through. Better still, leave the grille cloth to the amp makers and home hi-fi speakers that don't get rough use, and use metal mesh.
A third alternative, if you can find a source of supply, is a flat sheet of "expanded metal mesh" that is used for fencing and so forth, cut to the size of your entire baffle board. It is heavy and hard to work with and the sharp edges will slice your skin to ribbons, but it works great and will show off the cones of your expensive speakers. You may have to buy it in big sheets (check the yellow pages, make some calls) and cut it with a metal-cutting blade, or wire by wire with "diagonal" cutting pliers. If you get it unfinished (bare metal), you will have to lay it on newspaper and paint it (I use black epoxy spray paint, tough stuff but takes 1-2 days to dry, and you need to paint both sides). Either attach a (black or carpet-covered) frame of small-dimension wood to your baffle board, or use some rubber cabinet feet as spacers. Use black wood screws and black finish washers (sometimes called "cup washers") into the wood frame, or through the rubber feet into the baffle board. Just make sure the frame or rubber feet hold the mesh far enough from the face of your speaker(s) that it can't buzz against the face of the speaker gasket. This can require more baffle-board "setback," relative to the cabinet edge, than other types of grilles; I figure the amount of projection of my speakers in front of the baffle board, plus at least an inch. In this case I cut metal mesh to a size to cover the part of the baffle board that houses the speakers, then placed a carpet-covered piece of wood as a spacer/divider between that area and my horn, which doesn't need to be covered. I just like the look:
You've got a great-looking cabinet from the front, but it's still open in back. If you are using a crossover, in-line resistor (a good idea for some piezos, or for equalizing the level of large mismatches in woofer and tweeter efficiencies), L-pad or any other sort of internal electronic doodad, find a way to fix it securely to the cabinet back (I use Goop or silicone glue). If your speakers have slide-on terminals, you can find the connectors you need at Radio Shack or most any electronics-supply house, but make sure they are very secure and won't slide off as the cabinet is battered around. My woofers have spring-loaded terminals which will take bare wire. But nothing beats a securely soldered terminal connection; make sure your input-jack connections, at least, are securely and cleanly soldered.
I've spent a lot of time and money over the years buying or making speaker-connection wires from my amp to my cabinets out of 16, 14 and even 12-gauge wire, because those speaker wires carry considerable current and excessive resistance in them just wastes power. Then I've taken apart commercial cabinets and found the manufacturer used wire thin enough to floss with. This is a "weakest link" situation: use the heaviest stranded wire (18 or 16-gauge, probably) that you can still connect to the terminals you're using. I like the clear-jacketed stuff with one silver strand and one copper strand because it's easier to figure out phasing; I always connect the copper-colored wire to the "plus" side and the silver-colored wire to the "minus" side. There is a handy guide to wiring on this site and several others. Leave enough excess wire so that you can remove the back and look inside for that diamond ring you lost during assembly, but not so much that it coils around inside and buzzes against things. Route the wires between multiple drivers so that when the back is installed, none of the wiring can fall against a speaker frame and buzz. Little wire-routing clips screwed to an inside surface can help with this. A two-conductor "quick disconnect" plug can be handy for later back removal, but make sure it has generous contact areas and snaps together snugly or it's just one more thing to go wrong. Once you build a good speaker cabinet, you won't be removing the back all that often anyhow.
Glue and staple foam to the inside of the back panel, if you haven't already. Put that snug, press-fitting back into the cabinet, but before you secure it, plug the cab into an amp and test it. Listen for "hiss" coming out of all the speakers individually, a sure sign that they're wired up and functioning. Play something through them, but not so loud that you blow the back off and yank the wires loose as it falls. The voice of experience, there.
You could have secured the back with bolts and T-nuts, but all it really has to do is hang there and trap air in the box; it doesn't support anything, really. I use drywall screws and finish washers, and I don't even predrill the holes since drywall screws will pretty much thread themselves through the carpet, back and bracing (a regular nail and a couple taps will give you a fine starter hole for the screws). If you find yourself removing the cabinet back a lot, bolts and T-nuts are an easy retrofit someday. Space them however you want, but use enough so that sucker is secure. I spaced mine about 6 inches apart. A photo wouldn't tell you much because black screws and washers disappear against black carpet, which is exactly what you want.
