20.3.2
Intensity and sound pressure level
Sound power cannot be measured directly but must be calculated from
pressure measurements. If an imaginary sphere is placed around a
sound source (with the source at the center of the sphere), all the
energy from the source must pass through the sphere. Power flow
through a unit area of the sphere is the
intensity,
expressed in watts
per unit area. Intensity varies inversely as the square of the distance
from the source. The intensity and the sound pressure level are nearly
identical numerically if proper units are used. The ASHRAE Hand-
book uses watts per square meter and micropascals (
Pa). The mea-
suring system converts the pressure to decibels, corresponding to the
sound power levels of Table 20.1.
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