# Hearing and Noise by MikeJenny

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• pg 1
```									    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
 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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Hearing Measurement
    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)
    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.
      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
 Productivityis probably unaffected by noise except
 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
    In offices, coworkers’ conversations are the main
source of noise.
    Consider sound absorbers or masking noise.
        To reduce cumulative trauma:
2.   Modify the existing noise source
3.   Modify the sound wave
4.   Use personal protection

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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                              BETTER

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

    Reduce exposure
duration.
    Use earmuffs and
earplugs.

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