Also see noise .
In audio , noise is generally any unpleasant sound and, more technically, any unwanted sound that is unintentionally added to a desired sound. Ambient sound itself is a series of changes in air pressure transmitted in waves from the sound source to anyone with the sensory apparatus to detect the waves (human beings and other animals with ears, for example). Sound waves are expressed as a series of analog sine waves. The combination and blend of these waves gives sounds their individual characteristics, making them pleasant or unpleasant to listen to. Some sounds are combination of waves that are related to each other, as in the case of a pitched instrument that transmits one dominant frequency of sound wave and additional frequencies that enhance the fundamental or dominant frequency. An oboe is an example of a pitched instrument. Other sounds are made from a grouping of tightly spaced waves that do not have a specific fundamental frequency that is dominant over the others. A snare drum is an example.
In recording sound, noise is often present on analog tape or low-fidelity digital recordings. The standard audio cassette includes a layer of hiss on every recording. When doing digital recording, the conversion of a sound file from 16-bit to 8-bit adds a layer of noise.
White noise is a sound that contains every frequency within the range of human hearing (generally from 20 hertz to 20 kHz ) in equal amounts. Most people perceive this sound as having more high-frequency content than low, but this is not the case. This perception occurs because each successive octave has twice as many frequencies as the one preceding it. For example, from 100 Hz to 200 Hz, there are one hundred discrete frequencies. In the next octave (from 200 Hz to 400 Hz), there are two hundred frequencies.
White noise can be generated on a sound synthesizer. Sound designers can use this sound, with some processing and filtering, to create a multitude of effects such as wind, surf, space whooshes, and rumbles.
Pink noise is a variant of white noise. Pink noise is white noise that has been filtered to reduce the volume at each octave. This is done to compensate for the increase in the number of frequencies per octave. Each octave is reduced by 6 decibel s, resulting in a noise sound wave that has equal energy at every octave.