Noise is random or unwanted sound. Unwanted signals, such as static that interferes with radio transmission, are also referred to as noise. The sound we hear is caused by vibrations of air molecules. A tuning fork produces a pure tone, with only one frequency, or rate of vibration (see Tone). Musical instruments produce harmonic sound. Such sound contains many frequencies that are harmonically related (see Harmonics). Random sound has many frequencies that are not harmonically related.
Unwanted sound can be caused by any kind of sound—tones, harmonic sound, or random sound. Techniques to reduce unwanted sound include filtering and averaging. Filtering can eliminate certain unwanted frequencies. Averaging causes unwanted sound waves to cancel each other out through interference (see Interference).
