Shavano Music Online

Adding subwoofers to your PA system

NOTE: Let me say right up front that you only need to read this article if you are lucky enough that your group is getting bookings in large or outdoor venues.  If you are only playing small clubs, or just trying to “get tight” in your garage, you don’t need to worry about bi-amping and subwoofers and such just yet.  But if you have a good set of main PA speakers that are starting to sound strained at those large gigs, you might want to think about a next logical step.  One might be to just add more full-range speakers and/or amp power to what you have already. But if your main speakers are already capable of very loud midrange and high-range sound without distorting, subwoofers can make their lives a little easier by taking the low-end load off of them – and can improve your overall sound considerably by allowing you to project low frequencies your main speakers were never designed to handle in the first place.

An occasional contributor here, Larry Mundy has played in various rock/pop bands over the years and developed an interest in live-sound reproduction and do-it-yourself speaker projects.  Many of the projects in the articles on this site are in current use by his group,  www.yournamehereband.com.   You can send comments, questions, or scathing diatribes to Larry at lmundy@attbi.com

04/03 - Larry Mundy http://www.colomar.com/Shavano/subs.html


In our last spine-tingling episode, “A Speaker Cabinet Project in Words and Pictures,” we were making PA cabinets to be hoisted on poles and carry the main PA signal to our admiring fans.  As I said at the end of that article, the result worked great.   But those cabinets weren’t designed to handle the thundering low frequencies of bass instruments or full-range recorded music, and my guess is yours weren’t either.  At some point in a band’s evolution, it becomes convenient or even necessary to “mike” instruments, as well as vocals and maybe even a little CD player during the breaks, through the main PA.  This requires PA speakers and amps that can reproduce a full spectrum of sound.  For most folks with a decent PA setup, this means simply adding subwoofers and subwoofer amps to extend the low range of the overall system. 

By their nature, subwoofers are big, heavy things, with giant speakers in thick-walled cabinets, and need to be powered by big, heavy amplifiers.  Yes, this is a lot more stuff to load into the truck and lift onto the stage and set up and lift off the stage and cart home, over and over.  But for large venues, it’s pretty much a necessity.  One side benefit, though, is that a lot of the zillion-watt instrument amps can be left at home, if you have a truly powerful full-range PA.  The guitar player can get the tone he wants from that little vintage amp, and play as loud as he wants through the simple expedient of a microphone strategically placed in front of his little-vintage-amp speaker.  The bass player, the keyboard player, everybody can play a little softer through smaller, lighter equipment.

You’ve been to concerts; this is how the big acts work today.  They don’t have stacks of speakers and amps stacked to the ceiling behind them; they make their sound with relatively modest equipment and the PA system, which is the size of a suburban house, carries it throughout the arena.  The little amps onstage are almost like monitors and the musicians aren’t playing much louder than they would in a small club.  As a result they can probably hear fairly normal levels of conversation when the gig is over, a decided benefit compared to the old days when you could tell what size shows a performer played, by how deaf he or she was by age 30. 

What kind of PA do you need to play to a 20,000-seat arena?  The kind that you rent from pro sound companies who show up with crews and trucks and test equipment and do it all for you.  If you get big and famous enough to play a 20,000-seat arena, you won’t have to know anything about subwoofers because your business manager will do all the paperwork with the sound company and you can sit in a dressing room full of groupies and gripe about the expensive caviar and scotch the promoter provided, or rehash that lick from your record that you blew big time last night in Cincinnati.  And you will never again have to read articles like this.

But how about us normal humans playing outside or to a largish crowd, but having to pay for and carry our own equipment?  We may need subwoofers too, but more modest ones than the ones the pro sound guys hoist into the arena on cranes and stack eight-high.  Because this is a relatively expensive undertaking, I’d advise you start with just two, one on each side of the stage, with a single driver in each.   And make them of a reasonable size so that one or two people can carry them from the van, into the club and up the stairs to the stage, without dislocating both shoulders.  But unlike some of my other articles, this one won’t be a step-by-step construction project, for a very good reason: I already have a nice set of subs and don’t need to build any more, and I don’t have photos of my subs’ creation.  So this will be a more theoretical discussion, without all the photos of plywood and screws.  Just a lot of boring text.  Sorry.

