The values of quantities in the acoustical domain, namely sound pressure and power, are expressed in terms of their levels, considered in relation to reference values. The sound pressure level, expressed in decibels, is given by the following expression:

niveis pressao_1

In this equation, the square of the effective value of the sound pressure, for a specific time interval t, defined by (t 2 – t 1 ), is given by:

niveis pressao_2

The sound pressure level is usually determined and presented by frequency bands with a width of one octave or one-third octave, identified respectively by their nominal center frequency. As far as the sound power level of a specific source or equipment is concerned, it is expressed in dB(A) and is given by the following expression:

niveis pressao_3

Where W represents the value of the sound power of the source or equipment.  The reference values of the previous expressions are, respectively: for sound pressure – P ref. = 2 x 10 -5 pascals; and for sound power – W ref. = 10-12  watts.