| MadSci Network: Physics |
It depends. The low frequency of bass sound will bounce off of things (like walls) very well. The trick with "bouncing" the sound, is that you want the reflected sound to add to the original sound rather than cancel it out. The trick is all in the distance you put the sub from the walls, and also the distances between you and the subs and the walls!
Try doing a search on the internet for "subwoofer placement" or "speaker position". You'll find all sorts of articles that will probably teach you more than you ever wanted to know. Most articles will probably focus on placement for home theaters rather than in a car, but all the same principals still apply. Here's one good site to start with: http://www.sonicdesign.se/subplace.html.
One thing to keep in mind, is that reflected sound will never be as loud as the original. So the sound straight from the speaker will be the loudest, and reflections will be less. So you're probably best off trying to get the speaker pointed in the air, then get reflections to add to the sound. On the other hand, if pointing the speaker at something lets you get more total reflections, then it may be louder than straight in the air alone! So that's why I said that it depends. It will probably take a little trial and error to decide what works best inside your vehicle.
If you're really trying for the absolute maximum volume possible, go to Radio Shack and buy a sound meter. They cost around $50. You can use this to take objective measurements to determine whether location changes really make a difference or not.
Try the links in the MadSci Library for more information on Physics.