Chapter%2024:%20Hearing%20and%20Noise

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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
• 55 80 dBA ? annoyance
• gt 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.
• Losses can be temporary or permanent.
• Temporary threshold shift, TTS
• Recovery after 14 hrs of exposure lt 80dBA
• Permanent threshold shift, PTS (or NIPTS)
• TTS ? PTS
• Audiograms should be performed annually.

Normal Hearing
Conduction Hearing Loss
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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 lt1 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 lt3 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 (gt85 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
• 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
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.