What Is Noise?
Noise is unwanted sound which may be
hazardous to health, interfere with speech and
verbal communications or is otherwise
disturbing, irritating or annoying.
What Is Sound?
Sound is defined as any pressure variation
in air, water or other fluid medium which may
be detected by the human ear.
What Are The
Characteristics
Of Sound?
The two most important characteristics
which must be known in order to evaluate the
sound or noise are it's amplitude and frequency.
The amplitude or height of the sound wave
from peak to valley determines the loudness or
intensity. The wave length determines the
frequency, pitch or tone of the sound.
How Are These Characteristics Expressed?
The frequency of sound is expressed in
wavelengths per second or cycles per second
(CPS). It is more commonly referred to as
Hertz. Low frequency noise is 250 Hertz (Hz)
and below. High frequency noise is 2000 Hz
and above. Mid-frequency noise falls between
250 and 2000 Hz.
The amplitude of sound is expressed in
decibels (dB). This is a logarithmic
compressed scale dealing in powers of 10
where small increments in dB correspond to
large changes in acoustic energy.
What Are Octave Bands?
Standardized octave bands are groups
of frequencies named by the center
frequency where the upper limit
is always twice the lower limit of
the range. Test data for performance
of acoustical materials is
standardized for easy comparison
at the center frequencies.
Equipment noise levels and
measurement devices (dB meters) also
follow the preferred octave bands.
What Is The Difference Between
dB And dBA?
dB sound pressure levels are
unweighted. dBA levels are "A"
weighted according to the weighting
curves shown below to approximate
the way the human ear hears. For
example, a 100 dB level at 100 Hz will
be perceived to have a loudness
equal to only 80 dB at
1000 Hz. Other weighting
scales (C and B) are also
shown. The dBA scale is based
on a child's hearing and was
originally documented based
on actual hearing tests to
characterize the human ear's
relative response to noise.
Is Hearing Loss Permanent?
Yes! Permanent hearing loss occurs
when the tiny hair cells in the cochlea
(inner ear) are damaged or destroyed.
A healthy cochlea contains approximately
40 thousand hair cells which
are necessary to transmit sound vibrations
to the brain. Exposure to excessive
noise levels will damage the hair
cells resulting in permanent, irreversible
hearing loss.
The tables at left
show the additive effect
for adding equal and
unequal decibel levels.
Unless the two levels
differ by 10 or more dB
there will always be
some increase to the
higher level. Frequency
levels can also be
added together in a
similar fashion to get
overall dB levels.
For adding several decibel levels
of the same value: |
No. of Equal Levels | Add the Following
Amount to that Level
to Get the Sum |
2 | 3.0 dB |
3 | 4.8 dB |
4 | 6.0 dB |
5 | 7.0 dB |
6 | 7.8 dB |
7 | 8.4 dB |
8 | 9.0 dB |
9 | 9.5 dB |
10 | 10.0 dB |
N | 10 log N dB |
At the final total, round off to the
nearest whole number. |
|
For adding any two decibel levels to an
accuracy of about 1 dB: |
When Two Decibel Values Differ By | Add the Following
Amount to the
Higher Value |
0 or 1 dB | 3 dB |
2 or 3 dB | 2 dB |
4 to 9 dB | 1 dB |
10 dB or more 0 dB |
When adding several levels, start
with lowest levels first; continue
two at a time until only one final
value remains. |
|
Is A 5 dB Change Significant?
Sound Level
Change | Acoustic
Energy Loss | Relative
Loudness |
0 dB | 0 | Reference |
-3 dB | 50% | Perceptible Change |
-10 dB | 90% | Half as Loud |
-20 dB | 99% | 1/4 as Loud |
-30 dB | 99.9% | 1/8 as Loud |
-40 dB | 99.99% | 1/16 as Loud |
Yes! The pressure associated with the loudest
known sound is more than one billion
times that associated with the faintest sound.
Such a large range is unmanageable for
measurement purposes. Using a logarithmic
scale compresses the range to between 0 and
200 dB. At right, various sound level changes
are referenced to relative loudness and
acoustic energy loss. A 5 dB change is more
than a 50% change in acoustic energy!!!
Is Sound Power The Same
As Sound Pressure?
No! While both sound power levels (Lw)
and sound pressure levels (Lp) are both
expressed in decibels, the referenced standards
for each are different. More importantly,
the sound power level is the total
acoustic energy output of a noise source
independent of environment. Sound pressure
levels are dependent on environmental
factors such as the distance from the source,
the presence of reflective surfaces and other
characteristics of the room/building/area
hosting the source. Actual sound pressure
levels will always be higher than sound
power levels.
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