sound pressure level (SPL) : The loudness of an acoustic wave stated in dBindB that is proportional to the logarithm of its acoustic intensity. A sound wave progressing through air causes the instantaneous air pressure at any given point to vary above and below the barometric pressure in accordance with the waveform of the sound. This variation in pressure is used as a quantitative measure of the strength of the sound, and is called sound pressure. This is the quantity which a pressure microphone measures, and if it is expressed on a dBadB scale and referenced to a pressure of 20 Ápascals, it is called the sound pressure level. The amplitude dynamic range of human hearing goes from 0dB, or 10 -16 watt/sq.cm., the upper threshold of human hearing to 130dB or 10-4 watt/sq.cm., the threshold of pain, a factor of 1013 in range.