They're on your head everyday, delivering your favorite music, podcasts and audiobooks directly to your ears, but you may have given little thought to how your headphones actually do their job. Headphones are simply small versions of speakers, which use an electromagnet and a cone made of paper or other material to vibrate the air and create sound waves.
Principles of Sound Reproduction
Loudspeakers and their smaller siblings headphones both work on the same general principle, by converting electrical signals to sound waves. The music source sends the signal, in the form of an alternating electrical current, through a cable to the drivers -- miniature speakers -- in the headphones. The signal is sent through the voice coil in the driver, which generates a magnetic field. The voice coil is surrounded by a fixed magnet, and the alternating current in the voice coil attracts and repels it from the magnet. The voice coil is attached via a suspension material to the driver's cone, which moves back and forth between 20 to 20,000 times per second, changing the air pressure in front of it and creating sound waves you can hear.
Open Back and Closed Back
Over-the-ear headphones are broken down into two different types, those with open backs behind the drivers and those with closed backs. Open-back headphones allow for some ambient noise to mix with the music and tend to create a natural sound akin to listening to speakers. The open design does allow for sound to escape so others around you may hear what you're listening to, and loud outside noises are audible. Closed-back headphones shut out ambient noise as well as prevent others around you from hearing your music, but tend to have more bass sound and a less natural overall sound reproduction than open back headphones.
As headphones are miniaturized loudspeakers you can wear, earbuds are miniaturized headphones you insert into your ear. They function exactly the same way headphones do, just on a much smaller scale. Most earbuds have a sealed plastic back that makes them act like closed back headphones, though some do have vent holes to give them a bit of open back sound. Their small size is convenient for portability but often causes them to suffer from poor bass reproduction due to the small driver size.
Noise Cancelling Headphones
Noise cancelling headphones come in passive and active types. Passive noise reduction is accomplished by using a closed-back design with a seal around the ear that blocks out external noises. Active noise reduction headphones use a small microphone to detect ambient noise combined with noise cancelling electronics that create a sound wave that is 180 degrees out of phase with the ambient noise, negating the ambient sound and leaving only the music from the headphones. These headphones require a power source, usually in the form of a battery.