The Effects of Cabin Gain in Car Audio
Have you ever wondered why car audio systems can have so much bass as compared to a home stereo system? The reason has to do with space. Your home has a lot of it and your car audio system doesn't. Because there is so little space in a car the bass notes (which are long waves) build up inside the passenger area. To calculate the length of a sound wave you divide the speed of sound (in feet per second) by the frequency. At sea level, the speed of sound is approximately 1,127 ft/sec. For example, a 40 Hz note has a wavelength of approximately 28 ft (at sea level).
(speed of sound)/(frequency) = wavelength
(1,127 ft/sec)/(40Hz) = 28.175 ft
Since the length of the average car interior (including trunk) is in the 12 ft range the 40 Hz note will be longer then the car's interior. This is why notes below 70-90 Hz (depending on the vehicle) will have a greater output than the rest of the frequencies. Once this magic frequency is reached, bass output will increase by about 12 dB/octave below that frequency. This phenomenon is called cabin gain or "transfer function". So a smaller vehicle will have a greater cabin gain and should be able to have greater low bass than a larger vehicle. This is true for identical subwoofer systems with identical power. However larger vehicles are able to fit more subwoofers and amplifiers and so can out produce a smaller vehicle with limited space.
The transfer function works well because human ears are less sensitive to low bass. This natural bass boost helps to compensate for this. It doesn't matter what type of vehicle you have, the cabin gain will still apply. It will just be at a lower frequency with larger vehicles and at a higher frequency with smaller vehicles. Firing orientation of the subwoofer system has no effect on the transfer function of a particular vehicle. You can face the woofers in any direction or place them anywhere in the vehicle and the gain will be the same because it is only dependent on vehicle size. This is not to say that firing orientation or woofer placement will not have an effect on the subwoofer system output. It certainly does but this difference is not attributable to the cabin gain.
Bottom line: The cabin gain or transfer function will increase the low bass in a car stereo system over that of a home stereo system. The smaller the vehicle, the greater the gain