If noise measurements are to have any real value, allowing fair comparisons of the true noise contributions from different types of equipment, they should give a figure that is representative of what we hear, and the first step towards this is the use of a Weighting Filter , which emphasises some frequencies more than others. Additionally, it turns out that there is another property of noise that varies, which also affects its perceived loudness, and that is its amplitude distribution or 'peakiness'. Some noise spends much of its time around a mean level, without significant peaks, but other sources of noise contain short peaks which the ear seems to take special notice of, the loudness being more related to the peak value than the mean. Just as very high frequencies are barely heard, so very brief peaks also go unheard, and so just using a peak reading rectifier is not the answer. Instead a 'Quasi Peak' or 'almost' peak reading meter is needed, its 'attack' time tailored to the ears behaviour.
Early attempts to improve on basic noise measurement used what came to be
referred to as the 'A' Weighting curve, which had been derived from the
work of Fletcher and Munsen (1933) who investigated hearing variation with
frequency and found that they needed to plot a set of 'equal loudness'
contours because the ears frequency response was different at different
loudness levels. The curve representing loudness equal to that at 1kHz and
40dB SPL (sound pressure level) was known as the 40-phon curve, and was
(supposedly) adopted for weighting purposes. Later work by Robinson and
Dadson (1956), refined the method producing significantly different, and
more accurate curves however.
In 1968, two inventions began to make clear the inadequacy of A-weighting;
the introduction of FM Radio and the Compact Cassette. The latter in
particular, was found to sound a lot less noisy (10dB) with Dolby noise
reduction switched on, without measuring significantly better, and this
led to work on better ways of measuring noise. The BBC (British
Broadcasting Corporation) undertook a research project, culminating in BBC
Research Dept Report EL-17 entitled "The Assessment of Noise in Audio
Frequency Circuits", in which they studied the effectiveness of various
wieghting curves and rectifiers that had been devised by the world's
broadcasters, on all sorts of extreme noise sources, and they chose one
combination as being very effective. Later work refined this into what
became a world standard known as CCIR468, which, though it has gone
through versions 1 to 4 is still essentially unchanged, only the permitted
tolerances being altered.
Unfortunately, though CCIR468 was adopted by many major broadcasting
organisations, and also incorporated into standards from BS (British
Standards) and IEC (International Electrotechnical Commission) to EBU
(European Broadcasting Union), and for a while even became championed in
consumer reviews of Cassette Decks (notably by Angus McKenzie in the
1970's) it is now falling into disuse for one simple reason: in an age
when advertising rules, big figures are reckoned to sell products, even if
they are meaningless, and A-weighting gives bigger numbers!
This is something I am very keen to put right, before audio quality
measurement falls completely into disrepute. Measurements ARE valid, but
only if they are properly weighted, and CCIR weighting works very well
indeed. Lets all use it!