There are three basic components involved in adding subwoofers to your existing PA setup – a crossover, another amplifier, and the subs themselves.  You have probably heard, or certainly will if you keep reading this, about the concept of “biamplification” or more commonly “bi-amping.”  This just means running the same signal from a source (a mixer/preamp, probably) to two separate amps driving separate sets of speakers.  But usually the term also implies that the separate amps, and separate speakers, have different functions – most commonly, they cover different parts of the frequency spectrum.  So you can think of the additional components required as those shown in yellow on this diagram:

BIAMP.jpg (46052 bytes)

I know this looks like a lot of added complexity, but these are fairly simple devices you're adding, without a lot of knobs and switches and adjustable doodads.  I'll discuss each component in turn.

Passive vs. Electronic Crossovers.

Let’s talk about the first new item in the chain, the crossover.  You are probably used to the type of crossover found inside most hi-fi speaker boxes, and most full-range performance cabs.  It is some arrangement of capacitors and filter chokes (little tube-looking things and coil-looking things) and takes an incoming full-range signal from an amp and splits it electronically into low and high frequencies, feeding the appropriate frequencies to the appropriate speaker within a cabinet.  There is a lot of information on this site about design criteria for this "passive" type of crossover, and it is the perfect solution for distributing frequencies in a full-range cabinet, up to a point.  As you crank up the power-handling of your system, however, this type of crossover has some disadvantages.   One is that it is a passive load between your amp and speakers, and introduces some loss of efficiency.  Another is that it has limited power-handling ability; since it is handling the already-amplified current from your amplifier’s output.  While this can be overcome to some extent by using heavier-duty components, there is a point at which heavy-enough capacitors and chokes simply aren’t available or are so costly as to be ridiculous.  Next, each such design “crosses over” at a specified frequency or set of frequencies dictated by the capacitance and inductance values of the components and the impedance of the speaker loads, so they are nonadjustable and best suited for a single cabinet with a known, fixed array of speakers.  But the big drawback of passive crossovers is that they expect a single input from a single amp, which they then cheerfully divide by frequency.  If you are adding subwoofers to an existing PA setup, trust me, you need another amp for the additional power it provides.

An “active” or electronic crossover is simply a gadget that divides frequencies at line-level (i.e. as they come out of your preamp, mixer, whatever) before they reach the power-amplification stage.  That is, they split line-level signals by frequency, and spit out separate line-level signals for highs and lows (or even mids, if it’s a fancy one), ready for separate amplification.  And component-type electronic crossovers almost always have adjustable crossover frequencies to fine-tune their workings with the capabilities of the rest of your system.  Inexpensive electronic crossovers are not fantastically expensive, with one-unit rackmount versions starting at maybe $100 US or so (and up, of course, but even the relatively inexpensive ones do the job).  What you usually get at a lower price point is two channels of adjustable frequency division, so the unit can be used to separate two channels of highs and lows in a stereo system, or three channels of highs, lows and mids in a tri-amped system.

You may wish to put the electronic crossover and sub amp together in their own rack case, and only wheel them out for those larger gigs.  However, this means running a line-level cable from your mixer/preamp to the rack case, and then another set of line-level cables back to your main amp.  It’s more convenient, I think, to mount your crossover in the same rack as your preamp/mixer.  If you decided not to use your subs and sub amp for a smaller gig, you can just set the crossover to deliver full range to your main PA speakers – that’s the beauty of adjustable crossover frequencies.  You can even use it as a sort of filter to keep the lowest frequencies out of your main speakers, by setting the unit to send the very lowest frequencies (i.e. below 30, 40, 50 hz or whatever the low-end cutoff of your mains speakers might be), to a nonexistent sub amp.

It is possible, maybe even easy, to find a main amp or subwoofer amp has a built-in electronic crossover feature.  This may be a way to avoid buying another piece of equipment, although built-in crossovers tend to be nonadjustable and generally place the crossover point a little too high for my tastes.

Powering your subs

Even though subwoofers only reproduce a limited range of sounds, the physics of sound dictate that they be large and heavy because the lowest sounds are the most difficult to reproduce.  And reproduction of low notes takes LOTS of amplifier power, probably requiring an amp with an equal or greater power rating than you use for your main speakers.  So when I tell you that you can build a decent pair of subwoofers for around $500 US, or buy them prebuilt for maybe $600-$900, I’m talking about the speakers and cabinets only.  The power amp to drive them might cost that much again, or more.   Even if you drive your mains in stereo with a two-channel amp, look for a subwoofer amp that can be “bridged” into a separate mono signal at high output power; “stereo” is a silly concept with low-frequency sounds anyway since they are essentially nondirectional. 

