In acoustics and audio, a transient is a high amplitude, short-duration sound at the beginning of a waveform that occurs in phenomena such as musical sounds, noises or speech. It can sometimes contain a high degree of non-periodic components and a higher magnitude of high frequencies than the harmonic content of that sound. Transients do not necessarily directly depend on the frequency of the tone they initiate.
Transients are more difficult to encode with many audio compression algorithms, causing pre-echo.
Sonar
The term transient is used by military sonar operators to describe unexpected sounds emanating from another vessel such as operating machinery, a metal hatch being slammed, or the flooding and pressurization of torpedo or vertical launch tubes.