Occupational Noise Exposure ITEC 471 Spring 200

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Occupational Noise ExposureITEC 471Spring 2003
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Noise-Induced Hearing LossStatistics
According to the Bureau of Labor and Statistics
for 1999
There were about 372,000 newly reported cases of
occupational illnesses in private industry in
1999. Manufacturing accounts for three-fifths of
these cases. Disorders associated with repeated
trauma, such as carpal tunnel syndrome and
noise-induced hearing loss accounted for 4
percent of the 5.7 million total workplace
injuries and illnesses.
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The Physics of Sound

• Human ear detects variation in pressure
• Can be in any medium

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Sound Definition

• Energy that is able to be perceived by human ear
• Frequency range 20 Hz 20 kHz.
• Difference between sound and vibration.

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Noise Definition

• Unwanted sound
• Includes annoyance, not being able to hear
  wanted sounds.
• Noise is in the ear of the beholder.

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The curves shown are pictorial representations of
sound waves. Pitch is related to frequency and
loudness is related to the intensity of a sound.
Curve B represents a sound that has a higher
frequency - a higher perceived pitch - than the
sound represented by curve A because the
variations in air pressure, as represented by a
point on the curve, cross the axis more often.
The intensity of a sound can be shown by the
height of the curve. Curve C represents a sound
that has a greater intensity - a greater
perceived loudness - than the sound represented
by curve A.
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Terms

• Spectrum
• Broadband versus single frequencies, a.k.a.
  narrow band
• High frequency versus low frequency

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Sound Pressure

• Measuring the average of compressions and
  rarefactions zero
• Use root mean square or RMS
• Micropascal, newton per square meter, microbar,
  dyne per square centimeter
• Conversions 1 Pa 1 n/m2 10 microbar 10
  dyne/cm2

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Decibel

• Derivation minimum difference in loudness
  perceptible to human ear
• Decibel is proportional to sound power (amount of
  energy per unit time from a source as an acoustic
  wave)
• Unit of power watt
• Compare to reference amount (10-12 watt)

Lw 10 log W / Wo Where W sound power
(watts) Wo reference power (10-12 watts)
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Decibel

• The problem with sound power cant be measured
  directly
• Sound pressure is proportional to square of
  intensity

• Threshold of hearing 20 microPascal (reference
  tone _at_ 1000 Hertz
• Range of human hearing is HUGE - need a relative
  scale

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Decibel

• Equation for sound pressure level (in decibels)
• Lp 20 log p2 / po2 20 log p / po
• Where p rms sound pressure
• po reference rms sound pressure (20
  micropascal)

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Sound Pressure and SPL
Sound Pressure (?Pa)
Sound Pressure Level, dB
Example
20
0
Threshold of Hearing
20
Studio for sound pictures
200
2,000
40
Quiet office, audiometric booth
60
20,000
Conversational speech (3ft)
80
Very noisy restaurant
200,000
100
Looms in textile mill
2,000,000
20,000,000
110
Woodworking
120
200,000,000
Hydraulic press
140
Threshold of pain, Jet plane
2,000,000,000
180
Rocket-launching pad
20,000,000,000
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Anatomy Physiology of the Ear
Auricle(pinna)
Outer ear

• Hairs filter out particulate matter
• Collection of sound waves transmit to eardrum

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Anatomy Physiology of the Ear
Middle Ear

• Ossicles Tiny bones (malleus, incus, stapes)
• Primary function Transfer sound energy from
  outer to inner ear
• Amplification effect between eardrum and oval
  window

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Diseases of the Ear

• Out ear Sunburn and frostbite
• Middle ear Suppurative otitis media
• Inner ear Presbycusis (definition) diseases
  of the vestibular system- Menieres Disease
• Tinnitus-definition relationship to
  high- frequency hearing loss

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Anatomy Physiology of the Ear

• Acoustic (sound)
• Mechanical (middle ear)
• Hydraulic (cochlea)
• Electric (nerves to brain)

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Types of Injuries

• Acoustic trauma
• Noise-induced hearing loss
• Risk Factors Intensity (sound pressure level)
  type of noise (frequency/pitch) exposure pattern
  (recovery) duration (years) of noise exposure

