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May 1, 2006 Everyday Psychoacoustics For most of us, audio is mainly about listening to music. But the equipment and technology that make that possible have their own undeniable appeal, and many non-engineers delight in the few chances we have to look behind the panels and get some sort of handle on what’s going on in there.Sometimes, however, the handle’s a bit wobbly. Audio involves some very sophisticated electronic and digital principles, mixed in with that ol’ debbil psychoacoustics. It’s hardly surprising that some of the things that go on inside those boxes are difficult to grasp. Here are a few that seem to cause confusion. I’ll be with you in a millisecond Every Dolby Pro Logic surround decoder contains circuitry that briefly delays the surround-channel information -- typically by 20 milliseconds, but the delay is variable to accommodate different rooms. It may seem natural to suppose that this is a sort of "reverb," like that used to add a spurious sense of depth to old stereo (and mono) pop records. Nope. It’s a clever way to overcome one of the most sophisticated processes in human hearing. We detect the direction from which a sound is coming by two methods: through hearing differences in the sound’s level, and hearing the difference between the sound’s two separate arrivals at our two ears. The former is fairly imprecise, but it does mean that some spatial adjustment is possible using an amplifier’s balance control. We’re far more sensitive to differences in arrival time: If a sound reaches one ear a tiny fraction of a second before it reaches the other, it will be perceived as coming from a source closer to the first ear, even if the level of sound reaching the first ear is substantially lower than the level of sound reaching the second ear. A Dolby Surround decoder extracts a mono surround signal that has been mixed into the main audio signal, and ideally feeds that -- and only that -- to the surround speakers. Nothing’s perfect, however; often, the surround channels contain significant amounts of front-channel information. That might not matter, except that the surround speakers are often closer to the listening area than the front speakers, especially if you follow Dolby’s recommendation and place them beside rather than behind the seat. There is thus a risk of localizing some of the main-channel sound in the surround speakers. To prevent this, Dolby Pro Logic adds that 20ms delay to the surround channels so that, whatever the relative levels, the front signal will always arrive first. A delay of 20ms is roughly the equivalent of placing the surround speakers 20’ farther away from your chair than the front speakers. (Such trickery is not necessary with digital surround systems, which have virtually total separation between channels.) Putting Schwarzenegger in his place Just how sensitive we are to such time-related matters is evident up front as well. The original home surround decoders relied on ordinary imaging for putting the dialogue in the middle of the screen. That’s fine for whoever happens to be sitting on axis, equidistant from the speakers -- but only one person can sit in that seat at a time, and most of us watch movies with friends and family at least some of the time. For those sitting off axis, the distances from the front speakers are different: the sound from the closest speaker arrives first, and it’s at that speaker that we localize the sound. Adding a center-channel speaker in the middle helps a little, but not much. So Dolby Pro Logic extracts in-phase material (dialogue and other midscreen sounds), deletes it from the side speakers, and redirects it to the center channel. The result is that the dialogue appears to come from the center no matter where you sit, because it actually is being produced by its own speaker -- there’s no imaging about it. Sofa, so good Some sounds are very difficult to localize, and more and more designers of audio systems are taking advantage of that. What we detect mostly are acoustic events: changes in frequency or level, percussive noises, and so forth. However, steady-state sounds, such as prolonged tones, are hard to pinpoint even at fairly high frequencies, because the time cues are missing. The lower the frequency, the more pronounced this is, because the wavelengths of the sounds are very long compared to the distance between our ears -- a 50Hz wavelength measures about 20’ from peak to peak, compared to the 7" or so from ear to ear. As a result, the lower couple of octaves of the audio spectrum give almost no directional cues. There is thus no real need for the speakers that produce only the bass to be anywhere near the rest of the speakers. As long as some care is taken not to put the low-frequency units where they’ll interact unfavorably with the room surfaces, they can go almost anywhere -- behind the sofa, at the far end of the room, under the coffee table. And while in some systems there are good reasons to have multiple bass speakers, many fine-sounding home-theater installations have five (or more) tiny satellites for imaging and directionality but only one lone subwoofer. ...Ian G. Masters
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