One car that jumps to mind was owned by my friend Gord Stanszell, formerly an
instructor at Mobile Dynamics in Toronto, Ontario. The car was a well beaten Chevette.
Lots of rust holes in the floor, and perfect for bombing around in! The system
was comprised of an inexpensive Blaupunkt CD player, an old set of Panasonic tweeters, a set of factory 5 1/4" mids (from a jeep think), a set of MEI dual cone 6x9's and an Alphasonic amp. The mids and highs
were in custom kick panel pods, and the 6x9's were in the side walls behind the
front seats. The system didn't get down very low, nor did it play very loud. BUT, the imaging, placement and mid-bass attack were amongst the best I have heard. And of course it made my
quite expensive system (at the time) sound merely average. Gord and I talked about the design of the two systems (his and mine), and we decided once more, that simpler is better.
When it comes time to wire up your system, you truly must do this by trial and error. The acoustical phase relationships between drivers is critical to having a system that performs well. After judging the AudioThunder '97 show in Toronto, I saw one or two cars with obvious phase problems that could have been cured using this technique.
One vehicle that I measured showed a curve very similar to the red on in the graph below. The system was designed around a 4-way system. It had, per side, two 8's, a 6 1/2", 4" and tweeter. While the system was able to crank out 134 dB with only 4 eight inch woofers (quite impressive), it lost tons of points on frequency response. The problem was the speaker phasing.
While I am sure everything in the car was wired correctly (positive amplifier terminals to positive speaker terminals, etc..) the acoustic output from each speaker was not adding up. We know that if you have one subwoofer out of phase, the system produces little or no bass. By the same token, if two speakers in adjacent operating frequencies are operating out of phase, through the range in which they are both playing the same frequencies, their output will cancel instead of adding. The solution is to reverse the polarity of one of the speakers, so that it adds with the adjacent speaker. Why does this happen? Speakers are mounted on different angles or point in different directions on different planese. Subs firing rearward or downward, may not add with a mid-bass mounted in the door or front floor firing upward. So a simple electrical connection reversal will solve the problem.
In the case of the above truck, had they simply reversed the electrical connections on the 6 1/2" woofers, the system would have much more mid-bass, and been easier to listen to (male vocals, etc..). And they would have an excellent RTA score, rather than something in the range of 25 or so.
How can you measure or figure this out without an RTA? It's not a easy, but still can be done. You need to become intimately familiar with a particular piece of music, that has lots of (clean and clear) output throughout the frequency range you are concerned with. Now listen to the system with the driver wired with proper polarity, then with reverse polarity. One way will sound better than the other. This MUST be done for every driver in the system, including front and rear drivers playing the same frequencies. It also must be done for left and right speakers, to make sure they are adding. You need to start with the bass, mid-bass, midrange then tweeters. You can do the left / right test by determining the connection which produces the flattest frequency response, and the most bass output.
These tests can make some huge differences to even the simplest of systems.