I am assuming here that you will want to power your subwoofers with a separate, outboard amplifier.  It is possible to integrate a power amplifier into the subwoofer cabinet, as the hi-fi sub manufacturers do, and there are powered subs on the market for live music performances as well.  Actually, the “Ampzilla” project I wrote about here (www.colomar.com/shavano/asdfawgasg) is really just a powered subwoofer with horns to extend the high range, and no electronic circuits to filter out those high frequencies.  But since subwoofer drivers are heavy and require heavy cabinets, and powerful amps are likewise heavy, a really powerful powered-sub design has the potential to crash through the earth’s crust headed for China and cause volcanic disturbances in the molten magma.  Or at least, they can be extremely inconvenient to transport without a strapping crew of helpers. 

Also, there is the problem discussed in the Ampzilla article that really heavy-duty amps tend to have cooling fans and require flow-through openings in their cabinets.  This adds complexity to the cabinet design.  Amps that do not have fans are either very expensive, or in most cases, not very powerful.  Finally, you are probably powering your PA main speakers with an outboard amp, running ordinary speaker cables to them.  With powered subs, you would have to take off a separate, line-level signal from your mixer or electronic crossover and feed that to the amp in the powered sub, which is probably 10 or 20 feet away.  I don’t like to have line-level signals travel that far, at least not through unbalanced, two-conductor cables which are crossing AC lines and other cables in the nest of snakes on the stage floor.  The tried and true method is a separate power amp somewhere near your mixer and other power amp(s), where the line-level signals from your electronic crossover can reach it easily.

NL8MPR.jpg (26789 bytes)Your subwoofer amp should be just as powerful as your main amp, and maybe more, and have lots of ventilation or fan-cooling.   “Bridging” capability, for greater power into a monophonic load, is a good idea.  And most appropriately-sized amps will have either “banana plug” or “Speakon” connectors rather than ” phone jacks for speaker connections.  Use them, and make sure a similar-quality connector is on the subwoofers themselves.  The teeny tiny contact point that a ” phone plug makes in its jack is simply not enough to carry thundering bass signals reliably.  Speakon connectors lock positively and provide a secure connection.  Big, heavy cables are a good idea too.  If your subs are going to put out thundering, industrial-quality sound it makes sense to have heavy, industrial-quality components all along the signal chain.

The subs themselves.

OK, you have an electronic crossover, a heavy-duty amp and cables.  What do you use for subs?  As always, you have the choice to buy ready-made subwoofers, or to build your own and save some cash.   There is a huge variety of ready-made subs out there; most are similar in that they have a single, very heavy-duty driver – usually 18” in diameter, which yields far more “piston area” than a 15” speaker.  That driver will be housed in one of three types of cabinets – reflex, bandpass, or folded-horn.   Each has its advantages and disadvantages as I will discuss below.  The usual sorts of buying tips apply here – what is the manufacturer’s reputation, the type and length of warranty, the power rating of the driver/cabinet combination?  Larger music stores, or “superstores,” may have a try-before-you-buy option or at least the ability to audition the subs.  Try to pair them with an amplifier similar to what you’ll be using, and feed them a variety of bass signals.  Believe it or not, 4-string bass guitars going down to about 41hz don’t give subs all that great a workout, although a 5-string bass with its low “B” at around 31 hz is a possible test.  Prerecorded music is not a good test because the bass is frequently compressed in the mix, but there are test CD’s available in the $10-20 range that will give subwoofers a workout.  An electronic kick drum is a good test for deep, quick transients, but even better is a keyboard synthesizer with a half-octave below the typical bass guitar’s low “E”.  Fiddle with the synth to get a pure tone, no effects or modulation, and see how your intended purchase handles a quick, deep note.  There should be no after-ring.  Now try a sustained deep tone.  If you can feel the tone in your gut as much as hear it, and if the subs can extinguish a candle held in front of the speaker or ports, you’ve probably got a winner.  Plunk down a wad of cash for a pair of those babies and you’re in business, assuming you have a trailer or very large vehicle to get them home.

