WSHP-PRC004-EN44
Performance
Data Sound
What is Sound Power?
Sound power is a measure of the
acoustical energy emitted from a sound
source, and is an absolute value. As
discussed above, our hearing does not
perceive sound power directly, as there
is always some environmental medium
between the source and the listener.
However, from the standpoint of a
building designer, sound power is often
the preferred means of quantifying the
noise of a given unit because it is a
certifiable quantity. Using predictions are
used to tailor the design to the sound
pressure level requirements of the
building. The environmental effects that
must be taken into account when
converting sound power to sound
pressure for a specific location can be
lumped together and called the Transfer
Function.
Sound power should always be used
when making unit to unit sound
comparisons because it is a certifiable
absolute measure of the sound energy
produced by the unit. In contrast, sound
pressure is not certifiable because it is
difficult to ensure that each
manufacturer tests in precisely the same
environment. As in the case of sound
pressure, it is useful to express sound
power on a logarithmic scale.
What are NC and dBA?
Both NC and dBA are single number
descriptors used to represent perceived
loudness. Both scales take into account
the fact that people are more sensitive to
high frequencies than they are to low
frequencies
(see Figure PD-2 for
frequency range)
.
Noise Criterion (NC) is widely used to
quantify indoor sound. The NC level is
determined by the strength of sound
pressure across the 63 Hz to 8000 Hz
frequency range.
“A” weighted sound (dBA) is a single
number descriptor often used to define
sound in outdoor environments. For
example, local sound ordinances
typically regulate dBA levels at property
lines. Hearing-related safety standards
written by such bodies as the
Occupational Safety and Health
Organization (OSHA) also commonly
refer to A-weighted sound readings.
As a rule, “A” weighting is applied to
octave-band sound pressure data.
Applying “A” weighting sound power is
only appropriate in accordance with ARI
270. While NC and dBA are the most
popular, other single number descriptors
for sound are available including Room
Criterion (RC), NCB and other
weightings.
Figure PD-2 – Octave Band Center of
Frequency
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