Space...the Final Frontier Page 3

Part of the art of recording is in getting just the "right" mix of reverb and direct sound, and it usually turns out that the reverb content is below the level where masking can occur. Once a stereo recording is made, the amount of reverb on it is the same whether we listen to it in mono or stereo. But in stereo, the two-source reproduction unmasks some of the reverb that's obscured by the mono reproduction, making the sound fuller and richer.

Exactly the same thing happens when we reproduce that reverberant field from two more directions, such as from the left- and right-rear quadrants of the listening space; then, virtually none of the hall's ambience is subjected to masking by the direct sounds. The whole lower range sounds even fuller and richer than it does in stereo.

The amount of low-end difference between two-channel and multi-channel reproduction of large acoustical spaces is almost as great as that between mono and stereo. It has its greatest effect on sustained bass sounds, like those of bowed cellos and bass fiddles, which take on a lush bloom that two-channel stereo can usually only suggest. Yet most high-end speakers seem perfectly capable of reproducing that bloom without the help of surround channels. Why? Because of the miracle of equalization, that's why.

More real than real
One of the distinguishing features of high-end audio products is that they're designed to sound good rather than just to measure well. We all acknowledge this fact and even see it as something to be proud of, because it makes product design as much a noble art as a heartless technology. High-end loudspeakers haven't measured very well for some years now, and the bass difference between unequalized stereo and surround reproduction is a major reason.

Stereo loudspeakers with truly flat frequency response tend to sound weak through the midbass because much of the hall's ambient warmth is being masked. So speaker manufacturers, ever attuned to the aesthetic sensibilities of music-loving audiophiles, learned early on that a slight clockwise-response tilt---usually topped off with a treble lift (footnote 5) to offset an otherwise dull top---sold more loudspeakers, because it gave large-hall bass some of the richness and warmth that was missing when the speakers had a flat measured response. The tilt is relatively slight---amounting typically to only about a 3-4dB difference at 40Hz and 12kHz---but it's conspicuously audible and immediately earmarks any speaker that has it as a "high-end" product rather than a professional one (fig.3). In fact, today's high-end audiophile speakers and those used in theaters and recording studios almost constitute two different kinds of speakers: The consumer models are laid-back and full-sounding, while the pro monitors are, by comparison, up-front, cool, detailed, and relatively lacking in depth.

Fig.3 Anechoic frequency response of a typical high-end loudspeaker. The across-the-board clockwise tilt adds warmth and body; the top-octave lift compensates for missing detail.

Not everyone applauds this practice of "prettifying" loudspeaker sound. Purists argue that it's not an audio system's job to make recordings sound "better" than they really are, but to reproduce the recording as honestly as possible---the ongoing accuracy-vs-musicality debate. But if a recording of large-scale music---which I view as the ultimate challenge to a system---is reproduced through two channels rather than four (or more), it won't sound as realistic from an accurate system as it will from one that's been doctored up to sound more like real music.

Neither the mainstream audio nor the audiophile community has ever come to grips with this dichotomy: Engineers claim audiophiles are misguided (or worse) for preferring euphonically colored systems to accurate ones; audiophiles claim the engineering fraternity doesn't listen critically enough to the sound of their own systems. Neither camp has apparently entertained the thought that the source of their differences might be the differences between 90 degrees stereo and 360 degrees surround.

Footnote 5: I have long suspected that HF lift is the reason so many high-end audiophiles are plagued by minuscule amounts of signal distortion---it's the aural equivalent of a microscope. Most music lovers insist that audiophile systems sound tipped-up. That's because they are.