Transcript Noise Review

What is Noise?
 NOISE
is pressure change above and
below ambient pressure, occurring at rates
between approximately 20 and 20,000
cycles per second, Hertz (Hz)
 NOISE
and SOUND are physically the
same thing, with the term noise usually
implying absence of information and/or
undesirability
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Representation of Pressure Waves
Dr. Dan Russell, http://www.gmi.edu/~drussell/Demos/rad2/mdq.html
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Frequency, Amplitude, and Wavelength of a
Sound Wave
SOUND WAVE
AMPLITUDE
Peak
RMS
0.5
0.0
Atmospheric Pressure
-0.5
-1.0
C
l=
f
DISTANCE (one wavelength)
velocity
C=f•
344 m/sec @ 72 F
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Acoustic Quantities
 Pressure,
P (P2  energy, power)
Pressure amplitude measured in N/m 2 , Pascals
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(48 Pascals = 1 lb/ft )
atmospheric pressure  10 Pascals
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faintest audible sound  2 10 Pascals
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loudest tolerable sound  28-30 Pascals
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Decibels – a Useful Transformation
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Sound Pressure Level (SPL)
Use of deciBels
 deciBels,
dB, is a useful transformation
because it permits compressing one unit
that may cover a huge range into a smaller
numerical range
 note that a few dB is a large change in the
original unit
 useful for sound Intensity, Power, Pressure
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SPL vs. Sound Pressure
Sound pressure level (dB)
Pneumatic chipper (at 5 ft)
Textile loom
Newspaper press
Diesel truck 40 mph (at 50 ft)
Sound pressure (Pa)
120
110
100
90
80
70
Passenger car 50 mph (at 50 ft)
Conversation (at 3 ft)
Copy machine (at 2 m)
Quiet room
Air conditioning in auditorium
Quiet natural area with no wind
Anechoic chamber
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50
40
20
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Rock band
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1
0.5
Power lawnmower (at operator’s ear)
0.2
0.1
0.05
Garbage disposal (at 3 ft)
Milling machine (at 4 ft)
Vacuum cleaner
0.02 Air conditioning window unit (at 25 ft)
0.01
0.005
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0.002 Suburban area at night
0.001
0.0005
0.0002
0.0001
0.00005
0
0.00002
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deciBel addition (by table)
difference in dB:
0
1
2
3
4
5
6
7
8
9
10
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add to higher:
3.0
2.6
2.2
1.8
1.4
1.2
1.0
0.8
.6
.5
.4
.3
.2
.1
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deciBel addition (by table)
 it
is customary to rank order the dB values
to be added from largest to smallest
 for the largest two, find the difference, enter
the table in col.1, find value in col.2, and
add to largest of the pair being added
 add the result of the first pair addition to the
third value, get a new total
 add the new total to the 4th largest value,
get new total,etc.
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deciBel Addition (by table)
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Frequency Spectrum & Octaves
 acoustic
energy covers a range of frequencies,
and in varying intensity
 customary to divide the frequency spectrum
into octaves, half-octaves, or third-octaves
for measurment and hearing testing
 An octave is a range such that the top
frequency is twice the bottom frequency
 octaves are identified by center frequencies:
31.5, 63, 125, 250, 500, 1k, 2k, 4k, 8k, 16k Hz
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Loudness and Weighting Scales
 the
ear does not hear all frequencies with
equal response
 for equal energy the low frequencies do not
sound as loud, generally
 numerical measures at various overall noise
levels of the apparent loudness relative to
that at 1000 Hz are Weighting Scales
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Weighting Scales
 the
weighting scale for overall sound-level
of approx. 55 dB is the A-weighting
Frequency
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Weighting Scales
 because
the A-weighting was thought to
approximate the ear’s sensitivity, and:
 because A-weighted noise measurements fit
the hearing-loss data of the 1950’s and
1960’s reasonably well:
 ANSI, ACGIH, and subsequently OSHA all
specified that SPL’s should be measured Aweighted (and slow response), dBA
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Sound Measurement Equipment
 noise
(sound pressure level) meters
 dosimeters
 octave band analyzers
 sound intensity meters
 real time or spectrum analyzers
 impact meters
 vibration meters
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Noise Surveys
 Source
measurements
 Surveys
– Area measurements
– Workstation measurements
– Personal Dosimetry
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OSHA Noise Rules
time allowed
8
4
2
1
0.5
0.25
0.125
sound level, dBA
90
5 dB
95
100
105
110
115
120
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OSHA Noise Rules (continued)
 the
OSHA criterion of 90 dBA for 8 hours
was thought to prevent most hearing loss
 the 5 dB exchange rate, i.e. time is cut in
half if SPL increases 5 dB, was a
simplification of more complex data, and
assumes that the noise experienced is
interrupted several times per day
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Noise Dose
 each
line in the OSHA table represents
ALL the allowed noise above 90 dB for a
whole 8-hour day, i.e. 100% of the allowed
noise dose
 if people experience varying levels, dose is
calculated as:
 C1 C 2
Cn 

%Dose = 100

  
Tn 
 T1 T2
where C i = time experienced at a given level
and Ti = time allowed at the given level
Threshold Limit Value(R) - Noise
Sound level TLV time allowed
85 dBA
8 hours
88
4
91
2
94
1
97
0.5
100
0.25
103
0.125
(OSHA)
16 hrs.
10.6
7
4.6
3
2
1.3
Threshold Limit Value - Noise
 note
that the TLV not only assigns the 8-hour
allowed level to 85 dBA, but that the
exchange rate is 3 dB, i.e. time is halved if
the level goes up 3 dB
 this means that a given noise exposure
scenario will have a higher dose than under
OSHA rules, and that the calculated Leq will
be different
OSHA Hearing Conservation
 Initial
monitoring to find SPL in area
 if noise is above 85 dBA, hearing
conservation is required
 re-monitor if changes occur
 notify employees
 audiometric testing
 STS (Standard Threshold Shift)
– – 2k, 3k, 4k, avg. in either ear > 10 dB, compared
to an earlier audiogram
 Hearing
protection
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Some Important Hearing
Conservation Terms
 presbycusis
- hearing loss due to aging
 TTS - Temporary Threshold Shift (it is
generally thought that if TTS is avoided,
then PTS will not occur)
 PTS - Permanent Threshold Shift
 conductive hearing loss - loss due to
mechanical sound/vibration conduction
defect, usually in outer or middle ear
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Some Important Hearing
Conservation Terms (continued)
 sensorineural
hearing loss - primarily loss
due to damage to the neuro-mechanical
transducer system in the ear, the hair cells
in the cochlea
 Hearing Conservation Rules (OSHA) - the
main elements are: monitoring, audiometric
testing, hearing protection, training, and
record-keeping
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Conductive vs. Sensorineural
Hearing Loss
Conductive Loss
Sensorineural Loss
http://www.utdallas.edu/~thib/rehabinfo/tohl.htm
Some Important Hearing
Conservation Terms (continued)
 Hearing
Conservation Rules (OSHA) apply for persons exposed 85 dBA avg., or
dose 50%
– -Annual audiograms
– -STS - Standard Threshold Shift - an average
of 10 dB averaged at 2K, 3K, and 4K
compared to an earlier audiogram, in either ear
– -Hearing protection training and availability
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Noise Control Steps
 isolate
sources with enclosures
 modify path with barriers, absorption
 reduce solid-borne transmission
– flexible mounts, hoses, couplings on shafts
 substitute,
e.g. belt drives for gears, newer
quieter equipment for older
 receptor controls: PPE, and/or booths
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