Okay, those of you with sharp eyes, despite my cheesy digital camera, may have noted a couple little black rectangles on the sides of the cabinets, above the handles. Can't fool you, huh? This was a last-minute design addition, for which I have to thank the author of a book called Loudspeakers for Musicians. I don't know the author's name because I lost the book somewhere, but the principle is pretty simple, and plays off three undeniable truths:
(2) The "wing" placement of typical PA cabs creates a "sound hole" for listeners standing right in front of the stage, because the PA cabs are off to their sides and pointing behind them, so the guy who thought he got the best seat, gets the worst sound, unless you use little fill cabinets pointing forward at center stage (we've done that). In fact, I think a good argument could be made that wedge monitors should be bi-directional and have speakers on both sides for this reason. Maybe that should be my next project.
(3) The higher the frequency of a tone, the more directional it is. Your horns help disperse high frequencies a little, that's one reason they're shaped like they are, but you'll still be lucky to get 100 degrees of horizontal dispersion for the high notes (as opposed to low bass notes, which are fairly nondirectional and go in every direction like cockroaches). So the "expensive seat" guy, who is 90 degrees off-axis from the cabinets, may hear a little PA sound in the lower registers, but that sparkling tweeter misses him entirely.
The piezos in the photos are Motorola/CTS model KSN1141, which are also part of the "Powerline" series with horn preattached in a tiny, 3" X 7" format. I don't want them nearly as loud as the main horns, but I can adjust that with cheap resistors or L-pads. For this application, I place a 20 ohm, 20-watt resistor in series between the "+" lead of the main horn driver, and the "+" lead of the small tweeters, which are themselves hooked in parallel with the main horn, but in series with each other; series connection diminishes the output of multiple piezos. Their output is thus much lower than that of the main horn, which works for their purpose as a source of "fill-in" sound. I then install a switch so I can disconnect them entirely by opening their little part of the circuit, and an L-pad (variable resistor) so that I can adjust their level down even more (even "100 watt" L-pads can burn out at high output levels, but I have already severely reduced the current level going to these little tweeters, using the fixed resistors). The switches and L-pads are installed in a second jack cup on the back that was intended for two ¼" jacks, but works for this application too. These side tweeters would be stupid in any other sort of speaker-box application, where the cabinet might be at your side or behind you, but for typical PA-on-pole placement it works well.
Side-firing tweeters spread the sound around.
Since building these cabs, we have used them in various venues with no complaints about sound projection, quality or power-handling. On proper stands, they are tall enough to project the highs over the audience. They will take the full range of sounds from our PA. We even tried playing instruments through them and they were surprisingly good, with plenty of low-end punch and acceptable tonal balance. Biamped with separate subs, they are even better. They weigh 75 pounds apiece, which is a lot, but considering that the two speakers in each cabinet weigh almost two-thirds of that by themselves, and that they are now secured in three-foot-high braced and covered wooden cabinets, that's not too bad - most good aluminum tripod-type speaker stands will hold at least 80 to 100 pounds (at the same time, lifting these cabs onto a pole is definitely a two-person job). Set face-to-face in a trailer or van, my two cabs are not a whole lot bigger than a single 4-12" guitar cabinet.
From time to time, I check out my finished cabs, testing bolt tightness and gluing down rips or tears in the carpet. I remove the backs now and then to make sure all the wires are still secure and that no wasps or bees have flown in the port hole and made a nest in there (I've seen this happen). Over the years, the ports will allow dust and dirt into the cabinet interior, easily removed with a hose-type vacuum cleaner. A solidly-built cabinet can be recarpeted, or have its corners and/or grilles replaced, and look just fine for the rest of both of our lives. Because I make sure my baffle boards are removable, I have even fabricated new baffles for existing boxes to fit a different size or assortment of speakers.
I have a few more speaker-cab projects on my "to do" list, and I hope to describe them in other articles later, but you can apply these same construction techniques to just about any cabinet you build. Have fun and enjoy your finished work!
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