Ambience Recovery Factors
We can learn what is important to ambience reproduction by studying a good living room system. My own reference system is diagrammed in Figure 1 (see next page). The Magnepan MG-III speakers are tall flat-panel systems that radiate from both the front and rear, but not to the sides. The room has patches of absorption and a scattering of objects on all of the walls, but is basically live (reflective). The floor is carpeted and there's some ceiling absorption.
A realistic stage-width angle is defined by the speaker's relationship to the listener-about 40 degrees. The direct sound is the first to reach the listener. There are no reflecting surfaces to interfere. In fact, the first significant reflections only arrive at the listener after having traveled an extra 10 feet to the wall behind the speakers and back. This delays the reflections by about 10 milliseconds.
When the reflections do hit, they're strong, having been focused somewhat by the corners of the room. More reflections follow from the sidewalls and from behind. (The 10-millisecond gap between direct sound and first reflections has been called the initial delay gap, or IDG. This IDG is sometimes achieved with conventional speakers by absorbing potential early reflections with acoustic treatments.)
The sonic effect of a system like this is clarity and intimacy combined with excellent reproduction of the recording space, be it large or small. The system reproduces an accurate left-to-right image of sources as well as depth. To be ultra critical, I could ask for a little more feeling that the ambience is coming from the rear.
Why does it work so well? First, there is a strong, direct, reflection-free hit of left and right channels from the proper angles. This establishes the image locations on the stage and conveys undiluted sonic detail. After 10 milliseconds, strong repeats of the original sound arrive from the sides and rear of the listener. The key factors are: Left and right arrivals free of early reflections, an IDG of about 10 milliseconds and multiple delayed side arrivals.
Optimizing Ambience
Great ambience reproduction starts with a great stereo system. We don't add ambience to a mediocre system to make up for its deficiencies.
Start with the front sound only. It should sound spectrally well balanced without rear speakers or a rear subwoofer. Try to get a line of sight path from the speakers to the listener, and try to minimize reflections from adjacent surfaces. Directional speakers carefully aimed and sound absorption carefully placed can help accomplish this. Fortunately, most car interiors absorb the sound after the first reflections off window glass, giving a "quasi-reflections-free" first arrival.
Deciding on the width between the front speakers requires compromise. Wide locations give more side-arriving sound and thus more ambience. However, this usually creates an unnaturally wide stage, with images mainly on the near side and a hole in the middle. Instrument-panel (IP) mounting of speakers gives a better soundstage, but reduces ambience. The reduced ambience can be recovered by the car's rear speakers, if they simulate the listening room's delayed side reflections. So IP mounting may be the best choice. Avoid multiple fullrange left and right speaker systems because they give multiple arrivals that obscure detail in the same way reflections do.
Timing is the most important consideration for rear speakers when designing for ambience reproduction. Their sound should arrive at the front seat listeners after the sound from the front speakers. With luck, this can be accomplished by mounting them as far back as possible. Fortunately, in most cars, the rear package tray provides a longer sound path to the front seat listeners than the front speakers.