The do-it-yourself option.

Since they are so large and heavy, a good portion of the cost of store-bought subwoofers is attributable to their shipment, storage, manufacturer profit and warranty costs, marketing and so forth.  You can save considerable money building your own, but perversely, you will spend more for the speaker drivers themselves than the manufacturer does, because you’re not buying in lots of 1,000 or more.  You can still save a lot of money, but don’t try to do so by scrimping on the raw speakers themselves.

I wrote an article on this site about shopping for raw speakers, which I hope will help you shop for appropriate sub drivers if you undertake a do-it-yourself project.  For subs, what you want are drivers specifically designed for this sort of work, with big, heavy cones and magnets, long excursion, and large (at least 3”, 4” is even better) voice coils.  They should be rated for a few hundred watts of amplifier power.  These are for subwoofer use, so you don’t care that the upper-end frequency response may be very limited; you will probably set your crossover at 100hz or below anyway.   In the low range, speaker – cabinet relationships have just as much to do with resulting frequency response as the driver’s own response characteristics, so table-flat frequency response or the driver in “free air” is sort of irrelevant. You want the speaker-cabinet combination to be capable of thundering down to at least 30hz or so, which is easiest to do if the “Fs” (free-air resonance) of the speaker is somewhere near that.  It helps if the speaker has generous “Xmax,” or excursion capabilities.   But basically, you want these speakers to take enormous abuse and come back for more.  I’d recommend cast-frame drivers if you can afford them, both for the precision this offers in voice-coil centering and the assistance such frames lend to cooling.  Between two speakers of apparently-equal capabilities and cost, choose the one with the highest efficiency, or “SPL.”  It takes so much power to deliver pounding low frequencies, if you can help your amp by connecting it to a more efficient speaker, it will thank you forever and reward you with more headroom and cleaner sound.  And finally, there’s the “grunt test.”  Do you grunt when you pick one up?  That’s a good sign.

There are limits to how low, and how loud, any speaker can play.  In the subwoofer arena, the 18” speaker is king because its larger cone area can push much more air for a single speaker.  15’s have to work harder to reach the same output levels but are a good choice for economy of size if your requirements are more modest.  One 18” is usually better than two 15’s, though, because of the size and complexity a multiple-driver subwoofer introduces.

Some manufacturers make variants of their normal pro-audio woofers just for subwoofer use; for example, Eminence makes an “LF” variant of many of their speakers designed to have enhanced low-frequency abilities, usually at the expense of high-frequency coverage.  Electro-Voice, JBL, EAW and Selenium make fabulous, heavy-duty subwoofer drivers, but they are expensive, over $250US per speaker, and if you’re going to spend that kind of money you might consider the store-bought option more carefully.  Gauss used to make very heavy drivers but I understand they are out of business.  18-inch Peavey Black Widows are decent although not outstanding for this purpose, but they do have the advantage of field-replaceable coils and cones if you blow them up.  I personally think the best price-value point for building subwoofers at a controllable cost probably comes from Eminence, or one of the private house brands (like Parts Express’ “Dayton”) that I suspect are built by Eminence anyway.  Get prices and specs on 2 or 3 likely drivers, but don’t buy them until you decide what you’ll put them in.

Enclosure design.

 Sealed boxes.

closed box.jpg (16279 bytes)OK, let’s talk about subwoofer design, which is actually sort of interesting if you’ve had lots of coffee and have nothing better to think about.  Chances are you may have a “home theater” subwoofer in your house, which is usually a little black self-powered cube that does low-frequency work in your living room.  It probably has its woofer in a sealed, unported box and within its power and design limitations, and placed in a small room with little background noise, sounds just great as you watch videos of crashing waves or stomping dinosaurs.  It may have flat frequency response all the way down to inaudibility.  But sealed-box design is not a good choice for live sound applications.  Because the woofer is working against that sealed-box volume of air, the design is relatively inefficient.  Woofers designed for small, sealed enclosures have very compliant surrounds (usually foam or butyl rubber) to allow them to move easily and quickly – which also makes them more delicate.  Few pro-sound woofers are designed this way, because the manufacturers know the horrible abuse you and I will inflict on them.  If you want to project bass at high volumes in a largish room, a sealed-box design is a silly way to do that.

