Oct 2019 New Scientist
Noise cancelling technology like that used in the new Apple Airpods Pro and Bose headphones was originally invented in the early 1980s to prevent ear damage for pilots and was later used by tank crews in the US Army. Read this story from the 1992 New Scientist archive to understand how noise cancelling headphones work with the use of antisound.
The new Apple Airpods Pro use noise cancelling concepts which were invented in the 1980s to prevent ear damage for pilots
This article was originally published on 5 December 1992.
The American company Bose, best known for making large amounts of noise through its range of loudspeakers, has now developed a technique to protect people from noise. The company, based in Framingham near Boston, has just won a contract with the US military to make helmets for aircraft pilots and tank drivers which cancel out background noise with antisound.
“The US government pays out $200 million a year in compensation for hearing loss caused by military service,” says Amar Bose, professor of electrical engineering at the Massachusetts Institute of Technology and founder of Bose. “”Hearing loss is a common reason for early retirement of pilots, second only to psychological stress. There will also be an enormous market in industry for the noise cancellation system.”
People who work in noisy environments can wear earplugs, but then cannot hear what is said to them. Earplugs also do not shut out the low-frequency, throbbing noise that aircraft and helicopter pilots, drivers of tracked vehicles and people working close to heavy machinery have to endure. Very large mufflers are needed to block low frequencies.
Such background noise can be cancelled out with antisound. A microphone picks up the noise and an electronic circuit analyses it and very rapidly produces an opposite noise. Where the original sound wave has a peak, the antisound has a trough and vice versa, so the two cancel each other. But attempts to use antisound in a room or cockpit have not been successful. As the sound and antisound bounce off the walls they get out of synch, so the overall noise reduction is only around 5 decibels, which is barely audible. (A reduction of 10 decibels makes noises sound half as loud.)
Anti noise works by playing a copy of a sound wave with an inverted phase
Five years ago, engineers from Bose developed headphones with a built-in noise-cancellation system. They were designed for the pilots of the Voyager aircraft which flew around the world without refuelling but was very noisy. The headphones have now been modified for industrial use and will be built into military helmets.
The headphones contain miniature speakers so that other people can talk to the wearer. Next to the speaker is a small microphone which picks up the mixture of voice and unwanted background noise which has penetrated from the outside. A circuit processes the sound signal, picks out the unwanted noise, and then reverses it into antisound. This antisound is then fed into the speaker along with the voice signal so that the background noise is cancelled as it enters the ear.
Bose won a contract from the US Army to supply tank crew with noise cancelling headsets in 1993
Because the cavity between the headphones and the ear is so small, there is no room for low-frequency, long-wavelength sounds to reflect and get out of synch with the antisound. Sounds with frequencies above 1 kilohertz have wavelengths short enough to be affected by reflections but it is easier to block these wavelengths out of the headphones with sound insulation. The walls of the headphones are made of a heavy damping material, and they have rims made of a sack of silicone gel which seals the gap between the headphones and the head.
The result is a noise reduction of more than 20 decibels. Bose is now selling the headphone sets for around $1000. The company plans to develop a battery-powered version which airline passengers can plug into their personal stereo so that the music is not drowned out by engine noise. Recording engineers can use the same system to listen to a music recording being made in a concert hall without being distracted by the sound of the orchestra itself.
A 1998 advert for a Bose headset being used by tank drivers and crew