Weighting Filter

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Weighting filter

om Wikipedia, the free encyclopedia

weighting filter is used to emphasise or suppress

me aspects of a phenomenon compared to others, for

asurement or other purposes.

udio applications

each field of audio measurement, special units are

d to indicate a weighted measurement as opposed to

asic physical measurement of energy level. For

nd, the unit is the phon (1 kHz equivalent level).

udness measurements

he measurement of loudness, for example, an A-weighting filter is commonly used to emphasize frequencies around 3

Hz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is

ensitive. The aim is to ensure that measured loudness corresponds well with subjectively perceived loudness. A-

ghting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher-Munson

ual-loudness contour. The B and C curves were intended for louder sounds (though they are less used) while the D curve

sed in assessing loud aircraft noise (IEC 537).

lecommunications

he field of telecommunications, weighting filters are widely used in the measurement of electrical noise on telephonecuits, and in the assessment of noise as perceived through the acoustic response of different types of instrument

ndset). Other noise-weighting curves have existed, e.g. DIN standards. The term psophometric weighting, though

erring in principle to any weighting curve intended for noise measurement, is often used to refer to a particular

ghting curve, used in telephony for narrow-bandwidth voiceband speech circuits.

vironmental noise measurement

weighted decibels are abbreviated dB(A) ordBA. When acoustic (calibrated microphone) measurements are being

erred to, then the units used will be dB SPL (sound pressure level) referenced to 20 micropascals = 0 dB SPL. dBrn

usted is a synonym for dBA.

ile the A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many

nd level meters, it does not really give valid results for noise because of the way in which our ears analyse sound. We

considerably more sensitive to noise in the region of 6 kHz than we are to tones of equivalent level (see ITU-R 468

ghting for further explanation). A-weighting is also in common use for assessing potential hearing damage caused by

ontents

1 Audio applications 1.1 Loudness measurements 1.2 Telecommunications 1.3 Environmental noise measurement 1.4 Audio reproduction and broadcasting

equipment 2 Other applications of weighting 3 See also

4 External links

Ten things you may not know about Wikipedia.


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d noise, though this seems to be based on the widespread availability of sound level meters incorporating A-Weighting

her than on any good experimental evidence to suggest that such use is valid. The distance of the measuring microphone

m a sound source is often "forgotten", when SPL measurements are quoted, making the data useless. In the case of

ironmental or aircraft noise distance need not be quoted, as it is the level at the point of measurement that is needed, but

en measuring refrigerators and similar appliances the distance should be stated; where not stated it is usually one metre

m). An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state

1 m in anechoic chamber". Measurements made outdoors will approximate well to anechoic conditions.

weighted SPL measurements of noise level are increasingly to be found on sales literature for domestic appliances such

efrigerators and freezers, and computer fans. Although the threshold of hearing is typically around 0 dB SPL, this is in

t very quiet indeed, and appliances are more likely to have noise levels of 30 to 40 dB SPL.

dio reproduction and broadcasting equipment

man sensitivity to noise in the region of 6 kHz

ame particularly apparent in the late 1960s with the

oduction of compact cassette recorders and Dolby-B

se reduction. A-weighted noise measurements were

nd to give misleading results because they did not

e sufficient prominence to the 6 kHz region wherenoise reduction was having greatest effect, and

metimes one piece of equipment would even measure

rse than another and yet sound better, because of

fering spectral content.

U-R 468 noise weighting was therefore developed to

re accurately reflect the subjective loudness of all

es of noise, as opposed to tones. This curve, which

me out of work done by the BBC Research

partment, and was standardised by the CCIR and

r adopted by many other standards bodies (IEC,

I) and, as of 2006, is maintained by the ITU. Noise

asurements using this weighting typically also use a

asi-peak detector law rather than slow averaging.

s also helps to quantify the audibility of bursty

se, ticks and pops that might go undetected with a

w rms measurement.

U-R 468 noise weighting with quasi-peak detection is widely used in Europe, especially in telecommunications, and in

adcasting particularly after it was adopted by the Dolby corporation who realised its superior validity for their purposes.

advantages over A-weighting seem to be less well appreciated in the USA and in consumer electronics, where the use ofweighting predominates -- probably because A-weighting produces a 9 to 12 dB "better" specification, see

csmanship. It is commonly used by broadcasters in Britain, Europe, and former countries of the British Empire such as

stralia and South Africa.

ough the noise level of 16-bit audio systems (such as CD players) is commonly quoted (on the basis of calculations that

e no account of subjective effect) as 96 dB relative to FS (full scale), the best 468-weighted results are in the region of

8 dB relative to Alignment Level (commonly defined as 18 dB below FS) ie 86 dB relative to FS.

e use of weighting curves is in no way to be regarded as 'cheating', provided that the proper curve is used. Nothing of

evance is being 'hidden', and even when, for example, hum is present at 50 or 100 Hz at a level above the quoted

eighted) noise floor this is of no importance because our ears are very insensitive to low frequencies at low levels, so itl not be heard.

ther applications of weighting


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he measurement of gamma rays or other ionising radiation, a radiation monitor or dosimeter will commonly use a filter

attenuate those energy levels or wavelengths that cause the least damage to the human body, while letting through those

t do the most damage, so that any source of radiation may be measured in terms of its true danger rather than just its

ength'. The sievert is a unit of weighted radiation dose for ionising radiation, which supersedes the older unit the REM

entgen equivalent man).

ighting is also applied to the measurement of sunlight when assessing the risk of skin damage through sunburn, or when

essing the sun protection factor of sun-screen lotions. Light, UVA and UVB in sunlight have different effects, with

ard to both DNA damage and melanin stimulation, depending on the wavelength of the radiation. Weightedasurements are often in SU (sun units) or DUV (Diffey weighted UV irradiance). The UV index is also used.

other use of weighting is in television, where the red, green and blue components of the signal are weighted according to

ir perceived brightness. This ensures compatibility with black and white receivers, and also benefits noise performance

d allows separation into meaningful luminance and chrominance signals for transmission.

ee also

Weighting

A-weighting Sone Phon ITU-R 468 noise weighting Psophometric weighting Equal-loudness contour Noise pollution Noise regulation

xternal links

Noise measurement briefing (http://www.ptpart.co.uk/show.php?contentid=70) Calculator for A,C,U, and AU weighting values

(http://www.measure.demon.co.uk/Acoustics_Software/a_weight.html) A-weighting filter circuit for audio measurements (http://sound.westhost.com/project17.htm) Rane pro audio reference definition of "weighting filters" (http://www.rane.com/par-w.html#weighting_filters) What is a decibel? (http://www.phys.unsw.edu.au/~jw/dB.html) Weighting filter according DIN EN 61672-1 2003-10 (DIN-IEC 651) Calculation: frequency f to dBA and dBC

(http://www.sengpielaudio.com/calculator-dba-spl.htm)

rieved from "http://en.wikipedia.org/wiki/Weighting_filter"egories: Audio engineering | Sound

dden categories: All articles with unsourced statements | Articles with unsourced statements since February 2007

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