Hearing and Noise n Defining and understanding noise

Hearing and Noise n Defining and understanding noise & its effects ¨ complex problem ¨ not always intuitive ¨ critical for occupational health n Level of noise affects comfort, performance, and long-term hearing ¨ 55 – 80 d. BA annoyance ¨ > 90 db. A risk of hearing loss ISE 311 - 20 1

Anatomy of the Ear Figure 21. 1, pg. 414 ISE 311 - 20 2

The Organ of Corti ISE 311 - 20 3

Hearing Measurement n Audiograms test the air pathway and give total loss. Normal Hearing n Conduction Hearing Losses can be temporary or permanent. ¨ Temporary threshold shift, TTS n Recovery after 14 hrs of exposure < 80 d. BA Permanent threshold shift, PTS (or NIPTS) ¨ TTS PTS ¨ n Audiograms should be performed annually. ISE 311 - 20 4

Sound n 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. ISE 311 - 20 5

Decibels n Sound pressure and sound power are analogous to temperature and heat. ¨ Sound pressure level: SPL = 20 log 10 (P / P 0) ¨ Power watt level: PWL, d. B = 10 log 10 (W / W 0) = 10 log W + 120 ¨ When combining or subtracting noises, use the power formula. n n Doubling of power results in 3 d. B increase in noise level. Mean minimum level of hearing for the unimpaired-hearing population is 4 d. B. ISE 311 - 20 6

Calculating d. B ISE 311 - 20 7

Noise Measurement n 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. ¨ n Sound-level meters provide one number, combining various frequencies. ¨ ISE 311 - 20 Octave band analyzers provide detailed information. 8

Determining Machine Noise n n Measure noise level with machine running. Measure noise level with machine off. Calculate the difference. If <3 d. B, the background noise is too high for accurate measurement. Recall: when combining or subtracting noises, use the power formula. ISE 311 - 20 9

Example (Refer to table 21. 1, pg. 418) n Assume the vacuum cleaner and the disposal are being operated at the same time in a kitchen. What is the total sound level in d. BA? PWL = 10 log W + 120 PWLA = _____ WA = _________ PWLB = _____ WB = _________ PWLcombined = _____________ ISE 311 - 20 10

Effects of Noise n 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. n 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. ISE 311 - 20 11

Speech Interference Figure 21. 8, pg. 422 n Note: in loud environments (>85 d. B), earplugs improve speech transmission. ISE 311 - 20 12

Hearing n n n Hearing loss is a type of repetitive trauma (‘cumulative strain’. ) Hearing loss (over and above age-related) begins with exposure to noises over 67 d. B. 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. ISE 311 - 20 13

Noise Reduction n Office vs industrial environment ¨ ¨ n To reduce cumulative trauma: 1. 2. 3. 4. ISE 311 - 20 In offices, coworkers’ conversations are the main source of noise. Consider sound absorbers or masking noise. Plan ahead Modify the existing noise source Modify the sound wave Use personal protection 14

Plan Ahead n n Substitute less noisy processes. Purchase less noisy equipment. Use quieter materials and construction. Separate people and noisy equipment. Fig. 21. 11, pg. 427 ISE 311 - 20 15

Modify the Noise Source n n n Reduce driving force. Change the direction of the noise. Minimize velocity and turbulence of air. POOR ISE 311 - 20 BETTER from fig. 21. 14, pg. 429 16

Modify the Sound Wave n n Confine the sound wave. Absorb the sound wave. ISE 311 - 20 Fig. 21. 20, pg. 432 17

Use Personal Protection n n Reduce exposure duration. Use earmuffs and earplugs. ISE 311 - 20 18