The quantity we come across most frequently is dB_{spl} (spl stands for sound pressure level) which is defined as follows:

Equation 3.2. dB_{spl}

where P_{0} is the reference value for atmospheric pressure and is commonly identified at 0.00002 Pa= μPa (pressure is measured in Pascals). This value is considered to be the sound pressure level above which the human ear begins to perceive sound.

Let's take an example. The value 20 μPa is a reference value for sound pressure in an environment where there is absence of acoustic waves.

This means that a 20 μPa sound pressure level doesn't exert any sound pressure that the human ear can perceive. Vice versa, a sound pressure level of 10Pa generates a certain amount of dB_{spl} which gives the following result:

Equation 3.3. Calculation of a sound pressure

Reading the formula from a different perspective we could say that a sound pressure level of 114 dB_{spl} corresponds to an acoustic wave that generates a pressure level of 10Pa. The following figure illustrates some typical sounds and their intensity measured in dB_{spl}:

Typical dB

_{spl}values