1 ISE 311 - Ch. 24 Chapter 24: Hearing and Noise Defining and understanding noise & its effects complex problem not always intuitive critical for.
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1ISE 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
4ISE 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 Hearing Conduction Hearing Loss
5ISE 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.
6ISE 311 - Ch. 24
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.
8ISE 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.
9ISE 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.
10ISE 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
PWLA = _________ WA = _________________
PWLB = _________ WB = _________________
PWLcombined = _________________________
11ISE 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.
12ISE 311 - Ch. 24
Speech Interference
Note: in loud environments (>85 dB), earplugs improve speech transmission.
Figure 24.11, pg. 469
13ISE 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.
14ISE 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 ahead2. Modify the existing noise source3. Modify the sound wave4. Use personal protection
15ISE 311 - Ch. 24
Plan Ahead
Substitute less noisy processes.
Purchase less noisy equipment.
Use quieter materials and construction.
Separate people and noisy equipment.
16ISE 311 - Ch. 24
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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