"They are here" vs. "You are there"

...we try to produce the illusion in a listener of being in a "virtual" acoustic environment which is entirely different from that of the space in which he (or she) is actually located. We are thus attempting to achieve the long sought-after goal of making a listener in his living room hear sound as if he were in a concert hall.

The availability of modern electronic technology for processing acoustic signals digitally has transformed our ability to generate this illusion, almost irrespective of the environment (living room, office or automobile interior) which surrounds the listener. The approach that we take is to process acoustic signals prior to their transmission by loudspeakers. We undertake this processing in order to generate the illusion in the listener that sound is coming from a number of "virtual" sources in well defined spatial positions relative to the listener. Of course, the intention of conventional "stereo" sound reproduction by loudspeakers is to produce just such an illusion, but two channel stereophony is capable only of producing acoustic virtual source images over a very narrow range of spatial positions, these being restricted to positions in the plane of, and in between, the two loudspeakers used for reproduction. The use of modern signal processing techniques can remove this restriction, even when only two loudspeakers are used for reproduction.

A number of approaches to "3D Audio" have been developed in recent years, but few have correctly tackled the basic signal processing problem that has to be solved. This is the design of a processing scheme that ensures that the correct signals are produced at the listener's ears. In order to achieve such a goal, the processing scheme has to account for the effect on the signal of the loudspeakers, of the transmission path (including room reflections), and of the effect of the listeners head and torso on the propagation of sound to the ears. The central problem to be tackled is one of "inversion" where all these effects have to be "turned upside down" (and thus compensated for) before the signals are transmitted by the loudspeakers. This is a problem with many technical subtleties, but by tackling it correctly, it's solution can produce remarkable results.

We are accustomed to thinking of reflections as causing coloration and degrading clarity, and philosophically we don’t like the room adding to the recording something that was not originally there. But if the reverberant field is done right (which is something we can come back to), timbre is more natural and clarity is actually improved! That’s right, controlled tests have shown that speech intelligibility is improved by normal in-room reflections. Apparently the ear is better able to decipher complex sounds when it gets multiple “looks” in the form of reflections. The direction that reflections arrive from plays a role as well. Reflections that arrive from the same direction as the direct sound are more likely to be perceived as coloration than are reflections that arrive from the sides. And, reflections that arrive from the sides are more effective at imparting a sense of spaciousness and envelopment. One benefit of my recommended 45-degree toe-in is that it ensures a relatively large proportion of the reverberant energy will be arriving from the sides. The ear derives tonal balance from a weighted average of the incoming sounds, so the reverberant energy plays a significant role there. When the spectral balance of the reflections is very close to that of the first-arrival sound, perceived timbre is richer and more vivid. This is why we listen to grand pianos and choral groups in lively recital halls rather than in thickly-padded rooms. In my opinion the goal of high-end audio is to recreate, as closely as is practical, the perception of listening to live music.