How Do Headphones Work?

History

It seems like everyone you see these days has a pair of headphones plugged into his mp3 player or cell phone. Headphones are an essential modern accessory, but they've changed significantly from the first model, created in the 1930s by Beyerdynamic. Headphones are essentially small speakers designed to fit in or over the ear, so to understand how the headphones of today work, you should understand the technology behind speakers.

Speakers Use Electromagnetism to Generate Sound Waves
The human ear depends on a thin membrane, the eardrum, which vibrates in response to sound waves. The brain translates the vibrations into what we experience as sound. Speakers create waves of sound very much like a drum makes noise. When you strike a drum, the skin of the drum vibrates, creating sound waves. Speakers start with a cone, and, like a drum, a diaphragm of paper, fabric or plastic is stretched across the wide part of the cone, where it is fastened to a metal ring. The smaller end of the cone contains an iron coil sitting in front of a magnet. The coil is connected to the wires that connect the speaker to the stereo or music player. Sound from the source is translated into electrical signals that turn the coil into an electromagnet. The magnetized coil either attracts or repels the magnet behind it, which moves the coil back and forth. The motion of the coil pushes and pulls the diaphragm, creating waves just like a drum, and pumping sound waves out from the speaker into the room---or straight into your ear.

Today's popular earbud headphones work just like their larger speaker cousins. Whereas larger speakers have cones, an earbud's "cone" is a piece of flexible plastic that vibrates in response to signals from your music player. Underneath, a tiny metal coil sits above a magnet, just like a larger speaker. When your music player's coil is magnetized, the plastic moves, creating the sound waves that travel out of the headphones and into your ear.

Complementary Sound Waves Combine to Cancel Out Ambient Noise
A new, smaller type of headphone has become more common: noise-canceling headphones. They were invented by Amar Bose, the founder of the Bose Corporation, when he became frustrated with the noise of an airplane drowning out what he was playing on his headphones. Passive noise-canceling headphones use features such as circum-aural (around the ear) padding and high-density foam to prevent ambient sound waves from reaching your ears. Active noise-canceling headphones use a different method: the headphones actually emit sound waves to mask low-frequency sound waves of ambient noise, using a technique called destructive interference. By emitting sound waves that are mirror images of the ambient sound waves, noise-canceling headphones let you hear the audio content you want and leave out other, unwanted sound.