The physics of automobile subwoofer placement.

Factual Questions
1

Now, before we start, let me say that I am an acoustician, so I know the physics of sound very well. However, there is a phenomenon that perplexes me. I have installed several stereo systems in cars, and I notice that the subwoofer placement changes the bass response dramatically. Specifically, when the subs are placed so that the cones face the rear corners of the car (directly at the back of the taillights), the response is best.

Why does this happen? I am familiar with this happening (bass sounding louder) at particular locations within rooms, due partially to standing waves caused by parallel surfaces and large volumes, and also by the tendency of bass frequencies to accumulate in corners. But parallel surfaces and sharp corners do not exist to any large degree in a sport coupe, nor is the air volume very large. I suspect that the reason has to do with the structure of the car, and the excitement of resonant frequencies of the frame.

I know that bass seems louder within the car when the trunk is open, simply because there is less frame rigidity with the trunk open, lowering the resonant frequency, and making bass frequencies seem louder. But I was always taught that low frequencies are extremely non-directional (which is why stereo subs are unnecessary). So I’m stumped as to why placement within a rather small volume can so drastically affect how loud the bass seems within the car. Can any of you shed some light on this?

(By the way, I’m not one of those guys who likes to rock the block with my stereo. I design every system I build to sound good within the car, not for a quarter-mile radius. :D)

2

This is a completely non-technical reply, based on observation of folks around here who have subwoofers in their cars.

1.It is important that observers outside the car can hear massive buzzing on each bass note. Extra points for parts that vibrate off.
2.There should be no need for Kong-like beating of one’s own chest. The speakers will do that for you.
3.If the lines of the car fuzz out of clear focus during moderately loud passages, you’re getting it right.
4.If listeners outside the car can hear any melody at all, you have inadequate subwoofage.

I hope I’ve been of some help. Now, excuse me while I go buy stock in Beltone.

3

Excuse me, but I think I made it clear that my objective was sound within the car. I do not dispute the fact that there are certainly people out there who wish to cause structural damage to buildings within 100’ of their cars, but I am not one of them. Thank you.