Cornell University Ergonomics Web

DEA350: Ambient Environment: Acoustics and Noise

NOISE

Occupational Noise

Like other types of noise (traffic, aircraft), occupational noise has increased intensity and impact over last 30 years.

Sources of industrial noise

1. vibration of a structure or machine, or of their components
2. aerodynamic processes - primarily turbulence e.g. contact of high-speed, high pressure air jet with surrounding air in a blower.

Four basic categories of industrial noise:

1. product fabrication - molding e.g. glass bottle manufacturing, metal fabrication e.g. can manufacturing
2. product assembly
3. power generation
4. processing

Most of the noise generated by these activities is of lower frequency and such noise may affect both persons outside and workers inside the factory.

External Aspects

However, planning regulations have led to much noise generating industry being sited away from residential areas, therefore impact is often small, though one can still find exceptions to this.

Internal Aspects

Main occupational hazard is hearing loss - physiological deterioration of hearing due to destruction of hair cells in the organ of Corti, and decreased number of associated nerve fibers.

Types of hearing loss:

• Temporary threshold shift

• Noise induced hearing loss (NIHL) - Presbycusis - deterioration of hearing with age. Males have increased loss compared with females.

• Sociocusis - deterioration due to specific noise exposure.

General rule of thumb - every halving of the daily exposure allows an increase of 3 dB in noise level without increase in hearing hazard.

Estimated that average 24 hour exposure level of 70 dB (LEQ(24)) over a 40 year period leads to no NIPTS.

Average threshold level for an 8-hour work exposure with no significant exposure during the remaining 16 hours is 75 dB (LEQ(8)).

Pattern of hearing loss:

Presbycusis and Sociocusis + N.I.H.C. (PTS + TISL) plus tinnitus (ringing of the ears) which can become disabling + loudness recruitment - loudness increases much more rapidly with intensity than it does in normal subjects. Disabled person cannot hear someone who speaks normally but if they shout, they tell you that they are shouting. 

Temporary Threshold Shift (TTS)

Prevention of noise-induced hearing loss by noise reduction.

1. The source - solutions attempt to prevent noise being generated to start with e.g. replacing worn parts, lubricating dry parts, or changing design of system e.g. noiseless plumbing systems have controlled water velocity and special values, using noise-suppressing alloys etc. OR sound may be used to mask noise e.g. musak, music, or sound conditioning.
2. The sound radiator - reduce or isolate the vibrations from any possible radiating surfaces (sounding boards) e.g. using flexible connectors and mountings. Care must be taken to ensure that the mounting and the machinery do not resonate. General rule is that the forcing frequency must be (square root of 2) (natural frequency) of the mounting (frequency at which mounting resonates) before any improvement takes place; higher the ratio, better the isolation. Sounding board modifications - if vibration cannot be isolated then board modifications must be made e.g. cut board into several pieces, each smaller than wavelength of most undesirable spectral component of the noise, coating the board with vibration absorbing material.

3. Noise reduction between source and receiver  - three types of noise reduction measures before they reach the receiver can be taken:

   • Sound absorption - theoretically the ideal way to decrease noise. Sound energy is transformed into heat energy by passing waves through soft fibrous materials. Energy is lost through the frictional interaction of fibres. The problem with absorption treatments is that the sound must pass through a great deal of such materials before significantly decreasing sound energy. Absorption treatments usually more effective against high frequency than low frequency. Absorption material applied to walls of an untreated room decreases sound by 8-10 dB. Doubling thickness of material gives an additional decrease of 3 dB. Local absorption - absorbent curtains, sound baffles (strips hung from ceiling), screens etc. can locally decrease sound levels.
   • Sound insulation - two types of barriers commonly used: 
     • complete enclosure - of either sound source (e.g. printer box for W.P.) or receiver. BUT in modern offices often have problems with false ceilings. ii)
     • partial barrier - instead of stopping noise, this barrier deflects it away from receiver. Such barriers must be made of material thick enough to prevent any sound transmission through it.
   • Resonators - devices specifically for treating low frequency noise. Typically a reflective surface is placed in the path of sound waves, the reflected waves then cancel the incident waves e.g. in an exhaust silencer.

   Finally, any or all of these can be combined to decrease sound.

4. Noise reduction at the receiver - Typically this takes the form of ear plugs, ear muffs, or ear protectors of some kind. Well designed ear protectors may decrease noise by 40 dB. But these are often uncomfortable to wear over time and workers may find them annoying. Also, these impede the reception of important acoustical signals e.g. warning horns, sirens, bells.

