It was suggested that I explain the term "point source" I used in my post, so here is my explanation.

In this case the term point source refers to a point of origin in space. It doesn't imply that it takes up space, simply that it's an imaginary point of reference and could be anywhere, a car door, dash, rear deck etc. When sound reaches our ears it has an apparent point of origin. Our brains are able to interpret the stereo input from our ears and determine the location of the sound source. This is why hi-fi systems use two (stereo) channels. With two channels a recording engineer can fool our brains into perceiving a moving source, or even multiple sources. Since modern music program material is in stereo we aim to have only two points of origin, the left and right.

"OK so ideally my stereo system needs to have only two sound sources (left and right) to be accurate, what problems did I cause by separating my drivers?"
For this discussion I'll explain one channel since the principles apply to both. For each channel in most modern hi-fi audio systems, the job of reproducing music is tasked out to two or more drivers. Each driver covers a part of the audio frequency spectrum, and when the sound from each reaches our ears it should sound like it came from one point in space. However, the sound from each driver comes from a different point in space, so we don't have a single point source for this channel. The problem with this setup is a possible time delay due to differences in distance between the listener and the location of each driver.

"Who cares right? Due to the speed of sound and the relatively small difference in distance between the listener and each driver, the distortion in the music due to time delays will be negligible right?"
The problem lies in the region of the audio spectrum where the two drivers share program material. This occurs at the crossover point, and the width of the band in which it occurs is dependant upon the crossover slopes (ie 12db/octave...18db/octave...). The difference in path lengths cause a phase shift dependant on frequency (ie wavelength) and will cause cancellation at some frequencies, and summing at others. This phase shift is in addition to that caused by the active components in the crossovers.

The problem compounds when the second channel is taken into account because the path lengths between the listener and the drivers are again different from each other and often don't mirror the other channel because of the off-center listening position (unless you drive a McLaren:-). This will leave you with strange peaks and valleys in your in-car frequency response. The response problems were absent when you listened to the drivers in the store because they were likely installed very close together and in a vertical alignment which minimizes path length differences. Further more, you were probably standing half way between the left and right speakers giving you near perfect stereo.

Another problem is the apparent change in location of an instrument depending on the frequency of the note. A piano for example is capable of playing notes that span most of the audio spectrum. It is a strange thing when the piano player starts a scale and the apparent position of the piano moves from your feet to the top of your dash!

"So what can I do to achieve a 'point source' in my car?"
Ideally, only use one full range speaker for each of your stereo channels. Of course this isn't practical in car audio because the high volume levels (to overcome wind noise of course will inevitably result in excessive Doppler distortion. The next best solution is to use separate high and low frequency drivers and install them in the same point in space. Of course this is impossible, but co-axial(shared axis) type speakers come close. If you have your mind set on using separates then care should be taken to mount the drivers an equal distance from the listening position. In the automotive environment this is rarely possible. Furthermore, a passenger invited to listen to your "great system" will hear a different sounding system because of the different listening position. One way to reduce the interference between the high and low frequency drivers is to use steeper crossover slopes. While adding more phase shift, it will also narrow the band where they overlap.

Installing car audio should be more about minimal-izing problems inherent to the listening environment than about getting your hands on the most expensive gear. Like many have said before me: it's how you install it, not necessarily what you install. No particular hardware choice condemns anyone to poor sound. Care must be taken to balance ease of installation, budget, and available space. Then optimize the installation of what you have. I have seen countless poor installations of expensive gear that sound no better (more often worse) than far less expensive systems.

The KEF Uni-Q line mentioned by another poster is the best attempt at a point source I have ever seen. The tweeter was mounted right down in the center of the woofer's dust cap. I suspect this may have increased diffraction problems, but I still think they sound great. Co-axial designs are steadily improving and may replace component sets as the standard of quality in a few years. Hopefully the prices for the better units will fall too...

I'm not sure I've succeeded at explaining point source and how it applies to car audio here. Phase relationships are such a complex thing to explain, maybe someone with better technical writing skills could clarify my scratchings. If anyone has any questions feel free to email me. Perhaps a discussion on Doppler distortion, stereo imaging, or phase relationships should follow next week? Or is this getting into the physics too much and taking the fun out of the hobby?

Kevin

kev{remove




"EFFENDI" wrote in message ...
I.C.E. School now in effect.

Topic for this week is choosing speakers.

Many people choose component speakers with outboard crossover modules
for their sound quality and imaging advantages over other types of
speakers. Two way coaxials are popular for their ease of installation.
The debate is up in the air. What would you reccomend??


Please share your opinions/experiences/information/suggestions on
component speakers versus two/three/four-way (no, i'm not talking about
sex here) or coaxial speakers.


Lets help the newbies!

EFFENDI