The Sound of Silence

Continued from page 1

By Simson Garfinkel

July 9, 2003

How are these headphones better than their forerunners? Let me count the ways. For starters, that side package no longer hangs off the headset-instead, the electronics and batteries have been built in to the headset band. Just one long cord lets you plug in to a walkman, or you laptop, or the sound system of your airplane seat. And if all you are after is silence, you don't have to plug into anything at all: the cord detaches from the headset assembly.

The QC2 innovations extend beyond its electronics to its mechanics. Unlike other headsets, they fit well on both my head and the head of my six-year-old daughter. And unlike the QC1, the earcups now pivot to let the headphones fold reasonably flat. This makes it much easier to travel with them. Bose includes with the headphones a small, sturdy case that protects the unit, and even has a bunch of business cards for those pesky travelers who want to know what you are wearing-a clever marketing gimmick.

Clearly, though, the main thing going on with the QC2 headphones is the improved sound quality. Dan Gauger, a manager in the Bose Research and Systems Engineering group, explains how Bose got rid of the hiss. "The hiss you hear in active noise canceling headphones is just the self-noise' or noise floor' of the electronics," says Gauger. In other words, the hiss of the QC1 and the current generation of Sony headsets is actually noise that's made by the noise-canceling circuitry itself. "Take just about any audio system, turn it up very loud with no signal being played, and put your ear close to a speaker in a very quiet room and you will hear some hiss," says Gauger. That's the self-noise.

Reducing this noise required paying careful attention to all aspects of the system. But Bose is a quality operation; why didn't they eliminate the noise originally?

According to Gauger, the noise-cancellation technology was first developed for "pretty noisy places such as light aircraft cockpits and armored vehicles; in these situations the noise floor of the product didn't matter much and so we didn't put much effort into minimizing it." it wasn't until people started using the original QuietComfort headphones in relatively quiet environments to listen to beautiful music, that the self-noise started to become apparent at all. Of course, once you knew that it was there, it could become quite annoying. Now it's gone. Good riddance!

To compare Bose's new headset to the competition, I bought two Sony headsets-the MDR-NC5 ($79), which looks like the Bose units, and the MD5-NC11 ($149), which is a modified set of earbuds that fit inside the ear canal. I tried using them at home, in an office, and on the subway-the same sort of places that I had tried out the QC2.

Of the two, the NC11 does a slightly better job-probably because the rubber buds plug up your ears and prevent more background noise from seeping in. The NC5, on the other hand, didn't shape very well to my head. Even if it did, it wouldn't have blocked out much noise, as it doesn't have any sound muffling material. But ergonomics aside, both of the Sony headsets had the same electrical problem: they both injected so much of their own noise that I felt myself consciously choosing: Do I want to listen to the background noise or to the self-noise?

In the long run, I'd like to be able to do without the headsets entirely and have the active noise reduction built into my desk or into the walls. The "easy" way to do this would be with some sort of head-tracking device that would monitor where you are and generate the correct antinoise waveform to reach your position at just the right time. But I'm skeptical that this will ever work right. According to Julius Smith, associate professor of music and electrical engineering at Stanford University, active-noise suppression systems need to have a phase accuracy of roughly a quarter wavelength: after that, the antinoise starts adding to the background sound, rather than canceling it out. The size of this bubble of silence is roughly 35 centimeters for a 250-hertz tone (roughly middle C on the piano), which has a wavelength of about 1.4 meters. So if somebody is standing next to you and the thing is turned on, they will perceive your quiet as a series of unpleasant shrieks and squeals.

But another approach, says Smith, is something in development called "acoustic holography."

"A spherical array of microphones and speakers could do some very interesting things. Also, each speaker can also be used, in principle, as a microphone," Smith explains. "If the sound to be canceled comes from outside the array, I would think good results could be obtained." He adds, however, that he has "not seen an attempt to estimate the sound field somewhere and send antisound to that location."

Although such technology may be years away from the marketplace, in an increasingly noisy world, it's only a matter of time.