Trane Axiom GEH006, Руководство пользователя

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WSHP-PRC001-EN 103
As an example, of the effect of environment, consider the sound produced by a drum. Clearly, the drum is perceived to be
louder in a bathroom with hard tile walls than it is in the middle of a football field. Also, the farther one moves away
from the drum, the quieter it sounds. In each case, the vibration of the drum (the strength of the source) is the same; the
perceived differences in noise level are due to the environment.
Because of the extremely wide range of sound pressure perceivable by a person-typically five or six order of magnitude-it
is convenient to express sound pressure on a logarithmic scale. As a result, adding two equal sound sources together will
result in an overall increase of 3 dB. However, 3 dB is barely a perceptible increase in sound. It takes an increase of 10 dB
to be perceived as twice as loud.
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 mea-
sure 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 sensi-
tive to high frequencies than they are to low frequencies
.
See
Figure 52
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 ordi-
nances 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.
Performance Data
Sound
Figure 52: Octave band center of frequency