Sound power and acoustic properties of a room determine sound pressure and thus the perceived loudness in the room. Auditory impressions in the human ear are caused by sound pressure p which affects the eardrum. The threshold of hearing is located around 2 · 10−4 μbar; the threshold of pain is located around 2 · 10+2 μbar. Due to the complex range of 6 decimal powers and also due to the physiological perception of loudness, the sound pressure level was introduced as a logarithmic scale. The logarithmic ratio of a noise source’s sound pressure p to the hearing threshold pressure p0 is expressed as the sound pressure level LP (often also referred to as sound level) in decibels (dB).
Sound pressure (level)
with the reference pressure (hearing threshold) of