Weighting filter

A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.

A-weighting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher–Munson equal-loudness contour.

At the basis of sound measurement is the idea of breaking down an incoming signal based on its different properties.

[6][nb 1] The A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters (see ITU-R 468 weighting for a further explanation).

An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state "at 1 m in an open field" or "at 1 m in anechoic chamber".

[citation needed] A-weighted SPL measurements of noise level are increasingly to be found on sales literature for domestic appliances such as refrigerators and freezers, and computer fans.

A-weighted noise measurements were found to give misleading results because they did not give sufficient prominence to the 6 kHz region where the noise reduction was having greatest effect, and sometimes one piece of equipment would even measure worse than another and yet sound better, because of differing spectral content.

This curve, which came out of work done by the BBC Research Department, and was standardised by the CCIR and later adopted by many other standards bodies (IEC, BSI/) and, as of 2006[update], is maintained by the ITU.

Noise measurements using this weighting typically also use a quasi-peak detector law rather than slow averaging.

This also helps to quantify the audibility of bursty noise, ticks and pops that might go undetected with a slow rms measurement.

ITU-R 468 noise weighting with quasi-peak detection is widely used in Europe,[7] especially in telecommunications, and in broadcasting particularly after it was adopted by the Dolby corporation who realised its superior validity for their purposes.

[citation needed][neutrality is disputed] It is commonly used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.

This ensures compatibility with black and white receivers, and also benefits noise performance and allows separation into meaningful luminance and chrominance signals for transmission.