Weighting filter

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Image:Weighting curves.png

A weighting filter is used to emphasise some aspects of a phenomenon over others, for measurement or other purposes.

In the measurement of loudness, for example, an A-weighting filter is commonly used to emphasise frequencies around 3–6 kHz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is insensitive. The aim is to ensure that measured loudness corresponds well with subjectively perceived loudness, but A-weighting is ony really valid for relatively quiet sounds,and for pure tones, being based on the 40-phon Fletcher Munson equal-loudness countour. The B and C curves were intended for louder sounds (though they are less used) while the D curve is used in assessing loud aircraft noise (IEC 537).

In the measurement of Gamma rays or other ionising radiation, a radiation monitor or dosimeter will commonly use a filter to attenuate those energy levels or wavelengths that cause the least damage to the human body, while letting through those that do the most damage, so that any source of radiation may be measured in terms of its true danger rather than just its 'strength'.

Another use of weighting is in Television, where the red, green and blue components of the signal are weighted according to their perceived brightness. This ensures compatibility with black and white receivers, and also benefits noise performance and allows separation into meaningful luminance and chrominance signals for transmission.

In the field of telecommunications, weighting filters are widely used in the measurement of electical noise on telephone circuits, and in the assessment of noise as perceived through the acoustic response of different types of instrument(handset).

In each field of measurement, special units are used to indicate a weighted measurement as opposed to a basic physical measurement of energy level. For sound, the unit is the phon (1 kHz equivalent level).

While the A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters, it does not really give valid results for noise because of the way in which our ears analyse sound. We are considerably more sensitive to noise in the region of 6kHz than we are to tones of equivalent level (see ITU-R 468 weighting for further explanation).

This shortcoming became particularly apparent in the late 1960's with the introduction of compact cassette recorders and Dolby-B noise reduction. A-weighted noise measurements were found to give misleading results because they did not give sufficient prominence to the 6kHz 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.

ITU-R 468 noise weighting was therefore developed to more accurately reflect the subjective loudness of all types of noise, as opposed to tones. 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 is now maintained by theITU is universally used by Broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa. It looked set to take over from A-weighting in the 1970's, though it remained less well known in the USA where A-weighting still predominates.

It was widely used in the UK and Europe, especially in broadcasting, especially when it was adopted by the Dolby corporation who realised its superior validity for their purposes. It's advantages over A-weighting seem to be less well understood in the USA, where the use of A-weighting predominates.

Though the noise level of 16-bit audio systems (such as CD players) is commonly quoted (on the basis of calculations that take no account of subjective effect) as −96 dB relative to FS (full scale), the best 468-weighted results are in the region of −68 dB relative to Alignment Level (commonly defined as 18 dB below FS) ie −86 dB relative to FS.

The use of weighting curves is in no way to be regarded as 'cheating', provided that the proper curve is used. Nothing of relevance is being 'hidden', and even when, for example, hum is present at 50 or 100Hz at a level above the quoted (weighted) noise floor this is of no importance because our ears are very insensitive to low frequencies at low levels, so it will not be heard. Rather, it is the quoting of unweighted figures that is to be deplored. Marketing departments would much rather quote -96dB, as in the above example of a 16-bit system, than the properly weighted and sensibly referenced figure of -68dB which European broadcasters would insist on using!

Other noise-weighting curves have existed (eg DIN standards)and it should be noted that the term 'Psophometric Weighting', though referring in principle to any weighting curve intended for noise measurement, is often used to refer to a particular weighting curve used in telephony for narrow-bandwidth speech circuits.

A-Weighting is also in common use for assessing potential hearing damage caused by loud noise, though this seems to be based on the widespread availability of sound level meters incorporating A-Weighting rather than on any good experimental evidence to suggest that such use is valid.

A-weighted decibels are abbreviated dB(A) or dBA. When acoustic (calibraated microphone) measurements are being referred to, then the units used will be dB SPL (sound pressure level) referenced to 20 micropascals = 0 dB SPL. dBrn adjusted is a synonym for dBA.

A-weighted SPL measurements of noise level are increasingy to be found on sales literature for domestic appliances such as refrigerators and freezers, and computer fans. Although the threshold of hearing is typically around 0dB SPL, this is in fact very quiet indeed, and appliances are more likely to have noise levels of 30 to 40dB SPL.

The distance of the measuring microphone from a sound source is often "forgotten", when SPL measurements are quoted, making the data useless. In the case of environmental or aircraft noise distance need notbe quoted, as it is the level at the point of measurement that is needed, but when measuring refrigerators and similar appliances the distance should be stated (where not stated it is likely to be 1m). An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state 'at 1 m in anechoic chamber'. Measurements made out of doors will approximate well to anechoic conditions.


Contents

= Definitions

The gain curves are defined by the following transfer functions [1]:

A

<math>

G_A(s) = {k_A \cdot s^4 \over (s+129.4)^2 (s+676.7) (s+4636) (s+76655)^2} </math>

kA ≈ 7.39705×109

B

<math>

G_B(s) = {k_B \cdot s^3 \over (s+129.4)^2 (s+995.9) (s+76655)^2} </math>

kB ≈ 5.99185×109

C

<math>

G_C(s) = {k_C \cdot s^2 \over (s+129.4)^2 (s+76655)^2} </math>

kC ≈ 5.91797×109

D

<math>

G_D(s) = {k_D \cdot s \cdot (s^2 + 6532 s + 4.0975 \times 10^7) \over (s+1776.3) (s+7288.5) (s^2 + 21514 s + 3.8836 \times 10^8)} </math>

kD ≈ 91104.32

The k values are constants which are used to normalize the function to a gain of 1 (0 dB). The values listed above normalize the functions to 0 dB at 1 kHz, as they are typically used. (This normalization is shown in the image.)

Parts of this article from Federal Standard 1037C and from MIL-STD-188

See also


External links

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