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Originally Posted by AVI

As Scooby Doo would say ..... " Absorutely Rot " !!!!

In a concert hall .... ambient sound will reflect off the rear wall ...
In a car ..... ambient sound will reflect off the rear glass ...

Both of these scenarios create natural ambience .... No rear fill is needed.

Big difference between a concert hall and car: size of the acoustic space!

And it's NOT just a relative difference ... there's a real, tangible threshold involved, as defined by the Haas Effect. In a car, there's a real good chance that the natural reflections will not exceed the precedence effect (about 20msec, if memory serves) ... meaning that natural reflections will only tend to smear and confuse the front stage. In a concert hall, there's a real good chance that the natural reflections will exceed the precedence threshold.

So if you delay the rear fill by ~20msec or slightly more, you can create an apparent acoustic space that is much larger ... without confusing or ruining the front stage. This is a good thing Yes, we face the orchestra in a concert hall ... a classic argument against dumb rear fill ... but we also sense that the concert hall is bigger than about 25 cubic feet ... a very good argument in favor of smart rear fill

How to best accomplish it, is perhaps a discussion for another thread. You need a processor with time alignment, and the ability to bandlimit the rears. The difference signal can be formed either actively before the amp (trivial with balanced signals, not even hard with transformers and single-ended signals), or speaker-level after the amp (the classic Hafler matrix was the old, dumb example ... with no time delay, and even only crude attenuation and bandlimiting). But here's an interesting link for further reading:

http://www.mlssa.com/surround/surround3.htm

Of course, you may want to consider the options mentioned above ... Dolby Pro Logic II (invented by Fosgate with an automotive acoustic space in mind), DTS Neo:6, and the new Harman Logic7 processor for the car (which, by the way, is starting to get some very encouraging reviews). All of these options create a multi-channel signal from a 2-channel source, and make very intelligent use of speakers behind your head

I guess my main point is this : all "rear-fill" is not created equal It's important to distinguish what type, before we summarily embrace or dismiss the entire category.