PHYSICAL NOISE STRESSORS

Urban Stress

David Glass and Jerome Singer (1972)

2 social scientists, investigated stressor effects of noise on behavior. Although in general, noise is not the most important urban problem e.g. crime, drugs, poverty etc., if noise is a stressor, then it should be on a par with other supposed stressors e.g. crowding, and should lead to behavioral effects.

Conducted a series of experiments to establish stressor effects of uncontrollable and unpredictable noise on performance..

GSR effects

• Loud noise = 108 dbA (a scale weighted to high frequencies because these are more annoying to man).
• Soft noise = 56 dbA
• Decreasing GSR over four successive blocks =

  adaptation = short term coping.

But are there longer term effects? Do experiments of behavioral aftereffects of exposure to noise. Do experiments combining these with other stressors!

After noise exposure, subjects were presented with 4 puzzles - 2 soluble and 2 insoluble. Recorded number of trials attempted.

•  Results show that unpredictable noise leads to decreased persistence at solving insoluble problems, and increased impatience. Especially pronounced decrease with loud unpredictable noise.
• Increased annoyance of loud unpredictable noise over others is shown by increased proofreading errors made in this condition.
• Increased persistence at task with perceived control over noise compared with no perceived control.
• Similarly, decreasing errors of proofreading with perceived control. Therefore, noise is less of a stressor if you feel you have some control over it.

A different set of experiments included subjects given either control or no control over noise.

• Condition 1 - subjects have switch and experimenter tells them that flicing this will end noise, although subject should try to resist this as long as possible.
• Condition 2 - subject had no control over noise.

Traffic - main source of noise in cities.

Road Traffic Noise

1. Traffic parameters - speed and density; composition; fluidity (traffic lights etc.); driver behavior.
2. Road parameters - design (embankment, tunnel, cut, level); gradients and curves; type of surface; width.
3. Environmental parameters - distance and height from road of recipient; presence of natural or artificial screens; noise absorption possibilities; noise reflection possibilities.
4. Weather parameters - rain, snow, or dry conditions; wind direction and speed.
5. Dwelling parameters - orientation of living areas; noise attenuation through windows; window size.

Korte & Grand (1980) - Confederate on sidewalk in one of 2 conditions: i) clad in garish clothes with brightly colored balloons tied to nearby tree. ii) normally dressed but holding bright yellow teddy near a conspicuous advertisement saying "Attention - project in progress". Pedestrians who walked past confederate stopped by interviewer and asked whether they had noticed "anything unusual". Results showed that in high traffic noise conditions, there was a decrease in subjects who noticed unusual confederate and associated objects compared with low traffic noise. Also, pedestrian walked faster and gazed straight ahead for longer time.

Mathews & Canon (1975) J. Person. & Soc. Scientist, 32, 571-577. Increased level of background noise leads to decreased helping behavior. Subjects in lab exposed to 85 dB were less likely to help than those exposed to maximum of 65 dB. Then in field study, noise from loud lawn mower leads to decreased help. Possible explanation is 3 complementary effects:

1. noise as stressor
2. noise masking verbal communication
3. noisy situations aversive

General conclusions: Effects of noise may be similar to those of other urban stressors: short term direct effects, followed by adaptation, sometimes followed by aversive aftereffects because don't always have longer term aversive aftereffects. BUT continued exposure to stressors may lead to cumulative effects which appear only after stimulation is terminated.

Appleyard & Lintell (1972) Studied three streets in San Francisco. Heavy street - max. 900 vehicles / hour (avg. 15,750 per 24 hours) Moderate street - max. 550 vehicles / hour (avg. 8,700 per 24 hours) Light street - max. 200 vehicles / hour (avg. 2,000 per 24 hours) Noise levels - % time > 65 db. Heavy - 45% Moderate - 25% Light - 5%

Fairly similar populations and housing types. Results: Residents in light street had three times as many local friends and two times as many acquaintances as those in heavy street.

Cohen, Glass, & Singer (1973) Looked at children living in 32-story buildings built to span main highway (interstate 95) in Manhattan, NY. Looked at 54 children (29 male, 25 female) living on different floors of buildings. Tested children's auditory discriminations, reading ability, and couple of other tests.

Noise levels: Avg. outside = 77 dBA

Building floor

Results: Discrimination Correlation of +0.48 between floor level and auditory discrimination, that is, higher floors led to higher discrimination (i.e. increased noise leads to decreasing discrimination).

Effect of length of residence:
2 years

Reading - significant correlations between decreased discrimination and decreased reading performance as well, even when many other variables controlled for, e.g. mother's education, father's education BUT relationship between floor level (i.e. noise level) and auditory discrimination does not result from social class artifact.

Conclusions - Apartment noise level accounts for a substantial proportion of the variance in auditory discrimination and therefore in children's reading abilities.

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