And so it came to pass, on 16-07-96 20:27,
   that John Allen spake unto Matt Ion:
 >>  >>Most of the points are based on such criteria as installation
 >>  >> safety and quality (running 500W worth of amps on 16-gauge zip
 >>  >> cord that comes out the hood and through the door hinges is an
 >>  >> automatic zero :), sound quality (subjective to individual
 >>  >> judges' ears and, to some degree, tastes), imaging and sound
 >>  >> stage, and RTA measurements
 JA>    Hi Matt,            This is something I have never heard
 JA> mentioned with Car Audio, and I am having a hard time getting a
 JA> handle on it. In home audio when we speak of imaging and soundstage
 JA> we are referring to reporduction of such accuracy that you have
 JA> preceived width beyond your speakers, depth, height, and centering
 JA> as if you could point to the individual musicians/singers/sections
 JA> where they are playing from. The more accurate , the better the
 JA> system and setup. What is the goal in Car Audio for this. Is the
 JA> soundstage out somewhere beyond the hood or is the target for this
 JA> to place yourself in the musicians pit, playing with them? It is a
 JA> little confusing and I don't believe I would want The Village
 JA> People or The Stones dancing on my hood while I make my way through
 JA> city traffic. Cheers, John
Well ideally, you should not be able to directly locate the speakers 
(particularly the tweeters).  The center image should typically hover about 
eye-level over the dash.  It doesn't necessarily have to extend out to the 
front bumper, but I suppoe it could.
It's difficult if not impossible to get a sound stage as precise as you 
depict above, in the car environment... mainly because there is no center 
"listening position" in which the listener can position himself exactly 
between the speakers.  Such judgements are generally made from the driver's 
seat, so the trick is in shifting the imaging such that it's "right" from 
that position.
One of the biggest problems faces is, of course, path-length differences from 
the tweeters to the driver's ears.  Some systems place the tweeters in the 
ends of the dash, or at the tops of the doors.  This puts the left tweeter, 
in some cases 1/3 to 1/4 the distance from the driver's left ear, as the 
right tweeter is from the right ear.  It's almost impossible to get any kind 
of imaging from this without some heavy-duty processing (a few milliseconds' 
delay to the left tweeter will work, but will make the sound seem even more 
lopsided to the passenger).
One solution that's been seeing more use lately is the use of "WaveGuide" 
and similar type tweeters.  These are actually horn-loaded compression 
drivers designed specifically for the car.  The mouth of the horn is 
typically mounted just under the dash, aiming out at the person's knees (or 
upward from there), with the driver being tucked 'way up inside the dash at 
the other end of the horn.
In addition to the obvious advantages with horn-loaded tweeters (much higher 
sensitivity, requiring less power, and extended frequency range [generally 
800Hz to 20kHz, meaning no separate mids are necessary]), this extends the 
path length of BOTH drivers significantly (remember, we only care about the 
distance from the *driver* to the ear, not from the mouth of the horn to 
ear), so that the RELATIVE DIFFERENCE between the two distances is less.
Short of using relatively expensive horn tweeters, the best solution is to 
try to increase the distance from the tweeters to the listeners, which is 
usually done by placing them low in the kick panels.  I've found that doing 
this and then, in most cases, aiming them directly at the dome light, results 
is very good imaging for both driver and passenger (the path-length 
differences are offset somewhat by the off-axis level drops -- the driver's 
right ear is nearly on-axis with the right tweeter, which is farther away but 
will be louder, while his left ear is 45-60 degrees off-axis of the left 
tweeter, allowing the lower level to compensate for the proximity of the 
driver.
Ultimately, you'll probably get the best sound stage by wearing headphones 
anyway -- no leakage of the left signal to the right ear, and vice-versa :)
IKEA ... Swedish for "particle board."
--- Sqed/32 1.10/unreg
---------------