Lost in modulation
Brian Pereira | 15 October 2006
Sony ex-chairman Akio Morita enjoyed listening to his Walkman while playing tennis. Back in 1979, the Walkman player was a hit because it offered both portability and hi-fidelity audio. But the late Mr Morita wouldn’t have enjoyed his music on an airplane. On a flight to Europe in 1978, Dr Amar Bose was trying hard to appreciate the music on his standard issue airline headphone. But the soft notes were overwhelmed by the roar of the engines. And Bose wondered if headphones could actually reduce noise. Back-of-the-envelope calculations on that flight indicated it was possible. That led to Bose Corporation to set up an entire research group dedicated to noise reduction technology. It was only after a whole decade of research that the first Bose Acoustic Noise Cancelling headset was ready.
Serious music enthusiasts are investing in noise-canceling technology. In fact, the technology is aimed at those who listen to music while traveling. Some airlines have taken note and offer noise-canceling headphones on-board.
Noise canceling technology is available in two forms: Active and Passive. Active noise-canceling headphones are obviously more expensive. Headphones with active noise cancellation have special circuitry that generates a counter signal to cancel surrounding noise. Naturally, these headphones are battery-powered. Light, rechargeable batteries ensure that such headphones are comfortable to wear over extended periods. But it’s the circuitry within the ear piece that deserves all the credit.
Here’s a technical explanation of how active noise cancellation works. Miniature microphones in the headset pick up both ambient sounds as well as music coming through the headphones. It is assumed that most noise occurs at the low frequencies. So the mid- and high-frequency sounds are immediately filtered out of the signal, allowed to pass through, and you hear them.
The circuitry in the headset analyses the low frequencies in real time, and applies a technique called ‘phase inversion.’ A 180 degree inverted signal (see graph) is created. This neutralizes the frequencies, though not entirely, because you do need to listen to low frequencies in the music (bass sounds). And that’s how the noise in the headset cancels itself out. The Bose Quite Comfort series and Sennheiser’s PXC series of headphones employ active noise cancellation. The Bose Quite Comfort 3 headphone (US$ 350) is now smaller and lighter than its predecessors. It uses a rechargeable lithium-ion battery. The kit includes a battery charger, case, 1/4-inch stereo plug adapter, and a five-foot extension cord.
Passive noise cancellation or sound-isolating technology is used in special earphones that have a modified in-ear design. The nozzle on the ear piece goes deep into the ear canal. Effectively this is a ear plug that blocks out ambient sounds, but directs sound from its tiny speaker deep into the ear canal. So you only hear the music. The Shure E series earphones (see picture on the right) have sound isolating designs. Earphones with sound isolating technology can block 70 - 80 percent of ambient sounds. This means you’ll hear only the music while traveling in an aeroplane, and less noise from its engine. But be ready to spend anything between US$ 100 - 550 for these high-tech earphones.
Noise cancellation and sound-isolating headphones cannot block out 100 percent ambient sounds. But they significantly improve the listening experience, especially while commuting. These products are not yet mainstream; for now they are only for enthusiasts.