1. 1 ISE 311 - Ch. 24 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  > 90 dbA  risk of hearing loss

2. 2 ISE 311 - Ch. 24 Anatomy of the Ear

3. 3 ISE 311 - Ch. 24 The Organ of Corti

4. 4 ISE 311 - Ch. 24 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 HearingConduction Hearing Loss

5. 5 ISE 311 - Ch. 24 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. 6 ISE 311 - Ch. 24 Decibels Sound pressure and sound power are analogous to temperature and heat.  Sound pressure level: SPL = 20 log10 (P / P 0 )  Power watt level: PWL = 10 log10 (W / W 0 )  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. 7 ISE 311 - Ch. 24 Calculating dB

8. 8 ISE 311 - Ch. 24 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.

9. 9 ISE 311 - Ch. 24 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. 10 ISE 311 - Ch. 24 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 PWL A = _________W A = _________________ PWL B = _________W B = _________________ PWL combined = _________________________

11. 11 ISE 311 - Ch. 24 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.

12. 12 ISE 311 - Ch. 24 Speech Interference Note: in loud environments (>85 dB), earplugs improve speech transmission. Figure 24.11, pg. 469

13. 13 ISE 311 - Ch. 24 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. 14 ISE 311 - Ch. 24 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

15. 15 ISE 311 - Ch. 24 Plan Ahead Substitute less noisy processes. Purchase less noisy equipment. Use quieter materials and construction. Separate people and noisy equipment.

16. 16 ISE 311 - Ch. 24 Modify the Noise Source Reduce driving force. Change the direction of the noise. Minimize velocity and turbulence of air. POORBETTER

17. 17 ISE 311 - Ch. 24 Modify the Sound Wave Confine the sound wave. Absorb the sound wave.

18. 18 ISE 311 - Ch. 24 Use Personal Protection Reduce exposure duration. Use earmuffs and earplugs.