Folded horns.

It was discovered long ago that a sound source can be louder and more resonant if attached to a cone, or horn, that concentrates the sound waves and keeps them from diffusing into open air.  So early little sound-generation devices, like the mechanical ones in early record players, were hooked to big horns.  If you have ever seen the classic “RCA” logo with the cute dog cocking one ear toward the big brass horn, you know what I mean.  Horns are still the most efficient, effective way to go for speaker design, which is why your one-inch-diaphragm tweeter may well be connected to a flared, ten-inch horn.  For larger speakers, horns can only be approximated in the cabinet design, because a horn “tuned” to the lowest frequencies of most recorded music would have to be about 30 feet long and would be difficult to fit in your Nissan.  So there are various “folded”-horn cabinet designs which consist of a speaker firing into an increasing-circumference chamber of some sort, usually designed into the enclosure cabinet itself.  In most of these, the speaker mounts in an interior corner somewhere and the rest of the box approximates a horn folded up like a spiral seashell.  “Scoop” and “W” cabinets are just variations on this design.   Here are some folded-horn variants:

 2x15-FOLDED.gif (15938 bytes)   lr-36.gif (28759 bytes)   15-bass-scoop.gif (27525 bytes)

 I know, the photos don’t tell you much, because all the “horn folds” are inside the cabinet.  Here’s a diagram of a “scoop” horn, so you can see some “guts”:

scoopguts.jpg (16082 bytes) scoop.JPG (15902 bytes)

A proper folded-horn design is, without question, the most efficient type of subwoofer, particularly if it’s as big as Wyoming.  Unfortunately, most of them are at least as big as Rhode Island, because all that folded-up horn leads to monstrous cabinet dimensions.  I have seen folded-horn designs housing a single 18” speaker that would be comfortable accommodations for a family of four.  Even some of the compact “scoop” designs can be as large as your average clothes washer or dryer.   And all that interior air-passage construction can get fairly complicated.  Thus many people decide they can live with the somewhat lower efficiency of smaller-box designs.

 There are two basic types of bass-horn enclosure: front-loaded and rear-loaded.  This refers simply to which side of the speaker the horn is attached to, since sound waves come out of both sides of a cone speaker.  In a front-loaded design, the rear of the speaker is usually sealed in some sort of chamber with the horn sticking out the front, so it works something like a sealed box with a front horn augmenting the sound from the front of the speaker.  Any horn cabinet where the speaker is not visible is probably of this type, with the speaker itself in a sub-chamber inside somewhere.  This is probably the most accurate-sounding design but it can be fearfully complex to build, with little access doors for the speaker which is wedged up into a tiny space and a labyrinth of passageways.

A horn cabinet where the speaker itself is visible is (1) probably not a horn design in the first place, but maybe just a reflex cabinet with little horn-type “wings” in front of the speaker, like the classic Altec "Voice of the Theater"; or (2) a rear-loaded horn.  The idea is to have the front of the speaker radiate toward the audience, and the rear pressure waves travel through a “horn” chamber of increasing cross-section, until they come out the front, hopefully in phase with the main signal.  So it’s sort of like a reflex design in which the “port” is a folded-up horn that takes up most of the interior of the cabinet.  The traditional form is the “scoop” cabinet illustrated above, in which the speaker is at the top, and the rear waves traverse two or three “folds” and exit beneath the speaker.  In such a cabinet, the length of the “horn” is generally a little more than two times the height of the cabinet.   Trace the theoretical path of the back waves from the speaker in the “scoop” diagram above and you’ll see what I mean:

100.jpg (16602 bytes)A variant of this rear-loaded design is the “angled scoop,” in which the speaker is mounted lower and at an angle to the floor, enabling simpler construction because the horn “throat” is above the speaker and the entire periphery of the cabinet comprises the horn.  This type of cabinet has two disadvantages compared to the “scoop”: first, the more directional high-frequency waves from the speaker are pointed at the floor, so the extended range of the speaker which would come from the front of the cone is largely lost.  This is not terribly important for a subwooferThe second drawback is that there’s additional interaction between the down-firing speaker and the horn output, since they radiate into the same space, with frequency-response effects that are difficult to calculate.  But since the wavelength of bass notes is so long, and the speaker in a “scoop” design is so close to the horn mouth anyhow, this may not be a big problem either.  And it's probably the physically smallest folded-horn design.

