A weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome.
The four curves are applied to the measured sound level, 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) above the threshold of hearing (see A-weighting).
[2] This method, formerly known as CCIR weighting, is preferred by the telecommunications industry, broadcasters, and some equipment manufacturers as it reflects more accurately the audibility of pops and short bursts of random noise as opposed to pure tones.
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.
This ensures compatibility with black-and-white receivers, and also benefits noise performance and allows separation into meaningful luminance and chrominance signals for transmission.