Chapter 24 Hearing and Noise
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Transcript Chapter 24 Hearing and Noise
Chapter 24: 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
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Anatomy of the Ear
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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
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 = 10 log10 (W / W0)
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. 24.6, pg.465.)
Sone is the unit of loudness
for pure tones.
Sound-level meters provide
one number, combining
various frequencies.
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 24.1, pg. 464)
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 24.11, pg. 469
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
In offices, coworkers’ conversations are the main
source of noise.
Consider sound absorbers or masking noise.
To reduce cumulative trauma:
1.
2.
3.
4.
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Plan ahead
Modify the existing noise source
Modify the sound wave
Use personal protection
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Plan Ahead
Substitute less noisy
processes.
Purchase less noisy
equipment.
Use quieter materials
and construction.
Separate people and
noisy equipment.
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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
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Modify the Sound Wave
Confine the sound wave.
Absorb the sound wave.
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Use Personal Protection
Reduce exposure
duration.
Use earmuffs and
earplugs.
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