Hearing and Noise
Defining and understanding noise & its effects
not always intuitive
critical for occupational health
Level of noise affects comfort, performance, and
55 – 80 dBA annoyance
> 90 dbA risk of hearing loss
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Anatomy of the Ear
Figure 21.1, pg. 414
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The Organ of Corti
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Audiograms test the air pathway and give total loss.
Normal Hearing Conduction Hearing Loss
Losses can be temporary or permanent.
Temporary threshold shift, TTS
Recovery after 14 hrs of exposure < 80dBA
Permanent threshold shift, PTS (or NIPTS)
Audiograms should be performed annually.
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length of sound wave = speed of sound / frequency
Frequency: rate of oscillation of the sound
Pure tone: one-frequency sound
White noise: distribution of sound through the audible
Impulse sound: duration of <1 s
Decibels measure level of sound pressure.
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Sound pressure and sound power are
analogous to temperature and heat.
Sound pressure level:
SPL = 20 log10 (P / P0)
Power watt level:
PWL, dB = 10 log10 (W / W0) = 10 log W + 120
When combining or subtracting noises, use the power
Doubling of power results in 3 dB increase in noise level.
Mean minimum level of hearing for the
unimpaired-hearing population is 4 dB.
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Noise at different
frequencies have different
perceived loudnesses for
the same pressure level.
Phon is the unit of loudness
(see fig. 21.6, pg.419.)
Sone is the unit of loudness
for pure tones.
Sound-level meters provide
one number, combining
Octave band analyzers
provide detailed information.
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Determining Machine Noise
Measure noise level with machine running.
Measure noise level with machine off.
Calculate the difference. If <3 dB, the
background noise is too high for accurate
Recall: when combining or subtracting noises,
use the power formula.
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(Refer to table 21.1, pg. 418)
Assume the vacuum cleaner and the disposal
are being operated at the same time in a
kitchen. What is the total sound level in dBA?
PWL = 10 log W + 120
PWLA = _________ WA = _________________
PWLB = _________ WB = _________________
PWLcombined = _________________________
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Effects of Noise
Comfort and Annoyance
Workers must increase concentration.
Noise reduction may be required even if costs are
high and benefits are small.
Community reaction to industrial noise is variable.
Productivityis probably unaffected by noise except
for high mental tasks.
Speech interference is measured by words missed.
To reduce speech interference, reduce noise or
improve the message, the speaker, the
transmission system, or the listener.
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Figure 21.8, pg. 422
Note: in loud environments (>85 dB), earplugs improve
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Hearing loss is a type of repetitive trauma
Hearing loss (over and above age-related)
begins with exposure to noises over 67 dB.
Factors include noise level, exposure, duration,
gender, age, and frequency.
Some researchers have developed predictive
models, but …
We cannot identify sensitive ears prior to hearing
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Office vs industrial environment
In offices, coworkers’ conversations are the main
source of noise.
Consider sound absorbers or masking noise.
To reduce cumulative trauma:
1. Plan ahead
2. Modify the existing noise source
3. Modify the sound wave
4. Use personal protection
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Substitute less noisy
Purchase less noisy
Use quieter materials
Separate people and
Fig. 21.11, pg. 427
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Modify the Noise Source
Reduce driving force.
Change the direction of the noise.
Minimize velocity and turbulence of air.
from fig. 21.14, pg. 429 16
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Modify the Sound Wave
Confine the sound wave.
Absorb the sound wave.
Fig. 21.20, pg. 432 17
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Use Personal Protection
Use earmuffs and
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