Search by sound is the retrieval of information based on audio input.
There are a handful of applications, specifically for mobile devices that utilize search by sound.
Shazam, Soundhound, Axwave, ACRCloud and others have seen considerable success by using a simple algorithm to match an acoustic fingerprint to a song in a library.
A microphone is used to pick up an audio sample, which is then broken down into a simple numeric signature, a code unique to each track.
The practice of using acoustic fingerprints is not limited to just music, however, but other areas of the entertainment business as well.