Noises are irregular and disordered vibrations including all possible frequencies. The intensity of a sound I can be expressed in decibels with the standard equation:
where I0 is the quietest sound most people can hear, the threshold of hearing 10-12Wm-2. When the ratio is between two field strength (so that the power being transmitted is proportional to the square of the pressure, the formula is:
The decibel unit is often used in acoustics to quantify sound levels relative to some 0 dB reference. The reference may be defined as a sound pressure level (SPL), commonly 20 micropascals. The reference sound pressure (corresponding to a sound pressure level of 0 dB) can also be defined as the sound pressure at the threshold of human hearing, which is conventionally taken to be 20 micropascals. It can therefore be seen that a ratio expressed in decibels. A-weighted decibels are abbreviated dB(A) . When acoustic measurements are being referred to, then the units used will be dB SPL (sound pressure level) referenced to 20 micropascals = 0 dB SPL.
Canarina CUSTIC · noise dosimeters and modeling · export · introduction 5
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