Realisation of the unit of sound pressure
The Pascal is the unit of sound pressure and is realised at BEV by means of the reciprocity measuring method, which is a primary measuring method, in which the transfer factor is determined by comparing 3 microphones in pairs at different frequencies. The relation between electric voltage and sound pressure is called the transfer factor. Depending on the use of the microphone either in a pressure chamber, in free field or in diffuse field we differentiate between a pressure-, a free field- and a diffuse field transfer factor of the microphones.
In practice, sound pressure is indicated in proportion to the reference sound pressure of 20 µPa. The sound pressure level in decibel [dB] is the twentyfold decimal logarithm of the ratio between the effective value of sound pressure and the reference sound pressure, basically corresponding to the human hearing threshold.
Sound level meters are used for the measurement of sound pressure level. They are calibrated with sound calibrators.
The unit of sound pressure is disseminated by sound calibrators or other measuring microphones compared with the standard microphone.