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Car Audio Subwoofers - Designing for High SPL
last updated: 16 August 2010
The Subwoofer DIY Page

 

Designing and building a car audio subwoofer system for high SPL is some ways quite different to standard subwoofer design, as the emphasis here is on the production of the highest SPLs without any emphasis on smooth frequency response or linear output capability.  This page attempts to discuss the design and construction of such systems.

What can I expect from a high SPL system?
Well, the obvious thing to expect is high SPLs.  Unfortunately these high SPLs will likely come at the expense of smooth frequency response - the system is likely to be a "one-note" bass system, with very heavy emphasis on a narrow frequency range in the bass region. It it possible however to design a system that can feature both high SPLs and smooth frequency response - more on this later. 

What type of system should I use?
Three systems of systems can be used to create a car audio high SPL system: sealed, vented and bandpass. The other types of systems usually are not very good matches for the car audio environment

High SPL sealed systems
A car audio system that is designed to use sealed alignments to deliver high SPLs usually has multiple small sealed subwoofers, each employing the same brand and model of driver, and designed and constructed so that system's resonance frequency (Fb)  is as close as possible to the optimum resonance frequency (the frequency at which the combination of the alignment's anechoic response and the car's measured "cabin-gain" produces the greatest output). Typically this optimum frequency lies in the range of 50~60 Hz, but it's best to accurately determine it via measurement.  

 The main advantages of sealed systems designed for high SPL is that box size requirements are smaller and the current draw at the peak SPL frequency is lower (system impedance is at its highest, the value directly dependent on the driver's Qm), however you will likely have to use multiple boxes to achieve the SPLs capable of vented and bandpass systems which negates those advantages somewhat.

High SPL vented systems
A car audio system that is designed to use vented alignments to deliver high SPLs usually has one or more vented subwoofers, each employing the same brand and model of driver, and designed and constructed so that the system produces a huge peak in output at the optimum resonance frequency (as described above).

The main advantage of vented systems designed for high SPL is that less drivers can be used to achieve high SPLs. Another advantage of these types of systems is that they can usually be converted for "sound quality" use by simply sealing the vents. However current draw at the optimum resonance frequency is likely to be very high (system impedance will be at its lowest, the value dependent on the driver's Re), and as the drivers will be maximally loaded at the resonance frequency, there's a greater risk of the voice coils being burnt.

Bear in mind when designing your system that the resonance frequency of the system might shift at higher SPLs. This is caused by the vent starting to "shut down" as the velocity of the air through the vent starts to increase. This effect can be minimized by using as large a vent as possible in the design; the larger the vent, the lower the velocity of the air resonating in it.

The graph below illustrates the in-car frequency response of two systems.  The first system is a simple sealed system, and its frequency response is indicated by the solid red line.  The second system is a vented box using the same driver that's designed for high SPL.  There's a difference of 10dB at the car's resonance frequency - in other words, the output at that system at that frequency is the equivalent of three of the sealed systems driven by three times the amplification. In this particular case, sealing the vent will convert the system into a sealed system with Qb=0.6, which is fine for sound quality use. The vent can therefore be left sealed until it's time to take part in the SPL testing.  The bad news - this box takes up almost three times the space in the car.

High SPL bandpass systems
A car audio system that is designed to use bandpass alignments to deliver high SPLs usually has one or more bandpass subwoofers, each employing the same brand and model of driver, and designed and constructed so that the system produces a huge peak in output at the optimum resonance frequency (as described above).

The main advantage of bandpass systems designed for high SPL is that less drivers can be used to achieve high SPLs. One other possible advantage of these types of systems is that, if you design them so that the vented section can be removed, you will end up with a system that can be converted for "sound quality" use that doesn't take up as much space in your car as vented systems normally do. However, as with vented systems, current draw at the optimum resonance frequency is likely to be very high (system impedance will be at its lowest), and as the drivers will be maximally loaded at the resonance frequency, there's a greater risk of the voice coils being burnt.

Bear in mind that as the bandpass system also uses a vent, it will be subject to the same resonance shifting effect as a vented system at higher SPLs. This can be minimized by using as large a vent as possible in the design.

What should I look for in a car audio subwoofer driver to be used in a high SPL system?
If you are designing your system ONLY for peak SPL, then the most important characteristics for the driver are (1) high power rating, (2) high volume displacement (Vd). Linearity and frequency response are not as important, as the driver will be operating over a very narrow range of frequencies.

Any other things to consider?
You should consider how much power you can safely provide to your SPL subwoofer(s), then take steps to ensure that the amplifier(s) can provide that level of power and deliver it to the subwoofer(s) with minimum loss.  This would likely mean using the largest power cable possible to ensure that the voltage delivered to the amplifier(s) does not dip at peak power levels, and using one of the larger gauges of speaker cable to ensure that the impedance of the cable is significantly lower than that of the subwoofer(s).

You also need to ensure that your car's power system can provide the current required for the amplifier to provide the required power to the drivers.  This could require that the car's alternator be upgraded to one that can supply considerably more current. For example, it makes no sense using a 2kW amplifier in your design if the car's alternator isn't rated to deliver enough current to that amplifier for it to reach its rated output.

A good way to minimize the current draw is to use class D, or "digital" amplifiers for the bass.  These amplifiers tend to be much more efficient than other types of amplifiers commonly used for car audio.

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Brian Steele
16 August 2010