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4-4-6. NOISE
UNIT OF NOISE : DB (DECIBEL)
Sound pressure level with a weight corresponding to the
“A” scale in the noise level meter is referred to the noise
level. The unit is internationally expressed as “dB (A).” In
Japan, JIS regulations stipulate that “dB” or “phon” should
be used. We use “dB” in this catalog.
ACCEPTABLE NOISE LEVEL
Recommended noise levels for indoor regular noises
Facility
Applications
Noise Level
(dB)
Facility
Applications
Noise Level
(dB)
Concert Hall
21-30
Bed Room,
Hospital, Hotel
34-47
Broadcasting
Studio
21-30
Small Theater,
Conference Room 42 or Less
Large Theater,
Church
30 or Less
Factory,
Workshop
66-80
Class Room,
Library
38-47
Lobby,
Laboratory
47-56
Living Room,
Drawing Room
38-47
Restaurant,
Large Office
42-52
(Source: Handbook of Noise and Vibration Control)
What is NC curve?
This is a curve to indicate frequency-based acceptable levels,
proposed and utilized for noise evaluations in the U.S.
How to interpret NC curve?
The band level is determined by applying octave analyses
to the target noise, record the results in the NC curve to
determine the maximum NC curve in each band, which shall
be determined as NC value for the target noise.
PRECAUTIONS CONCERNING
NOISE LEVELS
The noise values (dB) of the product indications are
measured at regulated distances in anechoic chambers
as specified in JIS, etc.
The following points should be considered in selecting
proper products.
(1) Changeable due to environmental conditions
The measured values were determined in anechoic
chambers. In real settings, echoes will take place
from walls, floors, ceilings, etc. The measurements
may also increase due to the materials used and the
surrounding space. It is thus necessary to consider
echo coefficients. Noise levels may also be increased
by piping and placement of components.
(2) Propagation of vibration
Although products are designed to minimize vibration,
certain levels of vibration are inevitable, as rotating
mechanisms are contained. It is thus necessary to pay
close attention to the installation methods appropriate
to individual products.
(3) Sound synthesis
When multiple units are to be installed in the same
room, it is necessary to consider sound syntheses.
Particularly when multiple units of the same model are
installed next to each other, “humming” sound may
arise due to slightly different rotations.
Distance decay for sound
When the sound source is sufficiently small in
comparison with distance, or in the case of a point sound
source, by calculating the sound level “r
0
” (m) at a short
distance, the sound level L
0
(dB) at a long distance, “r” (m)
is determined as “L = L
0
- 20log (r/r
0
).”
Sound synthesis
The sound level may be expressed as, y = 10log x.
Thus, when L
1
and L
2
noises are combined, the synthesized
sound level (dB) is “L = 10log (10
L1/10
+ 10
L2/10
).”
