In audio, recording, and broadcast systems audio noise refers to the residual low level sound (usually hiss and hum) that is heard in quiet periods of programme.
In audio engineering it can also refer to the unwanted residual electronic noise signal that gives rise to acoustic noise heard as 'hiss'. This signal noise is commonly measured using A-weighting or ITU-R 468 weighting
Noise is often generated deliberately and used as a test signal. The two most common types of such noise are:
white noise, which has a uniform power spectral density at all frequencies
pink noise which has a power spectral density that falls at 3dB/octave with rising frequency. The pink noise is often more useful in audio testing because it contains constant energy per octave (and hence per commonly used 1/3rd octave), rather than a preponderance of energy at high frequencies. In other words it contains energy that is distributed geometrically rather than linearly.
There are other less common kind of noise:
black noise, a term with numerous conflicting definitions, but commonly refers to silence with occasional spikes.
blue noise, contains more energy as the frequency increases.
brown noise, mimics the signal noise produced by brownian motion.
gray noise, similar to white noise, but has been filtered to make sound level appear constant at all frequency to the human ear.
green noise, an unofficial term which can mean the mid-frequencies of white noise, or the 'background noise of the world'.
orange noise, an unofficial term describing noise which has been stripped of harmonious frequencies.
purple noise, contains more energy as the frequencies increases.
red noise, an oceanograhic term which describes ambient underwater noise from distant sources. Also another name for brown noise.
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