Download Chapter 24 Hearing and Noise

Hearing and Noise

Defining and understanding noise & its effects
 complex
problem
 not always intuitive
 critical for occupational health

Level of noise affects comfort, performance, and
long-term hearing
– 80 dBA  annoyance
 > 90 dbA  risk of hearing loss
 55
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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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Hearing Measurement

Audiograms test the air pathway and give total loss.
Normal Hearing

Losses can be temporary or permanent.

Temporary threshold shift, TTS


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Conduction Hearing Loss
Recovery after 14 hrs of exposure < 80dBA
Permanent threshold shift, PTS (or NIPTS)
TTS  PTS
Audiograms should be performed annually.
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Sound

Definitions
 Wavelength:
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
range
 Impulse sound: duration of <1 s
 Decibels measure level of sound pressure.
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Decibels

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
formula.


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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Calculating dB
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Noise Measurement

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
various frequencies.

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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
measurement.
Recall: when combining or subtracting noises,
use the power formula.
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Example
(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.

Performance
 Productivity
is 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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Speech Interference
Figure 21.8, pg. 422

Note: in loud environments (>85 dB), earplugs improve
speech transmission.
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Hearing





Hearing loss is a type of repetitive trauma
(‘cumulative strain’.)
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
loss.
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Noise Reduction

Office vs industrial environment



To reduce cumulative trauma:
1.
2.
3.
4.
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In offices, coworkers’ conversations are the main
source of noise.
Consider sound absorbers or masking noise.
Plan ahead
Modify the existing noise source
Modify the sound wave
Use personal protection
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Plan Ahead



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Substitute less noisy
processes.
Purchase less noisy
equipment.
Use quieter materials
and construction.
Separate people and
noisy equipment.
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.
POOR
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BETTER
from fig. 21.14, pg. 429
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Modify the Sound Wave


Confine the sound wave.
Absorb the sound wave.
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Fig. 21.20, pg. 432
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Use Personal Protection


Reduce exposure
duration.
Use earmuffs and
earplugs.
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