The sound pressure (p), Sound Pressure Level (SPL), is a form of energy that is caused by changes in air pressure and is subjectively perceived as loudness. Sound pressure is limited by the threshold of hearing and extends from it through 13 powers of ten to the threshold of pain.
Sound pressure is expressed in pascals or in newtons per square meter (N/m2) and, at 1 kHz, covers a sound pressure range from about '2*10^5` `N/m^2` to '2*10^-5` `N/m^2`, where 1 `N/m^2` corresponds to one pascal( Pa). In order to be able to represent this large dynamic range in a simplified way, the absolute sound pressure is given in logarithmic quantities as sound pressure level (SPL) in decibels, Decibel Sound Pressure Level ( dBSPL), which leads to the sound level (L). 0 dB SPL at 1,000 Hz corresponds to a sound level of `2*10^-5` `N/m^2`, which corresponds to the hearing threshold, and 120 dB corresponds to a sound level of `2*10^5` `N/m^2`. At this value, the pain threshold begins at higher frequencies. At lower frequencies it is about 130 dBSPL. One watt of acoustic power is equivalent to 107.5 dBSPL at one meter from an omnidirectional sound source.
A 10 dB increase in sound level corresponds to a doubling of the perceived loudness. If two sound sources with the same sound pressure are superimposed, the resulting sound pressure increases by 3 dB. This value of 3 dB also corresponds to the maximum sound pressure increase that two sound sources can achieve together. When adding sound levels, only the dB values are added.
The sound pressure decreases with the distance from the sound source to the quotient of the distance (1/distance). A doubling of the distance results in a sound pressure drop of -6 dB, which corresponds to the 50% value of the sound pressure of the sound source.
An important SPL value is the 94 dB SPL. This value is 94 dB stronger than the human hearing threshold at 1,000 Hertz and corresponds to the sound pressure of 1 Pascal. The 94 dB SPL value with a frequency of 1,000 hertz is used to test the output sensitivity of microphones.