Noise-Cancelling Headphones

Destructive interference at work!

Circum-aural, noise-cancelling headphones!

(By, Kevin Le)

Ambient, or background, noise is a music lover’s most despised pet peeve.  It ruins the sound quality of headphones and makes it impossible to immerse an individual into their music.  Amar Bose, the founder of Bose, had the same issue in 1978 when he found that the roar of the jet he was in disturbed his listening experience.  Right then and there, Bose grabbed a piece of paper and started making calculations.  Ten years later, noise-cancelling headphones hit the scene.

Noise-cancelling headphones are either passive or active.  When a noise-cancelling headphone is passive, that means that there is nothing actively soothing the incoming ambient noise.  Technically, all headphones are passive noise-cancelling headphones to different extents, simply because the material of headphones themselves block out sound waves, particularly high frequency waves.  However, the best passive noise-cancelling headphones are circum-aural (surrounds the ear) headphones which are specifically designed to eliminate ambient noise.  These types of headphones are packed with high density foam and other types of sound-absorbing materials.  Although this makes the headphones heavier, it does reduce ambient noise by 15 to 20 decibels (dB).  But, considering that Bose started designing his famous headphones on a plane, it is important to note that a jet engine creates about 75 to 80dB of ambient noise inside the aircraft.  The solution: active noise-cancelling headphones.

Active noise-cancelling headphones acts in the same way that passive noise-cancelling headphones do in the sense that they both use dense foam and other sound-absorbing materials to reduce ambient noise, particularly high frequency sounds.  The difference, however, is that active noise-cancelling headphones also eliminate lower frequency sound waves by actively erasing them.  This is achieved by creating a noise which perfectly mimics the incoming sound waves with one big catch: the waves being created by the headphones are 180 degrees out of phase with the incoming sound waves.  Consequently, the headphones create a phenomenon known as destructive interference, which is what happens when two opposite wavelengths balance and cancel each other.  To accomplish this, there is a built-in microphone which picks up ambient noise readings that cannot be blocked through passive means.  A set of electronics inside the headphone cup interpret the input sent by the microphone.  Then, this noise-cancelling circuitry creates an exact copy of the ambient sound waves.  Using this copy, the circuitry generates a wave 180 degrees out of phase, which is then sent to the speaker, who eliminates the ambient noise through destructive interference without affecting the normal audio.  However, one minor setback of active noise-cancelling headphones is that it is, well, active.  In other words, noise-cancelling headphones uses energy that must be added to the system to cancel out the ambient noise.  Active noise-cancelling headphones run on rechargeable batteries which need to be recharged occasionally, but a single charge can work for hours on end — although people might get annoyed when their headphones run out of batteries and lose the ability to play audio.

In the end, active noise-cancelling headphones work well – but not perfectly.  Compared to passive noise-cancelling headphones, active noise-cancelling headphones are able to add another 20dB to the reduction of ambient noise.  As a result, active noise-cancelling headphones only cancel about 70% of ambient noise – which is still a lot, no matter the location.  Additionally, people argue that noise-cancelling headphones muffles sound quality and that they can feel the air pressure changing within the headphones when ambient noise tries to sneak in.  Despite these drawbacks though, nobody ever wants to return to normal headphones which do not cancel out exterior sound waves.  Noise-cancelling headphones were dreamed about during a plane ride, and the final destination has been a dream for music lovers everywhere.

Posted in How Things Work