Noise

A signal is said to be noise if it is not correlated with itself at any non-zero time shift. A noise signal is the opposite of a periodic signal.

Noise may have a wide range of spectra. There are several types of noise important enough to have names. These are:

• White noise is noise in which the energy density is the same at every frequency. Here is some white noise.


• Pink noise is noise in which the energy density at a given frequency is inversely proportional to the frequency. Pink noise therefore has the property that the total energy is the same in every octave. Here is some pink noise.


• Brown noise is noise in which the energy density at a given frequency is inversely proportional to the square of the frequency. It is called "brown" because Brownian motion generates noise with this spectrum. Here is some brown noise.

In practice, many sounds are a mixture of periodic and noisy. Here is a sound composed of equal mixtures of pink noise and a 500 Hz sine wave.