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Expected hearing threshold levels in an
industrialized society, for males not exposed to
workplace noise, as a function of age. Average
of right and left ears. Age corrections after
Spoor (1967) have been added to actual median
HTLs of the 20-year-old non-noise-exposed workers.
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Expected hearing threshold levels in an
industrialized society, for females not exposed
to workplace noise, as a function of age.
Average of right and left ears. Age corrections
after Spoor (1967) have been added to actual
median HTLs of the 20-year-old non-noise-exposed
workers.
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Age 50 - 59
Age 40 - 49
Incidence of hearing impairment in percent of
population
Age 30 - 39
Age 20 - 29
90
100
110
General population and non-noise
A - Scale level in dB
The incidence of hearing impairment in the
general population and in selected populations by
age group and by occupational noise exposure.
Data represent working lifetime levels.
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Mean estimated NIIPTS (dB)
A-weighted sound level (dBA)
Estimated industrial-noise-induced permanent
threshold shifts at various frequencies produced
by 10 years or more of exposure to noise at the
indicated A-weighted level, hours/day, 250
days/year. After Passchier-Vermeer (1968).
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Non-Occupational Exposures

• Car stereos
• Hunting
• Power tools, lawn mowers
• Must factor these in when considering
  occupational contribution
• Opinions of audiologists and physicians is key
• OSHA wants 14 hours of no high noise exposures
  prior to audiogram
• Get this documented!

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Hearing Measurement

• Audiometry generation of pure-tone frequency
  and intensity are varied
• Bone conduction Test of sensorineural system
• More on audiograms next week

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Remember hierarchy of control for control of
noise hazards.

• Engineering controls
• Administrative controls
• Personal Protective Equipment (PPE)

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Engineering Controls

• Enclosures
• Sound-attenuating materials
• Mufflers, baffles
• Frequency-specific data is vital

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Administrative Controls

• Standard Operating Procedures

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Hearing Protection Comes In Many Forms.
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Earplug Trivia

• Earplugs are a good choice when noise is
  predominantly low frequency (125-250Hz)
• Earmuffs are a better choice with mid-range noise
  (around 1kHz)

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Comparison of the attenuation properties of a
molded-type earplug and an earmuff protector.
Note that the earplug offers greater attenuation
of the lower frequencies, while the earmuff is
better at the higher frequencies.
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OSHA Noise Reduction Rating (NRR)

• When using a dosimeter that is capable of
  C-weighted measurements, subtract the NRR from
  the C-weighted TWA
• When using A-weighted measurements, Convert the
  A-weighted dose to TWA, then subtract 7 dB from
  the NRR
• OSHA compliance manual says to apply a safety
  factor of 50 to manufacturer (lab-based) NRRs
  (OSHA CPL 2-2.35A)

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Keys to an Effective Hearing Protection Program

• Offer many varieties of hearing protection and
  make them available.
• Make sure whats offered is best for the spectra
  of noise in your facility.
• Train employees on how to insert into ear and
  educate them on occupational hearing loss.
• Develop a method to establish compliance with
  hearing protection usage.
• Have all high noise areas clearly marked.
• Periodically evaluate whats on the market.

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Conducting Noise Surveys

• SLMs and dosimeters
• Calibrator
• Extra batteries
• Clipboard, survey forms, camera, film
• Calculator and reference tables
• HPDs, safety glasses, shoes, etc.
• Microphone windscreens

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Sound Level Meters

• Type 0 Laboratory Standard
• Type 1 Precision
• Type 2 General Purpose
• Type S Special Purpose
• For types 0, 1, and 2, weighting filters A, B,
  and C as well as the fast and slow time constants
  are required (ANSI)
• Type 1 and 2 instruments must measure a pulse of
  100 microseconds (or less)

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Dosimetry

• Best way to evaluate dosages
• Spot checks with SLM will identify high noise
  areas but you must account for worker mobility
• Statistics Is one dosimetry survey accurately
  characterizing worker exposures on a weekly,
  monthly or annual basis?
• Can estimate dosages in cases where you cant
  measure 8-hours
• Worst case extrapolate noise to entire shift
• Best case assume zero exposure for rest of shift