No folded horn is even close to perfect, anyhow.  A perfect horn would be round in cross-section and expand at a carefully calculated rate.  Fitting a round-but-folded horn (think of a tuba) in a square box would be terribly wasteful of space and very difficult to do, so folded horns are simply an approximation of a proper horn, squeezed into a square or rectangular box.  This gives them rectangular cross-sections, sharp angles, and varying rates of flare, yet they still work. 

Horns are good at projecting low frequencies out into a large audience, because the sound waves stay "focused" longer as they emit from the horn, as opposed to exiting a ported cabinet into undirected, open air.  If the computer program you are using tells you your chosen speaker requires a cabinet the size of a bedroom anyhow, you may want to consider the complexity of a horn design.  There are a number of horn-design plans available on the web or from driver manufacturers.  For most working musicians, this is more effort than it’s worth.  If so, consider a different driver that can live happily in a smaller box that’s easier to construct.

Ported boxes – bandpass and reflex.

If you don’t want to undertake the complexity of a folded horn, a ported design is the way to go.  Ported designs aren't as efficient as horns, but properly designed, the cabinet port emits or reinforces certain low frequencies, allowing you to achieve higher sound pressure levels from a given driver.   But don’t get the mistaken impression your life is now simple.  There is more than one type of ported design, and you will have to do a little math to design your cabinets.   Okay, a lot of math.

The two most common types of ported subwoofer enclosures are known as “bass-reflex” and “bandpass.”  “Bass-reflex” is simply a box with a front-facing speaker in it, and some sort of vent hole or port to help reinforce low frequencies.  Chances are that your bass player’s speakers are built on this model, as are most home hi-fi speakers today.  The PA cabinets illustrated in my earlier article are “reflex” designs, and in their simplest form can be illustrated like this:

 reflex.jpg (15972 bytes)HS-18medium.jpg (46183 bytes)

 A simple “bandpass” enclosure is actually two boxes in one, with the speaker mounted in a hole between the two chambers of different size and not visible from outside (except in car-stereo bandpass boxes where they just can’t stand for that expensive fluorescent-orange speaker to be hidden, so they make one enclosure wall out of plexiglass).  The sound emanating from the speaker comes outside through a port or ports in one (“4th order bandpass”) or both (“6th order bandpass”) of the chambers.  On the outside, it looks like a big sealed box with little portholes in it, because that’s what it is.  The more common 4th-order bandpass enclosure looks like this:

 bandpass.jpg (15383 bytes)

So which ported design do you want for your subwoofer?  Tough call; both have their advantages and disadvantages.  A bandpass enclosure is so named because the effect of the speaker’s positioning between the two chambers of air, which provide a different quality of air-pressure “spring” on either side of the cone.  This restricts the speaker’s frequency response to a narrow range – the “passband,” hence the design’s name.  If the design is done right, that narrow range sits pretty much where we want it to for a subwoofer, i.e. in the lowest octave or two.  Because the speaker’s work is confined to the passband and it is not trying to reproduce higher frequencies, it has a slightly easier life, but this is like saying that you prefer a brief stroll in a category 3 hurricane, to a walk in a category 4 one.

A bass-reflex design, on the other hand, is “tuned” only for its low-frequency response, and the speaker is free to reproduce whatever high frequencies it is capable of, which is why this is the design of choice for full-range bass guitar cabinets and scads of other applications.   Whether this is good or bad depends upon your application.  For a more full-range application, a ported reflex cabinet is the only way to go.  But for subwoofer use, since you will need an electronic crossover anyway to direct the lows to your subs and and mids and highs to your mains, you may never use or hear the higher frequencies a reflex design is capable of, so you might as well chase the promise of higher efficiency or better speaker loading offered by a bandpass design, right?  Well, maybe.   But a bandpass design may roll off higher frequencies a little more rapidly than you want, and can exhibit frequency-response irregularities even within its narrow passband.  If the chambers are too small (known technically as an “S” of 5 or 6), the frequency response can exhibit “camel humps” at the resonant frequency of each chamber and low frequencies can be cut off at too high a point (a 6th-order cabinet design has even bigger camel-humps).  If the chambers are larger (an “S” of 7), this smooths out the response and low-frequency response is extended, at the expense of size.  From a construction-complexity standpoint, a bandpass enclosure requires more internal work, to separate and seal the two chambers from each other, but less external work because there is no external grille. 

