1 ISE 311 - Ch. 24 Chapter 24: Hearing and Noise Defining and understanding noise & its effects complex problem not always intuitive critical for.

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

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Anatomy of the Ear

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The Organ of Corti

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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

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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 = 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.

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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. 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.

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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 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 = _________________________

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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 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.

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Speech Interference

Note: in loud environments (>85 dB), earplugs improve speech transmission.

Figure 24.11, pg. 469

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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 ahead2. Modify the existing noise source3. Modify the sound wave4. 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.

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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

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Modify the Sound Wave

Confine the sound wave. Absorb the sound wave.

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Use Personal Protection

Reduce exposure duration.

Use earmuffs and earplugs.