The logarithmic compression of sound pressures leads to the development of the decibel scale. The decibel is a unit of sound pressure level. It is a logarithmic unit that is defined by 10 times the common logarithm of the squared ratio of sound pressure value of interest relative to a standardized reference sound pressure of 20 micro Pascal. In absence of any other reference value, sound pressure level is always referenced to 20 micro Pascal. Squaring the ratio of the pressures yields a relative energy ratio. For instance, if the squared ratio of P sub1 over P sub 0 is12, the sound energy of the pressure value of interest would be 12 times the energy associated with the reference pressure.