Sound pressure decreases with increasing distance from the sound source. Furthermore, sound pressure can be reflected and absorbed as well. In this respect, sound pressure is not a definite parameter for a sound source.
Sound power level
A sound source's "acoustic" power is characterised by its sound power level LW which does not factor in room absorption. It is derived using the measured or perceived sound pressure level and the room’s sound absorption.
The sound power level LW is defined as the logarithmic ratio of sound power P and reference sound power
P0.
The connection between sound power level and sound pressure level is expressed using the following equations, which apply to diffuse sound fields:
or
weighted according to hearing sensitivity (see above).
.
LWA |
sound power level in dB(A) |
LPA |
sound pressure level in dB(A) |
A |
equivalent sound absorption area in m2 |
V |
room volume in m3 |
T |
reverberation time in s, for guidance values see table |
Table 3.154: Guidance values for A-weighted sound pressure level and reverberation time
The comfort level of a room is strongly influenced by the relationship between sound sources and acoustic interior, e.g. size and shape as well as sound absorption of floors, walls, ceiling and objects within the room. A room with short reverberation times (containing many or large-format surfaces with strong sound absorption) does not only seem quieter, people will also speak more softly while in the room. This is why minimum reverberation times are the objective for work rooms, and sound-absorbing surfaces, e.g. acoustic ceilings, are often built in.