Sometimes, depending on the characteristics of your chosen subwoofer speaker, there is a notable difference in size between the two types of enclosures.  With most of the potential speaker drivers I’ve considered, I designed both bandpass and reflex enclosures to have identical roll-off levels at 30hz.  Inevitably, the bandpass design was about 20% larger in overall size to meet this criterion.  These would be big, heavy things anyway, so that 20% may loom large in your planning.  Also, with no bandpass limit on high-frequency response, if you graduate to larger and more powerful subwoofers someday, a reflex subwoofer design might be pawned off on your bass player.  This is not to say a bandpass design isn’t as good or better; it’s just usually the case that a reflex design is a little more flexible, and maybe a few pounds lighter and/or inches smaller, at the potential cost of a little bit of efficiency.

Computer-aided design

Every aspect of the enclosure you build is important to the performance of your sub – the dimensions (i.e. internal air volume), design, porting (if any), lack of resonance, all that stuff.  To design and build a decent subwoofer, you either need a computer and speaker-design software, or a stack of legal pads and a fondness for advanced algebra.  I’d advise that you go the computer route; you probably have one anyway since you’re reading this online.

There are a number of speaker-design programs out there for the PC (there probably are for the Mac as well, I don’t know).  Several of them are freeware or shareware; there is even at least one entire speaker-parameter calculator web site that doesn’t even require that any software be loaded onto your computer.   Since they’re free to download, I’d recommend you look at two or three shareware or freeware programs and see if you find them easy to use.  WinISD seems to be pretty popular, although everything is expressed in metric measurements and I’m one of those metric-impaired Americans, so personally I use a set of programs from AJ Design (one for bandpass, another for reflex) that cost about $15 U.S. and can be switched from metric to English measurements, or vice-versa, from the main menu.  They have some other relative shortcomings but work just fine for me.   And as noted in a previous article, Parts Express’ PEBox is a great tool for calculating reflex-type enclosures, but only for speakers Parts Express sells.  Eminence Designer is good if you have Eminence Drivers, but they’re pretty proud of that program; I think it’s about $70, when you can get similar or identical results for free.

Most of these programs will require that you input your speaker’s vital statistics, or “parameters” such as Qts, Vas, Fs, and so forth.  You don’t have to know what any of these mean, just find them in your speaker’s literature or on the web or wherever such things are found.  Any decent pro sound driver will have such information available.  Check carefully whether the parameters that require measurement, are in millimeters or inches, and that you are feeding your program the information in the format it expects, or you can get some wild and horrible results.

Most such programs will digest this information through a complex maze of formulas, and maybe ask you the number and diameter of ports you plan to use (check electronics-parts sources to see what diameters and lengths are available), and will propose a box design specifying suggested internal volumes, port lengths, and in the fancier programs, box dimensions.  The really fancy ones will let you “lock in” one or more box-side dimensions and calculate the other one(s) for you to preserve the required internal volume, which can be handy if you envision something tall and skinny, or short and squat, or whatever.  Goof with the parameters, box types, whatever; you can design legions of subwoofers right there on your screen with nothing invested but your time.  If you haven’t bought speakers, you can get the parameters for the ones you’re considering and plug them into the computer model, to see what suits your fancy.  You will find very quickly that a driver that doesn’t have a low “Fs” won’t subwoof all that well.  Try a driver with an Fs rating in the 20’s or very low 30’s.

Building boxes.

I’ve already written and illustrated an article about speaker-box construction, and a sub project is no different, so I won’t belabor the steps here.  Use thick (3/4”) plywood, as high-quality and void-free as you can afford.  Your computer program will tell you that you need lots of interior volume, and one way to achieve this without unwieldy height or width is to design very deep cabinets – 20” to 24” of depth is enormous for most speakers, but not at all unusual for subs.  Use lots of screws, glue, and cabinet bracing.  This box will spend its life, literally, shakin’ all over.

