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?
What type of system should I use?
High SPL sealed systems
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
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
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?
Any other things to consider?
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.