So what is 3-dimensional sound? Is it real and can it be achieved with stereo speakers?
With well-placed speakers and some clever engineering tricks, it’s possible to create a “surround sound” audio experience that mimics real life. In fact, most high-end systems have something close to that as a goal; to recreate the sound actually recorded at the original event.
In fact, most people involved with high-end audio are familiar with the idea of soundstage – which is the ability of the stereo system to create a palpable 3-dimensional image behind the loudspeakers. This image can be enhanced by turning off the lights in the listening room or closing ones eyes. Properly done, the acoustic image is divorced from the loudspeakers and freely floats in space giving one the illusion of being at the event itself.
An even simpler example is the center channel of any stereo system. That center channel is, of course, an illusion created by equal sound coming from the left and right loudspeakers and reaching your ears at the same moment in time.
One of the drawbacks of stereo 2-channel audio attempting to reproduce a 3-dimensional image is the requirement that the listener be in a perfectly positioned location in order to get the full benefit of that sound field. A good example of this is Q Sound, an older technique used to produce true surround sound from 2-channel recordings. A great example of this is Roger Water’s Amused to Death CD. If you get your head positioned perfectly in the center, you can appreciate sound enveloping the listener.
Most surround sound is typically produced via multiple speakers and associated with home theaters, rather than high-end audio systems. However, there has been some development in bringing 3-D sound from 2-channel sources.
The story starts with stereo itself. Years ago there was only mono. As the story goes, Alan Blumlein (a British engineer) and his wife were attending one of the new “talkie” movies that had been recently introduced (before World War II). Sitting in the sudience he noticed an actor walk across the screen (from left to right) but his voice didn’t follow the actor (mono). He turned to his wife and said “I can fix that”. His idea was to make an acoustic version of a stereo viewer (you may remember the old stereo 3-D viewers of the day). It’s really a similar principal and he patented the idea. For years, stereo has dominated the audio industry until the advent of multi-channel audio (Quadraphonic) which fizzled out in the 1970′s as a primary audio experience and resurfaced later as surround sound focused primarily on home theater.
Using phase cancelation techniques, it is possible to cancel the sound of one speaker while playing from the other and thus fooling the ear into believing there is really only one source of sound. The problem with all these systems is the same problem the basic two-channel audio system has: the sweet spot requirement. To appreciate the cancellation technique employed in pseudo surround systems, the listener must either be wearing headphones or positioned perfectly between the loudspeakers. Listeners off-axis do not appreciate the effects.
Recent innovations include the Yamaha soundbar, the Bose 3-D television and the Bang an Olufsen $85,000 television set. These innovations are quite complicated but appreciate a wider sweet spot. Recently, there has been even more work released.
Princeton University professor Edgar Choueiri has announced a new system that, while not addressing the sweetspot problem, does address a tonal balance problem common to many of these systems..
“With surround sound you get a feeling of sound around you, but you can’t fool someone into thinking that a person has walked up to you to whisper into your right ear, or that a fly is flying around your ear,” said Choueiri, a mechanical and aerospace engineer.
Choueiri says he’s developed a true 3-D audio system that could revolutionize your next movie experience because it is much truer to life.
People who have experienced a demonstration “had a very positive reaction, almost shocked,” Choueiri said. “People are used to stereo or surround sound and suddenly they hear a person going around their head with a scissors giving them a haircut. They have visceral reaction.”
3-D sound does more than merely surround you: It enables your brain to paint a three dimensional image of an event. For example, when listening to a symphony orchestra via just two loudspeakers, you’ll be able to hear the bass playing on one side of the concert hall, the violin on the other, just as you would in the concert hall itself. In real life, though, the sound of the both the violin and bass would combine and reach your left and right ears at different times. Your brain analyzes these impulses near instantaneously and determines exactly where the sound must be coming from.
Sound too simple? Indeed. The trick in producing 3-D audio lies not in the recording, but in the playback.
If you play these recorded queues back in two regular loudspeakers, your right ear will hear the queues intended both for right and left ears, as will your left ear. This phenomena is called interaural crosstalk.
“These queues get corrupted when your left ear hears the right speaker and your right ear hears the left speaker,” Choueiri said. “This is called crosstalk. And without cancelling the crosstalk the queues get mixed up and your brain won’t get the information it needs to hear in 3-D.”
Choueiri used well known phase cancellation filtering techniques to reduce the interaural crosstalk, but what’s unique is his results. The real tricky part — and this is where Choueiri and his team at Princeton’s 3D3A Laboratory came in: The filter had to maintain a sound’s integrity.
“Previously any attempt to do crosstalk cancellation resulted in strongly coloring the sound to the point where you would not accept it as tonally correct,” said Choueiri. “Any listener would just realize that’s not a piano anymore. That sounds like something else. Our contribution was crosstalk cancellation without coloring the sound. That’s what makes this technology commercially feasible and acceptable to the human ear.”
In other words, the piano still sounds like a piano.
3-D sound will enter the commercial market within a few months, Choueiri said. He declined to give details, but suggested that a company that now has a product on the market without 3-D audio will bring out the next generation of that same product with 3-D audio.