Industrial Noise

acoustic tramua
-injury to the sensorineural elements of the inner ear
-produced by one or a few exposures to sudden intense
-blasts, explosions, direct trauma to the ear or head
noise-induced hearing loss
-cumulative permanent loss of hearing
-always sensorineural type
-months or years of hazardous noise exposure
-affects both ears equally
any pressure variation that the human variation can detect
nonaudible acoustic phenomena recognized by touch or feeling
undesirable effects of sound
-masking of wanted sounds (speech)
-auditory fatigue
-damage to hearing
unpleasant sound
likely to be harmful
sound waves
elastic medium: air, water, or steel
number of times/second that a vibrating body traces one complete cycle of motion
perceived as pitch
cycles per sound
sound frequency's composition
determining factor in the level of annoyance
distance measured between two analogues points on two successive parts of a wave
measured in feet or meeters
bending of sound around obstacles
-if obstacle has smaller wavelength, obstacles will not stop than noise
-if obstacle has larger wavelength, sound is scattered in many directions (shadow)
-tf walls only help with short wavelegths
speed of sound
velocity at which the analogous pressure points on successive parts of a sound wave pass a given point
-always = product of wavelength and frequency
c = fw
rms sound pressure
squaring the value of sound pressure disturbance at each instant of time
values are then aded and averaged over a given time
threshold of hearing
the weakest sound that can be heard by a person with very good hearing in an extremely quiet location
threshold of pain
greatest sound pressure that can be perceived without pain
relative scale
most convenient to use
dimensionless unit used to express the logarithm of the ratio of measured quantity to a reference quantity
sound power (W)
amount of energy per unit time that radiates from a source in the form of an accoustic wave
sound level (Lw)
expressed in decibels relative to the reference power of 10^12 watt (Wo)
depends primarily on sound pressure, but also frequency
-subjective human response to sound pressure and intensity
equal-loudness contours
results of experiments designed to determine the response
weighted sound scale
series of correction factors that are applied to sound pressure levels on an energy basis as a function of frequency
damage risk criteria purpose
define maximum permissible noise levels
sound level meters
-weighting networks -metering system
responds to sound pressure variations and produces an electrical signal that is processed by sound level meter
has wide freq range
used for measuring sounds that differ greatly in level
weighting networks
controls sound level meter response at various frequencies
metering system
after electrical signal from microphone is amplified and sent thru attenuators and weighting networks, it is used to drive metering circuit
proportional to electrical signal applied to it
octane-band analyzers
breaks down total noise into various frequency components
noise dosimeters
practical way to measure noise exposure for workers that move around and are exposed to different noises
sound surveys
2 categories:
1. source measurement
2. ambient noise measurement
source measurement
collection of acoustic data for the purpose of determining characteristics of noise radiated by a source
ambient noise measurements
used to study a single sound level or to make a detailed analysis showing hundreds of components of a complex vibration
preliminary noise survey
conducted by a audiologist, industrial hygienist, safety and health personnel, or qualified engineer
if you have to shout at a distance of 3 ft or less, should be surveyed, muffled sound or ringing in ears are experienced
not in depth
detailed noise survey
Does a noise problem exist?
determines TWA exposures
1. obtains specific info on noise levels
2.develops guidelines for establishing engineering and or admin controls
3. define areas where PPE is required
4. determine areas where audiometric testing is needed
General classes of noise exposure
1. continuous noise
2. intermittent noise
3. impact type noise
noise control programs
1. control source (most desirablemodifying equipment)
2. modification of noise path (shielding or enclosing source, creating distance)
3. enclosures (sound booth)
4. control measures
5. engineering
6. admin controls (rotating)
7. PPE
bone conduction
the sound that can reach the inner ear even if canal is completely closed
sound will always be able to bypass PPE
classes of hearing protection
1. enclosures
2. aural inserts or earplugs
3. superaural protectors, or canal caps
4. circumaural protectors, or earmuffs
Noise reduction rating
developed by EPA
earmuffs: subtract 25% from label
formable earplugs: -50%
all other plugs: -70%
threshold audiometry
determines an employee's auditory threshold for a given stimulus
need a reference point before hearing can be abnormal
air conduction
air path of sound at the earphones to ear drum
who should be examined?
all employees
otherwise liable for all hearing loss
second test 9-12 months after
conclusions can be drawn from the average changes in a group
effective programs
-medical surveillance
-qualified personnel
-suitable test environment
-calibrated equipment
-adequate record keeping
adequate records
hearing threshold values
noise-exposure history
identification - name, ssn, sex, age
noise exposure regulations
not to exceed 85 dba in an 8 hour period
baseline and annual
standard threshold shift (STS)
amendment as an avg shift/loss in an ear of 10 db or more
record keeping
noise exposure records kept for 2 years
maintained for employees duration
includes name and job classification of employee