Hearing and Noise

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					    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
                 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
                   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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    Plan Ahead
        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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