Holophonic Sound

3D audio with just two speakers

I’m old enough to remember when the term high fidelity still meant something—it set apart audio equipment that had been deliberately engineered for faithful sound reproduction and a high signal-to-noise ratio from cheaper, cruder devices. At a certain point, though, pretty much everything was considered “Hi-Fi”; the new buzzword was stereo. Having equipment and recordings with two discrete channels of audio—conveniently matching the average number of human ears—was seen as the new sign of audio competence. Then there was the shift from the analog world of tubes, tapes, and vinyl to digital—a new standard of audio quality. A few years later, yet another phase: an increasing number of carefully positioned speakers and subwoofers to simulate the 360° audio field of the cinema. And now we’re using sophisticated digital signal processing to get better results with fewer speakers, but most people still seem to think two is too few.

For Those Who Have Ears to Hear…

While the number of speakers in the typical living room has increased since the days of my youth, the number of ears on the typical head has not. Humans somehow have the ability to locate the source of a sound spatially with only two inputs; even with a single ear, most people can pinpoint the direction a sound is coming from. Crucially, this sound-locating ability is not restricted to a single plane; we can also determine if a sound is coming from above, below, or anywhere in between (not to mention in front and behind). No home audio system I’ve ever seen (or heard) addresses the Z axis (up and down)—and neither, for the most part, do cinemas; for the most part, the only way to experience truly 3D sound artificially is to go into a special environment such as San Francisco’s Audium where speakers are physically placed above, below, and all around you.

If humans can determine the location of a sound anywhere around them with just two ears, it’s reasonable to imagine there must be some way of reproducing spatially accurate sound with just two speakers. But what’s the trick? What can ears and brains do that microphones and speakers can’t?

The Ears Have It

A large part of what enables people to identify the position of a sound is attributable to the hardware—the unique shape of the ear folds and ear canal. Because sounds coming from one direction will be reflected and channeled into the ear canal with slightly different characteristics than sounds coming from another direction, the brain is able to use these subtle clues to unconsciously create a mental picture of where the sound must have originated. While digital signal processing equipment can add depth and spatial separation to a stereo signal, there’s a much different and older approach to solving the problem: a method of recording known as binaural audio. A binaural recording is made with two microphones and a two-track recorder, just as a stereo recording would be. The difference is that the microphones are placed inside a dummy head—shaped just like a human head, complete with rubbery ears, sinus cavities, and so on. The microphones are right where the eardrums would be, so the signal they pick up is much closer to what ears would hear. The resulting recording—always most effective when heard through headphones—produces a vastly more accurate spatial rendition than would be achieved by using a pair of conventional microphones.

A well-executed binaural recording can sound shockingly realistic, even if the sound quality itself is not pristine. But binaural recording is appropriate only for live recordings; it’s also inconvenient, expensive (some pro-quality dummy heads retail for over US$8,000), and, frankly, just plain weird—all of which, along with the fact that you need to listen through headphones for maximum impact, helps to explain why you don’t encounter such recordings very often.

Hooked on Holophonics

But there’s a clever, patented variation on binaural recording that claims to go far beyond the simple microphones-in-the-dummy-head approach. It’s called holophonic recording, and the realism it produces, especially in the up/down dimension, is uncanny, eerie…even—as a friend of mine likes to say—freakadelic.

Ordinary holograms are produced by mixing reflected laser light with a second beam hitting an object from another angle; the resulting interference pattern of the two waves is what’s actually recorded on film. Expose the film to the same wavelength of light again, and a 3D image emerges from the interference pattern. Italian inventor Hugo Zuccarelli wondered whether a similar process could be used to record sounds, since after all, sound waves can form interference patterns with each other just as light waves can. His holophonic process starts with a type of binaural dummy head, but it reportedly records the interference pattern formed by mixing the sound with an inaudible, digitally superimposed reference signal. Zuccarelli believes that the human auditory apparatus, when listening to sounds, adds the same signal to the input, effectively decoding the interference patterns previously recorded. All that may sound like a bunch of mumbo-jumbo—and Zuccarelli certainly has detractors who claim “holophonic” sound is nothing more than binaural with maybe a few bells and whistles. All I can say is: hearing is believing. The holophonic recordings I’ve heard are simply remarkable—much more impressive than conventional binaural recordings—but I encourage you to listen for yourself (remember, use headphones!) and form your own opinion.

There is, of course, a little snag. As with all binaural recordings, holophonic sounds lose most of their spatial characteristics when played through ordinary speakers (though Zuccarelli has designed a special speaker system that enables holophonic sounds to be appreciated even outdoors by a large audience). As things stand now, you won’t be able to enjoy a holophonic soundtrack on your home theater system—no matter how many speakers it has—unless you and everyone else watching the film wear headphones. Nevertheless, a number of recording artists, including Michael Jackson, Stevie Wonder, and Pink Floyd, have employed holophonic technology in recordings or concerts, and it has also been featured in both films and commercials. Holophonic sound may be slow to catch on as a mainstream technology, but it’ll make your Surround Sound system sit up and go “Wow.”

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on July 17, 2003, and again in a slightly revised form on October 12, 2004.

Image credit: U.S. Patent and Trademark Office