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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 measurement as opposed to a basic physical measurement of energy level. For sound, the unit is the phon (1 kHz equivalent level).

Sound

Sound has three basic components, the wavelength, frequency, and speed. In sound measurement, we measure the loudness of the sound in decibels (dB). Decibels are logarithmic with 0 dB as the reference. especially in telecommunications, and in broadcasting particularly after it was adopted by the Dolby corporation who realised its superior validity for their purposes. It is commonly used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.

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) i.e. −86 dB relative to FS.

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 supersedes 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, since different wavelengths have different biological effects. Common examples are the SPF of sunscreen, and the UV index.

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
  • Weighting curve
  • Sone
  • Phon
  • ITU-R 468 noise weighting
  • Psophometric weighting
  • Equal-loudness contour
  • Noise pollution
  • Noise regulation
  • A-weighting
  • B-weighting
  • C-weighting
  • D-weighting
  • G-weighting
  • M-weighting
  • Z-weighting

Notes

References

  • Noise measurement briefing
  • Calculator for A,C,U, and AU weighting values
  • A-weighting filter circuit for audio measurements
  • AES pro audio reference definition of "weighting filters"
  • What is a decibel?
  • Weighting filter according DIN EN 61672-1 2003-10 (DIN-IEC 651) Calculation: frequency f to dBA and dBC