Designing and building a car audio subwoofer is in some ways very similar to designing a subwoofer for your home, and in some ways very different. This page attempts to discuss the design and construction of car audio subwoofer systems.
Why should I build a car audio subwoofer?
The reasons for building a subwoofer for your car remain the same as the reasons for building one for your home: typically the main loudspeakers, which in car audio systems are usually 4"-6.5" drivers mounted in the front doors or in the dash, are simply not capable of reproducing the lower bass frequencies cleanly at any significant volume. If these loudspeakers are relieved from trying to play the lower bass frequencies, they will sound better at higher volume levels. And, as is the case with home subwoofers, a car audio subwoofer can be located anwhere within the listening area, though the car's trunk usually ends up as the final location for the subwoofer for practical reasons.
What type of system should I use?
As with home audio systems, there are five basic types of subwoofers systems that can be used: sealed, ported, bandpass, passive radiator and transmission line systems. However, the best type of subwoofer system for in-car use is usually the sealed system, as (1) the box size requirements are usually the lowest for sealed systems, and (2) the 12dB/oct rolloff of a sealed system is a perfect match for the free 12dB/octave rise in response below 60~80 Hz that you get in a car because of cabin gain. The combination of these two conditions typically produces a frequency response curve that remains flat almost to DC (0 Hz). Most sealed systems designed for car audio use employ a Qtc of 0.8-0.9, to minimize box size requirements and maximize power handling, while still providing acceptable results.
What about the other systems?
The other systems can be used to create a car audio subwoofer, but the parameters of the design will have to be adjusted to compensate for cabin gain. In other words, the box calculations given on this site for these systems should not be used when designing a subwoofer for your car. The design spreadsheets provided on this site can be used to model the effects of cabin gain on ported and bandpass systems, and you can therefore use them to create a ported or bandpass system to match your car's characteristics. Typically, a ported system designed for car audio use has a smaller box size requirement and lower resonance frequency than a ported system for home audio use using the same driver. In the case of bandpass systems for car audio use, the front chamber ends up being larger and the resonance frequency ends up being higher. In both cases, the aim is to produce the best power-handling with the flattest response possible when cabin gain is taken into consideration.
Displayed above are the predicted frequency response curves for two systems designed for car audio use, using a driver with the following parameters: Vas: 68 litres, Fs: 35 Hz, Qts: 0.47. The red curve indicates the predicted anechoic response of a sealed system (Vb=25 litres, Qb=0.91) using this driver. The dashed red line indicates the predicted in-car frequency response of this system. The effects of an 80 Hz 12dB/oct filter are also included. The blue curve indicates the predicted anechoic response of a ported system (Vb=25 litres, Fb=33.5 Hz) using this driver, and the dashed blue curve indicates the predicted in-car frequency response of this alignment. Again, the effects of an 80 Hz 12dB/oct filter are included. As can be seen from the graph, the curves are very similar; the primary advantage of using this particular ported system is the reduced excursion requirements to produce the same SPL within the system's passband.
What should I look for in a car audio subwoofer driver?
The most important characteristics for a car audio subwoofer driver are (1) box size requirements, (2) Peak linear SPL capability, (3) price, and (4) behaviour at excursions over Xmax (e.g. does it overload gracefully, or does it start making really ugly noises). The first two you can get from the driver's T/S parameters. Unfortunately, the last one you can only find out by listening to the driver when mounted in an appropriately-sized box.
12 October 2018