ITU-R 468 (originally defined in
CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to
noise measurement In acoustics, noise measurement can be for the purpose of measuring environmental noise or measuring noise in the workplace. Applications include monitoring of construction sites, aircraft noise, road traffic noise, entertainment venues and neighb ...
, widely used when measuring noise in audio systems. The standard,
now referred to as ITU-R BS.468-4, defines a
weighting filter
A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.
Audio applications
In each field of audio measurement, special units are used to indicate a weighted measure ...
curve, together with a
quasi-peak
A quasi-peak detector is a type of electronic detector or rectifier. Quasi-peak detectors for specific purposes have usually been standardized with mathematically precisely defined dynamic characteristics of attack time, integration time, and dec ...
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
having special characteristics as defined by specified
tone-burst tests. It is currently maintained by the
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
who took it over from the CCIR.
It is used especially in the UK, Europe, and former countries of the
British Empire
The British Empire was composed of the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts esta ...
such as Australia and South Africa. It is less well known in the USA where
A-weighting
A-weighting is the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured ...
has always been used.
M-weighting
ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The s ...
is a closely related filter, an offset version of the same curve, without the quasi-peak detector.
Explanation
The
A-weighting
A-weighting is the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured ...
curve was based on the 40 phon
equal-loudness contour
An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and ...
derived initially by Fletcher and Munson (1933). Originally incorporated into an ANSI standard for
sound level meters, A-weighting was intended for measurement of the audibility of sounds by themselves. It was never specifically intended for the measurement of the more random (near-
white
White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
or
pink
Pink is the color of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, politeness, ...
) 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
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
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
The Commonwealth of Nations, simply referred to as the Commonwealth, is a political association of 56 member states, the vast majority of which are former territories of the British Empire. The chief institutions of the organisation are the Co ...
countries, where engineers were heavily influenced by
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test methods. Telephone companies worldwide have also used methods similar to 468 weighting with quasi-peak measurement to describe objectionable interference induced in one telephone circuit by switching transients in another.
, 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. A-weighting was not giving consistent results, especially on
of high frequencies was resulting in increased noise readings that did not correlate with subjective effect. 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.
Experiments in the
...