Brüel & Kjær 2240 Guide De L'utilisateur page 45

sound:
sound pressure level:
Any pressure variation that the human ear can detect. Just
like dominoes, a wave motion is set off when an element
sets the nearest particle of air into motion. This motion
gradually spreads to adjacent air particles further away
from the source. Depending on the medium, sound extends
and affects a greater area (propagates) at different speeds.
In air, sound propagates at a speed of approximately
340 m/s. In liquids and solids, the propagation velocity is
greater – 1500 m/s in water and 5000 m/s in steel.
The level in decibels of the pressure variation of a sound.
See also decibel.
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