Basic acoustics

Definition of sound

Sound is defined as a mechanical compression and rarefaction or a longitudinal displacement wave that propagates through a medium, solid, liquid or gas. The speed of this propagation depends on the type and temperature of the medium. It does not depend the pressure of the medium. In dry air at 20° C the speed of sound is approximately 343 m/s.

Some basic formulas

Relation sound power and sound pressure

Lw = Lp +10 log A
Sound power (dB) =
sound pressure (dB) + 10 log (surface in m2)

A-weighting

Very often the dB(A) is used, the A is a correction(filter) based on the human ear.

Hz	31,5	63	125	250	500	1000	2000	4000	8000
dB	-40	-26	-16	-9	-3	0	+1	+1	-1