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Weighting Filters

• A, B and C filters originally based on
  Fletcher-Munson curves - characterizing equal
  loudness perception for pure tones of variable
  frequency (not on hearing loss)
• A-weighting curve is an approximation of equal
  loudness perception characteristics of human
  hearing for pure tones relative to a reference of
  40dB SPL at 1 kHz
• A-weighting (coincidentally) provides best
  estimate of potential noise-induced hearing loss

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Fletcher-Munson curves with weighting filter
response overlay.
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Frequency-response attenuation
Frequency-response attenuation characteristics
for the A, B and C-weighting networks.
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Octave Bands

• Used as an analysis tool for engineering control
  of sound, in testing hearing, and for selection
  of HPDs, among other things
• Center is geometric mean of upper and lower
  limits (e.g., center of 44 - 88Hz band is the
  square root of 44 x 88)

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Octave-Band Mean Frequencies and Corresponding
Band Limits ANSI S1.11 - 1986
Geometric Mean Frequency of Band (Hz)
Lower Band Limit (Hz)
Upper Band Limit (Hz)
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31.5
44
44
63
88
88
125
177
177
250
354
354
500
707
707
1,000
1,414
1,414
2,000
2,828
2,828
4,000
5,656
5,656
8,000
11,312
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What does OSHA say?

• 29 CFR 1910.95
• gt85 dB-A TWA means employees must be on Hearing
  Conservation Program
• Controversy Are OSHA standards protective
  enough?
• 90 dB-A TWA is PEL

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Noise Formulas Sample Problems
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Adding Decibels (by schedule)
Difference in Decibel Values
Add to Higher Value
0 or 1dB
3dB
2 or 3dB
2dB
4 to 9dB
1dB
10 or more
0dB
Sample Problem
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Adding Decibels

• Decibels are logarithmic values and cannot be
  added or subtracted algebraically

Sample Problem
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Finding Noise Dose
D 100 (C1/T1 C2/T2 ...Cn/Tn)
where D dose in percent allowable C noise
exposure duration T allowable duration for a
specified level in dBA
Sample Problem
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Finding Allowable Exp. Duration (T)
Table G-16 is based on the relationship
T 8/ 2 (L-90)/5
where T allowable time, in hours, at the
level (L) L the A-weighted sound level(dBA)
Sample Problem
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Finding a TWA from a Noise Dose
TWA 90 16.61 log (D/100)
where TWA 8-hour time-weighted average
noise exposure D noise dose in percent
Sample Problem
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Finding Average Noise Levels(similar to TWA
except t ? 8 hours)
LA 90 16.61 log (D/12.5 t)
where LA average noise exposure over time t
(hours)
t noise exposure duration (hours)
(unrelated to table G-16)
D noise dose
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An employee has an OSHA dose of 164 over a
10-hour shift. What is the average sound
pressure level (dBA)?
LA 90 16.61 log D/(12.5)( t)
90 16.61 log 164/(12.5)(10)
90 16.61 log 1.312
90 1.96
92dBA
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Finding T (time allowed at a given noise level)

• Use table G-16 or G-16A in 1910.95 Appendix A
• Or use formula

T 8 / 2 (L-90) / 5
Where L is the measured A-weighted sound level
and T is in hours
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OSHA Noise Dose (Appendix A)

• A dosimeter will typically provide dose
• When using a sound level meter, dose must be
  calculated

Dose (C1/T1 C2/T2 C3/T3 Cn/Tn) x 100
Cn exposure time at a given level Tn time
allowed at that noise level

• Noise under 90dBA is not counted for hearing
  protection (29CFR1910.95(b)(1)
• For hearing conservation, noise under 80dBA is
  not counted (29CFR1910.95(c)

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Finding OSHA 8-Hour TWAs

• Use table A-1 in 1910.95 Appendix A
• Or use formula

TWA 16.61 log (D/100) 90 where TWA
8-hour time-weighted average sound level and D
accumulated dose in percent exposure.