ITU-R 468 noise weighting

It was never specifically intended for the measurement of the more random (near-white or pink) noise in electronic equipment, though has been used for this purpose by most microphone manufacturers since the 1970s.

The human ear responds quite differently to clicks and bursts of random noise, and it is this difference that gave rise to the CCIR-468 weighting curve (now supported as an ITU standard), which together with quasi-peak measurement (rather than the rms measurement used with A-weighting) became widely used by broadcasters throughout Britain, Europe, and former British Commonwealth countries, where engineers were heavily influenced by BBC test methods.

Developments in the 1960s, in particular the spread of FM broadcasting and the development of the compact audio cassette with Dolby-B Noise Reduction, alerted engineers to the need for a weighting curve that gave subjectively meaningful results on the typical random noise that limited the performance of broadcast circuits, equipment and radio circuits.

Early efforts to produce a better weighting curve led to a DIN standard that was adopted for European Hi-Fi equipment measurement for a while.

While it has been said (incorrectly) that the difference is due to a requirement for assessing noise intrusiveness in the presence of programme material, rather than just loudness, the BBC report makes clear the fact that this was not the basis of the experiments.

The real reason for the difference probably relates to the way in which our ears analyse sounds in terms of spectral content along the cochlea.

This behaves like a set of closely spaced filters with a roughly constant Q factor, that is, bandwidths proportional to their centre frequencies.

Dependent on spectral content, 468-weighted measurements of noise are generally about 11 dB higher than A-weighted, and this is probably a factor in the recent trend away from 468-weighting in equipment specifications as cassette tape use declines.

It is important to realise that the 468 specification covers both weighted and 'unweighted' (using a 22 Hz to 22 kHz 18 dB/octave bandpass filter) measurement and that both use a very special quasi-peak rectifier with carefully devised dynamics (A-weighting uses RMS detection for no particular reason[citation needed]).

Rather than having a simple 'integration time' this detector requires implementation with two cascaded 'peak followers' each with different attack time-constants carefully chosen to control the response to both single and repeating tone-bursts of various durations.

This ensures that measurements on impulsive noise take proper account of our reduced hearing sensitivity to short bursts.

It now finds fresh relevance in the measurement of noise on computer 'Audio Cards' which commonly suffer clicks as drives start and stop.

Weighting the distortion residue after removal of the fundamental emphasises high-order harmonics, but only up to 10 kHz or so where the ears response falls off.