Weighting curve
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A Weighting curve is a graph that is used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly know example is in sound level measurement where a specific set of weighting curves known as A, B, C and D weighting are often used. Measurements of sound intensity do not correspond to perceived loudness because the human ear is less sensitive at low and high frequencies, with the effect more pronounced at lower sound intensities. The four curves are applied to the measured intensity, for example by the use of a weighting filter in a sound level meter, to arrive at readings of loudness in Phons or in decibels (dB) (see A-weighting).
[edit] Weighting curves in Electronic Engineering, Audio and Broadcasting
Although A-weighting is frequently used when measuring noise in audio circuits, a different weighting curve, ITU-R 468 weighting is preferred by most broadcasters, and some equipment manufacturers as it reflects more accurately the audibility of random noise as opposed to pure tones. Psophometric weighting is used in telephony and telecommunications where narrow-band circuits are common.
[edit] Other applications of weighting
Acoustics is by no means the only subject which finds use for weighting curves however, and they are widely used in deriving measures of effect for sun exposure, gamma radiation exposure, and many other things. 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, 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.
