Cell-phone muffler squelches street noise.

A technique that quiets noise by producing counteracting sounds may soon mute the background din that often plagues cellular-phone conversations.

Researchers at Lucent Technologies in Arlington, Va., reported last week that they have developed a modified cellphone prototype that combats low-frequency, ambient noise, such as wind and traffic sounds. The antinoise circuitry can squelch sound at frequencies below roughly 1,200 hertz, reducing the noise to as little as one-eighth of its untreated loudness, says Michael A. Zuniga, leader of the development team.

Zuniga described the nascent technology last week at the 136th biannual meeting of the Acoustical Society of America in Norfolk, Va.

Cell-phone makers have expressed interest in conducting more extensive testing of the innovation, he says. The producers then might license the technology from Lucent, which no longer manufactures or sells telephones. The noise-fighting phones "could appear in Japan next year, depending on how things go this fall," Zuniga says.

"It is absolutely a very exciting application," says Irene Lebovics, president of NCT (Noise Cancellation Technologies) Hearing Products in Stamford, Conn. "Walking on the street or riding in a car, you're always subject to low-frequency I noise. [It] has a masking effect on speech," she adds.

Zuniga says his team experimented with two approaches to the ambient-noise problem, both of which involve detecting noise with an extra microphone and generating sound waves that cancel it. One method, a feedback approach, reads noise signals directly at the ear. Since feedback can cause squeals, the researchers had to use less-than-optimal circuit settings to avoid them, yielding marginal noise reduction.

The other method, called feed-forward, positions the microphone about 2 centimeters from the ear, so that it detects ambient sounds before the ear does. The microphone converts the sound to electric signals, which are then sent to a programmable, digital microchip custom-made by Fujitsu Microelectronics in Dreieich-Buchschlag, Germany.

After analyzing the signals, the chip races noise-canceling waveforms to the phone's speaker, which converts them to sound just in time to counteract arriving noise. Because sound exists as compression waves in the air, opposite peaks and dips of compression from the speaker nullify target noise in the cavity between the phone and the ear, Zuniga explains.

The modifications also include internal hanges to the earpiece so that the speaker produces effective compression patterns despite differences in ear size and shape, he says.

The performance of the feed-forward system was, "across the whole range of users, pleasantly, pleasantly surprising," Zuniga says. He and his colleagues both measured the noise reduction electronically and heard positive reviews from five test subjects who tried the prototype phone.

Using sound to cancel sound is not a new idea. For decades, acoustics designers have been developing products with what they call active noise reduction, such as headphones that protect the ears against loud noises or make it easier to hear an in-flight movie on an airplane.

Cellular phones, however, are a new frontier, says Graham Eatwell, a consultant and president of Adaptive Audio in Annapolis, Md. "As an active-control person, I think it's very interesting," he says of the Lucent design, which has several patents granted or pending.

In particular, Lucent's use of digital circuits might signify that such circuits are ready for use in other battery-operated, portable equipment, he adds. Designers of such products previously shunned digital circuits as too slow and power hungry. "One very big market is hearing aids," says Eatwell. "That's the one everyone has their eye on."