Audio Noise Reduction
In audio post-processing, noise gating reduces steady noise sources such as rumble from LP records, hiss from audio tape, static from a radio or amplifier, and hum from a power system, without greatly affecting the source sound. An audio signal such as music or speech is broken up into many frequency bands by a collection of overlapping band-pass filters, and if the signal amplitude in any one band is lower than a preset threshold then that band is eliminated from the final sound. This greatly reduces perceptible background noise because only the frequency components of the noise that are within the gated passbands survive.
The technique was implemented in real-time electronics in some audiophile record players as early as the 1980s, and is now commonly used in audio production post-processing, where software to Fourier transform the audio signal can yield a very detailed spectrum of the background noise. Common digital audio editing software packages such as CoolEdit and Audacity include easy-to-use digital noise gating code: the user selects a segment of audio that contains only static, and the amplitude levels in each frequency band are used to determine the threshold levels to be applied across the signal as a whole.
Noise gating works well when the static is steady and either narrowly confined in frequency (e.g. hum from AC power) or well below the main signal level (15 dB minimum is desirable). In cases where the signal merges with the background static (for example, the brushed drum sounds in the Sun King track on the Beatles album Abbey Road) or is weak compared to the noise (as in very faint tape recordings), the noise gating can add artifacts that are more distracting than the original static.
In the context of a multi-microphone recording session, noise gating is employed to reduce the leakage of sound into a microphone from sources other than the one the microphone was intended for. One example involves the mic-ing up of a drumkit. In most multi-mic drum recordings one microphone will be used to capture the snare drum sound and another to capture the kick drum sound. The snare microphone will output a signal composed of a high level snare signal and a lower level kick drum signal (due to the further distance of the kick drum from the snare microphone). If the threshold level of the noise gate is set correctly a snare drum signal can be isolated. To fully isolate the snare drum signal the release rate has to be quite fast which can cause the tail end of the snare sound to be 'chopped off'. This can usually be remedied by the inclusion of one or more overhead microphone, which can act as a general 'audio glue' for all the other gated sources.
For vocal applications on stage an optical microphone switch may be used. An infra-red sensor senses if somebody is in front of the microphone and switches on the microphone.
Read more about this topic: Noise Gate
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