Above: Noise-cancelling headphones (© istockphoto.com/Alysta)
Can’t hear your music on the bus? Just need some peace and quiet to study? Noise-cancelling headphones (or earphones) may be just the thing for you! These devices use both passive and active cancellation to reduce background noise.
Fast fact: The idea for noise cancellation comes from Dr. Amar Bose. On a flight to Europe in the late 1970s, he was bothered by the noise from the jet engines when he listened to music using the in-flight headphones.Have you ever put your hands over your ears just before your friend blurts out a movie spoiler? That’s passive noise cancellation: your hands block sound waves (or spoiler waves!) from entering your ear. Headphones typically do this by using large amounts of sound-absorbing material to prevent background noise from entering your ear.
|Figure 1. Simple sinusoidal wave, or sine wave. Click to enlarge (Omegatron)
Active noise cancellation is a technology that attempts to completely eliminate background noise. To understand how, we first need to learn how sound travels. Mechanical vibrations displace molecules in the air in a periodic motion, forming a sound wave. The sound wave consists of areas of compression (where air molecules are tightly packed together) and areas of rarefaction (where molecules are farther apart). These compressions and rarefactions correspond to the “peaks” and “troughs” of a sinusoidal wave, also called a sine wave (see Figure 1).
Sound waves are defined by their frequency (the number of cycles in a given period of time) and amplitude (the distance between peaks and throughs). When two waves with the same frequency and amplitude meet they can be in phase, in antiphase, or out of phase (see Figure 2). If the waves are in phase (the waves overlap perfectly), the amplitudes add together. This is called constructive interference and the result is a louder sound. If the two waves are 180 degrees out of phase, or in antiphase (one is at its peak when the other is at its trough), the amplitudes cancel each other out. This is called destructive interference, and the result is no sound. If the waves are less than 180 degrees apart, they are out of phase, but not in antiphase. In this case, the result is a combination of both constructive and destructive interference.
|Figure 2. Interference of two waves. The two waves on the bottom left are in in phase, causing constructive interference (top left). The two waves on the bottom right are in antiphase, causing destructive interference (top right), Click to enlarge (Haade, modified by the author)
Active noise-cancelling headphones take advantage of destructive interference to eliminate background noise. These headphones are equipped with a microphone, a circuit board, an oscillator, and a battery. The microphone samples ambient noise (background noise), and the circuitry detects the amplitude and frequency of the ambient noise. The oscillator is an electronic circuit that produces repetitive, oscillating signals (a bit like a pendulum). In the case of noise-cancelling headphones, the oscillator emits sine waves in antiphase to the ambient noise, effectively cancelling out the ambient noise before it enters your ear. The battery powers this entire operation.
Fast Fact: Sound intensity is measured in decibels (dB). A whisper registers around 30dB; a jet engine can be as high as 150dB.Active noise-cancelling technology is regularly used in factories and airports where loud noises occur on a constant basis. This is because long-term exposure to noise exceeding 85 dB (the noise produced by a large truck) can cause permanent hearing damage. People often listen to music at dangerously high volumes to drown out background noise. But by using effective noise-cancellation headphones, you can get the full effect of being at a concert without blasting the volume.
So what’s the downside? Headphones using active noise cancellation are generally bulkier, heavier, and require battery changes. But that’s not even the worst part! On average, they cost 10 times more than other headphones. Can’t bear to hear the price? Cover your ears, and maybe you’ll find that passive noise cancellation is more affordable, and just as effective!
General science websites
What is passive noise cancellation? (Learning About Electronics) The Nature of a Sound Wave (The Physics Classroom) How Do Noise-Canceling Headphones Work? (Mental Floss)
Noise Induced Hearing Loss (Health Canada)
Acoustic Noise Cancelling headphone technology (Bose)
Silverthorn DU. 2004. Human Physiology: An Integrated Approach. 3rd edition. Pearson Education Inc., San Francisco, CA.