Shavano Music Online

    Sub Woofer;
    4th Order Bandpass Calculator
    Information

    10/98 - Jens Moller - http://www.colomar.com/Shavano/4th_help.html
    Text updated 3/02

    Construction Data

    Diameter = This is the actual diameter of the speaker in inches (ie. if its listed as a 12 inch speaker, enter 12 here). This information is used to calculate port diameter and enclosure dimensions. Dimentions will be altered based on bracing size - the diameter is needed to make sure that the physical speaker/driver can be installed in the cabinet. Actual speaker cone size is smaller than the speaker diameter..

    Bracing = This is the size of the wood bracing used in the cabinet. The default is 0.0 (ie. none). Common sizes are .75 inches (3/4 inch square strips) and 1.5 inches (1 1/2 inches square strips). These would be used at all of the corners/edges of the construction. Specifing a bracing size will alter the suggested cabinet size to account for the volume that the bracing takes up internally. Please enter measured size. You can choose to give this size in inches or mm.

    The cross section is what we are interested and assume that the material used is wood (pine or similar material) and is cut into material that has the same height and width. You are providing that the height or width. The length will be determined by the program.

    Thickness = The measured thickness of the material you plan to build your cabinet out of. Defaults are .75 (3/4) inches for drivers less than 12 inches in diameter and 1.0 inches for drivers greater than or equal to 12 inches in diameter. Reccomend that high density material be used. Be careful of plywood - it often contains voids within the wood plys that can buzz or do very un-musical things at random resonant frequencies. The Thickness is used to generate the measured sizes of the box pieces.

    Speaker (Driver) Data

    Qts = Driver Q at its resonance frequency

    Fs = Driver resonance frequency

    Vas = Equivalent air compliance for the driver (in either Cubic Feet or Liters)

    Cabinet Parameters

    S = Bandpass Ripple factor. Lower values of S will have degraded transient response, but wider bandwidth and a smaller box size. Valid range 0.4 to 0.7. 0.7 is often optimal. User definable.

    F3 = Low Frequency roll-off value. User chooses this value and the rear (sealed) box is tuned to this value. Based on S, the high frequency range is determined. 30 to 40 is a reasonable start point. User definable.

    Port Diameter = Optionally, you can define a port size from 1.0 to 8.0 inches - default value is 2 inches. User can select to enter this data in either inches or mm.

    Using the results

    This program attempts to extrapolate one or more sets of cabinet designs based on the speaker driver diameter and the calculated internal area for both the Sealed and Ported boxes. It makes the piece that you would mount the driver on square, and builds outward from there. This keeps it simple when it comes to construction.

    The Sealed box needs to be sealed from the inside. I reccomend silicon sealant and that this portion of the box glued together with the the driver being mounted from the Ported box section. The Ported Box section must have a removable portion to allow for this. Take into account the area taken up by any bracing used.

    The program generates both a 1 or 2 port design. You pick which one works best for you. Many people use plastic PVC pipe from the hardware store to make ports. Some people buy pre-made plastic ports. Because of this, the program allows you to enter the inside dimension of the port material and bases its calculations on this. For very small woofers (4 to 8 inch), you can specify ports down to 1 inch in diameter.

    The program also generates a Suggested port diameter. You may find the frequency response is better if your port material is similarly sized. 3 to 4 inches is a common size in production Sub-Woofers using 10 to 18 inch speakers. You may want to use a slightly longer port initially (2 inches or 50 mm) and cut it down to size as you tune the cabinet to an individual driver.

    Port Length is from the very front of the port to the very end of the port, where the port begins on the outside surface of the speaker cabinet.

    You must restrict the frequency range entering a 4th Order Bandpass cabinet. It does respond to frequencies outside the Sub-woofer range that this program attempts to optimize for - the frequency response outside that range is very poor. Please use either a Passive or Active Crossover mechanism to restrict this range. Do not allow the Crossover to exceed 200 Hz.

    Avoid use of Poly (plastic) cone Sub-Woofers for Pro-audio use - they don't hold up well. Expect to drive these speakers hard and over-rate the wattage handling capabilities by 50 percent or more. Paper or metal cones are your best choice. Surrounds can include any of foam, accordian pleated paper or accordian pleated cloth. Unless you put a plexi-glass window in your Sub-woofer so that people can see the speakers, no-one will care what these look like.

    Xmax describes the excursion rate of the voice coil. The larger the better for Sub-Woofers.

    SPL

    A speakers SPL (Sound Pressure Level) indicates how loud it is for a given signal at 1 meter from the source. The higher that number, the louder your system will be. The side effect is that often speakers with Higher SPL values are not as faithful in sound reproduction as those with more average SPL values. If your Sub-Woofer is for Pro-Audio sound re-enforcement, its less of an issue than if its for home audio. Remember, for each 3db of increase in volume level output, your Power Amplifier must provide 2 times the power to achieve it. This means that if you buy speakers with an SPL of 89db, you'll need 2 times the power to drive them as loud as the same size speaker that is rated with an SPL of 92db. This would imply that the higher the SPL value the more volume level you can get out of your Power Amplifiers. This is a true statement.

    Large Woofers often range in SPL from 85db to 100db - in theory, you would need 16 times the power driving the 85db unit to equal the 100db unit. It would be much cheaper to buy higher SPL drivers. In general, many Car Audio Sub-woofers have an SPL around 90db. Many PA/Instrument speakers have SPLs above 96db. Your challenge is to find one that has the low frequency characteristics that you need at the highest SPL level possible. Unlike Home or Car Audio systems, you may be driving a very large room (possibly outdoors) with your systems - you need all the SPL you can get.

    Box Configuration

    When using a single driver, wire the speaker in phase. If using 2 drivers (Isobaric - also called Push-Pull), the driver within the Ported box must be wired out of phase. This will cause the 2 drivers to move back and forth correctly - when the Sealed Box driver pushes out, the Ported Box driver will move inward and visa-versa.

    The connector for the speaker wires should appear in the Ported Box section. Pull wires from there into the Sealed Box and use a Silicon based sealer to make sure that the hole where the wires come thru do not allow air to pass.

    Wiring diagram for Isobaric configurations; Where the Black Speaker is in the Sealed Box and the Red Speaker is located in the Ported Box.

    NOTE: In the Isobaric configuration, the program does not take into account the volume of space occupied by the driver within the Ported box. You may need to add acoustic fill material to this portion of the Sub-Woofer to compensate (or increase the volume by 3 to 5 percent) - if you do, staple it in and keep it away from the port openings.

    What to do when you get difficult or strange results

    Some Speakers are not designed to work well in a Sealed Box. Unfortunately, some of these are Instrument/P.A. speakers. The problem is that they were not designed to be Sub-Woofers and have higher values of Qts (.4 or higher) and very low values of Vas (under 2.0 cubic feet). Its not that they can't be made to work, its just that there are probably others that will be far more effective. Speakers that are designed to be Sub-Woofers are your best start point.

    If your ports are very long, this is another indicator of a possible bad match in the choice of the Speaker

    I won't suggest any brand or driver. I will suggest that you get catalogs that list many speakers (Part Express has some of this info Online) and work from there.

    Application information

    All calculations are done using cubic feet or inches. Any data entered as liters or mm are converted to their equivalent values and the operations are returned. You occasionally see rounding errors when entering your data in liters or mm.

    The CGI program is written in C (originally on Silicon Graphics Irix 6.2, but now runs on a Linux server). Use of it in this Web Based form is granted to anyone with Web access free of charge. The application may not generate optimal results for your needs. This program has not been tested against every possible speaker driver and carries no warranty for its usability.

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