SPI-STACKMEDIUM.jpg (30344 bytes)Consider whether you might wish to mount “top hats”on top of the cabinets, which with an inexpensive metal pole can position your main speakers at above-crowd height on top of the subs.  Most premade subs will come with these.  If I am building subs, I will sacrifice some additional interior volume to run a strip of 3” bracing, on edge, across the interior of the top near where my top hat goes, because otherwise, the weight of heavy main speakers (75 pounds each, in my case) will be focused on an area of the top panel which is probably a foot away from the nearest vertical support.  I want the top of the cabinet to be strong enough to support at least a very small elephant, and I want the pole mount to be in the geographical center of the top for maximum stability.  A piece of “angle iron” would probably have been a good choice too.  I know that supporting 75 pounds doesn’t sound like a tall order, but remember that it will be focused on a single pole 1-3/8” in diameter.  And I will not gently lower that 75 pounds onto the pole-top; I will fumble around holding that 75-pound speaker in the air until I get the hole and pole lined up, and then it will KLUNK! down onto the pole, creating the momentary weight equivalent of dropping a car on a housecat.  I’m going to give my cat a little reinforcement at the impact point.  And if the speaker you will polemount isn’t quite well balanced, don’t put it up there, or the whole thing might well topple and hurt somebody. 

 The top hats you use, 1-3/8” or 1-1/2”, will be dictated by the size of the polemount sockets on the bottom of your main speakers, because obviously they both should be the same.  “Quik Lok” makes an adjustable 1-3/8" pole, model S-203, which your music dealer can order for you.  But if you will use the same mains with the same subs gig after gig, you can just get a fixed-length pole.   One source is Airparts, Inc. (http://www.airpartsinc.com/steeltube.htm) who will cut tubing to length and ship it to you.  But this is the same tubing used for aircraft, race-car and custom-motorcycle construction, so the chances are good you can find it locally.  I’d recommend tubing of at least .060 wall thickness for strength.

Cabinet buzzes and rattles, and interior resonances, are the bane of subwoofer construction.  Make sure all panels fit well and are screwed and glued in place.  You may want to consider making the baffle nonremovable, since you will have a huge speaker hole for access to the interior anyway.  Foam or similar sound-absorbing material should line the entire cabinet interior.  Here, too, consider using “Speakon” connectors for cable connection.  Rear-panel jack plates or “cups” designed to fit Canon-type connectors will fit Speakons as well.

rearcaster.jpg (12488 bytes)Your subs will be big and heavy, and even heavier if main speakers are polemounted above them.  Casters are a good idea to help move your subs around, but they must be very heavy-duty to support all this weight.  An alternative is the corner-mounted recessed caster, like those sold by TCH (www.tchweb.com) as its part # 511-2296800; these allow you to roll your subs around at an angle, and them drop them onto more-solid “feet” when they are in place.  Or you can get the same effect on a budget by mounting a pair of non-swiveling casters at the bottom of the cabinet back, and bolting on an inexpensive pair of gate-type handles on the top rear.

The covering material you use is up to you; I’ve written about both “Ozite”-type carpet and spray-on “truck bedliner” finishes, and both will work fine.  I’d recommend a metal mesh grille, either full-face or just covering the driver cone, but in either event secure it so that it won’t buzz or vibrate loose.

Crank ‘er up.

OK, so you have subwoofers, an electronic crossover, a subwoofer amp, and some heavy-duty cables to connect them all.  Set the crossover point initially at about the point your computer program told you the subs’ high-end response would start rolling off, but still above the low-end rolloff point of your main speakers.  Use some prerecorded material with significant bass content to balance the relative loudness of the subs and mains; your crossover may have the capability to do this or you may have to use the “volume controls” on your power amps (the one that’s too relatively loud can be throttled back a little; the least-loud one should be running wide open to maximize the headroom and capabilities of the system).  Subwoofers shouldn’t introduce any new feedback concerns since their operating range is below that of most stage-type microphones.  You have now extended the range of your PA system, and you should find that your main speakers sound cleaner and louder.  Your main amp is not working as hard and has more headroom, and the lowest frequencies are no longer taxing your main speakers.

 


NOTE: We do not work on Home or Car Audio. We work only with Pro-Audio applications. We cannot help you with Home or Car Audio questions.

Questions or Comments about this article? You can write Larry Mundy and if he's not fiddling with some silly project he'll try to get back to you.

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