Science of the sound of silence

EBACE Convention News » 2007
May 9, 2007, 9:58 AM

The prototype personal stereo was famously developed in 1978 to help Sony founder Masaru Ibuka listen to opera while airborne on business trips.

The same year found another electronics pioneer, Dr. Amar Bose, on another business trip frustrated in his attempt to enjoy airline-supplied audio entertainment by the background noise that is one of air travel’s perennial banes. He started to work out how headphones might be used to reduce the background noise and eventually formed a dedicated research group to explore noise reduction technology.

Development of what became the Walkman was fairly straightforward. Sony chairman Akio Morita saw the commercial potential of Ibuka’s device and in February 1979 initiated commercial development with a launch deadline of June 21 that year.

The monaural recorder modified by Sony’s tape recorder division to create a stereo playback machine for Ibuka was combined with a new design of open-air headphones from the company’s research laboratory, and the player went on sale July 1. Sony went on to sell more than 150 million copies over the next 15 years, producing more than 300 different models.

Perfecting Bose’s noise reduction technology was less straightforward. The principle of active noise reduction is well known today: it involves measuring the frequency and amplitude of unwanted noise and generating an anti-phase equivalent to cancel it out. It is applied in some airliners and military aircraft flight decks using speakers behind cabin liners. But achieving the same effect in a headset light and comfortable enough to be worn for protracted periods was another matter.

In fact, development took more than a decade, though prototypes were supplied to the U.S. Air Force in 1985 and tested in December of the following year by Dick Rutan and Jeana Yeager on their nine-day flight round the world in the Rutan Voyager.

Passive headsets with sound-absorbing material in tightly fitting earcups can reduce high- frequency noise effectively, but the high clamping force required can make them uncomfortable and they do little to keep out low-frequency noise. Active noise reduction augments the earcup speaker with a microphone to sense the low-frequency noise and electronics to generate the anti-phase signal.

In 1989, Bose launched its first acoustic noise-canceling headset, although this was intended for use by pilots rather than passengers. EBACE visitors can experience the effect on Bose’s exhibit here (Booth No. 520), where a sound generator enables them to hear the difference the headset makes.

There are no new models on display, although the original design has seen periodic refinement over the years and a QuietComfort variant for airline passengers appeared in 2000.

The current Aviation Headset X model was launched in 1998. It introduced the TriPort technology which uses small acoustically calibrated reservoirs of air within the earcups to boost performance by making the speaker drivers more effective.

Dr. Bose’s idea has not spawned the same diversity of designs as the Walkman. There is an installed version that takes its power from the airplane and two portable options, one with a straight cable and twin plugs for pilots who fly multiple aircraft, the other with a curled cable and U174 plug for helicopter pilots. Since 2003 the portable models have incorporated AdaptiSense circuitry, which monitors power demand and adjusts the voltage to maximize battery life. The result is at least 40 hours of life from two AA alkaline batteries.

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