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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 55 – 80 dBA  annoyance > 90 dbA  risk of hearing loss. Anatomy of the Ear.

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Hearing and Noise

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  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

  2. Anatomy of the Ear Figure 21.1, pg. 414

  3. The Organ of Corti

  4. 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 < 80dBA • Permanent threshold shift, PTS (or NIPTS) • TTS  PTS • Audiograms should be performed annually. Normal Hearing Conduction Hearing Loss

  5. 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.

  6. 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.

  7. Calculating dB

  8. 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.

  9. 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.

  10. 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 = _________________________

  11. 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 highmental 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.

  12. Speech Interference • Note: in loud environments (>85 dB), earplugs improve speech transmission. Figure 21.8, pg. 422

  13. 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.

  14. 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

  15. 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

  16. 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

  17. Modify the Sound Wave • Confine the sound wave. • Absorb the sound wave. Fig. 21.20, pg. 432

  18. Use Personal Protection • Reduce exposure duration. • Use earmuffs and earplugs.

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