Sounds in excess of 120 dB may cause immediate irreversible hearing impairment, besides being quite painful for most individuals. The sound pressure level of audible sounds ranges from 0 dB through 120 dB. The unit used to express the sound pressure level is the decibel, abbreviated dB. Where log stands for the logarithm to the base 10 (ordinary logarithm). If we call Pref the sound pressure of a just audible sound and P the sound presure, then we can define the sound pressure level ( SPL) Lp as The fact that the ratio of the sound pressure of the loudest sound (before the sensation of sound is changed into pain) to the sound pressure of the lowest one is about 1,000,000 has led to the adoption of a compressed scale called a logarithmic scale. In order to reduce the amount of digits, frequencies above 1,000 Hz are usually expressed in kilohertz, abbreviated kHz. This rate is called frequency and is expressed in Hertz (abbreviated Hz), a unit equivalent to a cycle per second. Another important difference is that the atmospheric pressure changes very slowly, whereas sound pressure is rapidly changing, alternating between positive and negative values, at a rate of between 20 and 20,000 times per second.
It is not the magnitude the only difference between atmospheric pressure and sound pressure. This is much the same as the case of some gentle ripples on the surface a swimming pool. For instance, unbearably loud sounds may be around 20 Pa, and just audible ones may be around 20 m Pa ( m Pa stands for micropascal, i.e., a unit one million times smaller than the pascal). However, sound pressure has usually a value much smaller than the one corresponding to the atmospheric pressure. Then we can define sound pressure as the difference between the actual instantaneous pressure due to sound and the atmospheric pressure, and, of course, it is also measured in Pa.
This pressure amounts to roughly 100,000 Pa (the standard value is 101,325 Pa). It is measured in a SI (Système International, i.e., International System) unit called Pascal (1 Pascal is equal to a force of 1 Newton acting on a surface of 1 square meter and is abbreviated 1 Pa). First we have the atmospheric pressure, i.e., the environmental air pressure in absence of sound.
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