Dynamic microphones produce a voltage proportional to the sound pressure (measured in Pascals (PA)). Sound pressure level (SPPL) is the logarithmic ratio of the sound pressure  to a reference pressure (voltage in the microphone) known as the threshold of hearing.  SPL is measured in decibels (dB). A doubling (X2) of sound pressure leads to a 6dB increase in SPL, a ten fold (X10)  increase causes a 10dB increase in SPL while a X100 increase in sound pressure causes a 20dB increase in SPL because it is a logarithmic scale.