Weighting filter: Encyclopedia - Weighting filter

Weighting filter

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

Weighting filter - Audio applications

In each field of audio 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).

Weighting filter - Loudness measurements

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. A-weighting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher-Munson equal-loudness contour. 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).

Weighting filter - Telecommunications

In the field of telecommunications, weighting filters are widely used in the measurement of electrical noise on telephone circuits, and in the assessment of noise as perceived through the acoustic response of different types of instrument (handset). Other noise-weighting curves have existed (e.g. 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.

Weighting filter - Environmental noise measurement

A-weighted decibels are abbreviated dB(A) or dBA. When acoustic (calibrated 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.

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). 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. 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 not be 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 usually one metre (1 m). 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 outdoors will approximate well to anechoic conditions.

A-weighted SPL measurements of noise level are increasingly 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.

Weighting filter - Audio reproduction and broadcasting equipment

Human sensitivity to noise in the region of 6kHz 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 the ITU 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.

Weighting filter - Other applications of weighting

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'. The Sievert is a unit of weighted radiation dose for ionising radiation, which supercedes the older unit the REM (Roentgen equivalent man).

Weighting is also applied to the measurement of sunlight when assessing the risk of skin damage through sunburn, or when assessing the sun protection factor of sun-screen lotions. Light, UVA and UVB in sunlight have different effects, with regard to both DNA damage and melanin stimulation, depending on the wavelength of the radiation. Weighted measurements are often in SU (sun units) or DUV (Diffey weighted UV irradiance). The UV index is also used.

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.

See also

• Weighting - Root page
• A-weighting
• ITU-R 468 noise weighting
• Equal-loudness contour
• Noise

Adapted from the Wikipedia article "Weighting filter", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki