Essential elements of music are usually given as melody, rhythm, and harmony. The fact that music takes place in an acoustic space (either a real one or one electronically generated) is simply assumed.
We cannot assume, however, that this critical sense of space (or ambience) is automatically reproduced from a recording. Even if we get high volume, uniform frequency response, and low distortion, the music will sound flat, dry, and uninteresting without ambience.
The "real thing" is the sound at a live concert. If we're talking about studio production, the real thing is approximated by the sound of the master tape playback in the control room. The real thing is also the sound the artist wants us to experience. Ambience is an important part of it. We can go beyond reproduction and create a range of echo and surround effects from the wonderful to the bizarre. This can be entertaining, at least for a little while, but, for now, we'll be concentrating on the pure goal of accurate ambience reproduction.
Ambience: What It Is
The live performance has a soundstage in front that contains all of the musicians or other sources of sound. This stage is a relatively narrow angle in comparison to the entire 360 degrees of room that surrounds the listeners. The walls, floor, and ceiling reflect stage sound back in a way that tells us we're in a room, but without affecting perception of the narrow stage and the location of the sound sources on it.
The sonic characteristics that define the acoustical space of the performance-the concert hall or whatever-are what we call ambience. Ambience gives a richness and spaciousness to the music, but it doesn't contain directional information about the sound sources. Ambience comes from all directions, not just the front. The sound coming from the back is all ambience.
Surround sound is not ambience reproduction. The musicians don't surround us when they play-they stay on the stage. (Yes, I know that there are some music performances that surround the audience, but I'm sticking to the 99 percent of conventional performances with the musicians staying on stage.) Car audio systems usually have front and rear stereo speakers, and this layout usually gives a 360-degree soundstage. This is a problem.
Reverberation is not ambience reproduction. Recorded reverb can be heard over the cheapest mono radio, but this does not qualify as ambience. Stereo helps because the reverb sound takes on some of the critical spaciousness factor. Side and rear speakers can finish the job, providing a sense of spaciousness heard over the full 360 degrees. Adding echo from an artificial reverberation device is simply not compatible with the concept of ambience reproduction.
Generating a sense of spaciousness alone is not ambience reproduction. Connect stereo speakers in reverse polarity and you'll hear an extreme-but phony-spaciousness. This effect is often called "phasiness." It's sometimes used in the midrange of boomboxes and other stereo sets with closely spaced speakers. Avoid phasiness; it produces more headaches than ambience.
Commercial recordings are intended for homes, not cars. Recording engineers control the blend of ambience in a live recording or add artificial reverb in a studio recording. There are two important points here; the ambience information is in the recording and the artists intend it to be reproduced over the best home stereo systems. The problem of reproducing ambience in a car is not the lack of concert-hall acoustics. Rather, it's the lack of living room acoustics.