Cornell University Ergonomics Web
DEA3500: Ambient Environment: Acoustics and Hearing
point source - e.g. voice, printer
area source - numerous close sources
line source - e.g. busy road
For point source, "inverse square law" i.e. sound intensity varies inversely with square of distance. Each time distance is doubled, dB decreases sound intensity. For line source (because sound waves are cylindrical), doubling of distance decreases sound intensity by 3 dB. For area source, theoretically no decrease in intensity with increased distance.
Nerve deafness: damage or degeneration of hair cells in organ of Corti in the cochlea (inner ear). Such hearing loss is typically uneven, and there is greater loss of higher frequencies than lower frequencies. This is the type of hearing loss normally found with aging and once degeneration has occurred it usually cannot be remedied.
Conduction deafness: caused by some problem in the outer or middle ear that affects transmission of sound waves to the inner ear. Conduction deafness is more even across frequencies and doesn't result in complete hearing loss. This type of deafness can be arrested or improved with hearing aids. Conduction deafness is only partial hearing loss because sound waves can be transmitted to inner ear by conduction through bones of the skull.
Presbycusis - hearing loss due to normal aging process. Typically this is greater for men than women.
Sociocusis - hearing loss due to non-occupational noise sources e.g. household noises, TV, road traffic, discos, etc. (criteria for occupational/ non-occupation fuzzy). Some debate where sex differences due to sociocusis or presbycusis.
Nosocusis - hearing loss due to a pathological condition.
Temporary Hearing Loss - After exposure to continuous noise of sufficient intensity, some temporary hearing loss which is usually recovered a few hours or days after exposure e.g. pop concert phenomenon. This hearing loss is usually measured 2 minutes after the end of exposure and is termed the temporary threshold shift at 2 minutes (TTS2)
Permanent Hearing Loss - Repeated exposure to noise of sufficient intensity produces a permanent threshold shift (PTS). This is the same thing as noise induced permanent threshold shift (NIPTS). PTS is usually noticed at around 4KHz and with further exposure, gradually hearing loss spreads over wider frequency range.
100% noise dose = 90 dBA
Point at which employer must use feasible engineering/administration controls to decrease noise exposure.
1981 (OSHA) - more than 2.9 x 106 workers with TWAs > 90 dBA, 2.3 x 106 workers with TWAs > 85 dBA.
e.g. worker experiencing 95 dBA for 3.5 hours, 105 dBA for 0.5 hour, or 85 dBA for 4.0 hours. These fall within individual OSHA permissible time limits (see table 16-3). But total noise dose = 100% x · partial doses
= 100 (3.5/4.0 + 0.5/1.0 + 4.0/ 16.0)
= 100 (.875 + .5 + .25) = 100 (1.625)
Total dose = 162.5 which is equal to 93.5 dBA which exceeds permissible level!
Noise dose - exposure to any sound level > 80 dBA causes listener to incur a partial dose of noise. Partial dose = time actually spent at sound level / max. permissible time at that sound level.
The TWA is the sound level that would produce a given noise dose if an employee were exposed to that sound level continuously over an 8 hour work day.
Action level - a noise dose of 50% (TWA = 85 dBA) is point at which employer must implement continuing hearing conservation program which includes exposure monitoring, audiometric testing, hearing protection, employee training, and record keeping.