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Written by Robert E. Sheriff, MS, CIH, CSP, President
November 7, 2018
The OSHA Noise Standard (29CFR1910.95) states when the 8-hour noise exposure exceeds 90 dB-A, an effort to control noise through engineering methods must be attempted if the 90 dB-A level is exceeded.
Hearing protection alone is not an acceptable means of permanent OSHA compliance to the noise standard. The reason is simple, the poorest means of protection, whether to noise or other health hazard, is to utilize personal protection (in this case, hearing plugs or muff). The best protection is to eliminate the excessive noise or isolate the individual from the noise source. That means some form of engineering control—design changes to the source or separation of a worker from the machine.
There are a vast assortment of possibilities depending upon the noise source and the amount of energy (in the form of noise) that is produced. Remember that noise is energy and multiple sources compound the energy produced. For example: two (2) machines side-by-side each producing noise of 88 decibels each when combined that is operating at the same frequency doubles the energy output resulting in a noise level of about 91 decibels. That by itself may be confusing because doubling the noise energy only increased the noise level by 3 decibels (dB). Noise energy is expressed on a Logarithmic scale; 3 dB is a doubling the noise energy that is emitted.
In reducing noise by engineering means sounds easy to only have to reduce it by 3 dB. Not so easy since we have to reduce or absorb one-half of the energy to get a reduction of 3 dB.
There is another problem. Frequency. Noise is almost always a combination of frequencies—each of which will require different methods to reduce the noise. The different frequencies are actually different wavelengths—lower frequencies have longer wavelengths—higher frequencies have shorter wavelengths.
When sound (noise) is combined, the energy can bounce off surfaces and accumulate to increase the overall noise level. A good example is a factory with concrete floors, concrete walls and metal ceilings present great surfaces for sound to bounce off of back into the factory. The noise level is likely to be greater than the noise from any single source.
The methods of noise control are thus dependent on the noise frequency and the noise volume.
One of the first means of engineering controls is to interrupt the path of the noise from the source to the worker. This is best achieved where higher frequencies are involved by blocking the path through acoustical insulation that is effective for a specific frequency range. Higher frequencies may be absorbed but the lower frequencies (longer wavelength) will just go around the absorbers.
Another means of blocking the noise path is to interrupt the path of noise that bounces off walls and ceilings again by acoustical insulation. It doesn’t work on floors because most acoustical absorbers are soft and porous to absorb the intended frequencies—not good for traffic.
Another means of reducing the reverberant noise off walls and ceilings is to deflect the noise using rough surfaces to break it up—not as effective as absorbing it but it still has some value.
One of the most effective means of reducing lower frequency noise is to break up the transmission path of the sound energy that is being absorbed by a larger surface that can also vibrate—such as the floor. Lower frequency vibration from machinery can be absorbed by installing rubber pads—or other flexible mounting—to the base thus preventing the floor from being a transmitter. Any type of mounting that mismatches the material can reduce noise transmission: steel to rubber, steel to wood, wood to plastic.
Another method of control is to enclose the noise source itself—such as a machine or a motor. Obviously, this may not be practical in some cases because of the need to access the equipment. Also machines, especially motors, produce heat which must be released but the opening can allow the noise to escape (remember both heat and noise are energy). Where exhaust is at a single point of muffler is an effective control.
Often changes in frequency of sound emitted by adjacent sources can be helpful since that doubling of noise is frequency dependent.
Another effective method to reduce worker noise exposure is to isolate the worker from the noise source. Isolate/Enclose the machine or put the worker in a control room. control rooms are common and most effective in places such as power plants where a machine does not have to be personally operated or monitored.
Distance between work and noise source may also be effective but remember that the noise in an enclosed area with hard surface walls, floors and ceilings may cause reflected noise to make the entire area equally noisy.
In summary the possible methods of engineering control of noise include:
Noise Absorption – By walls, ceilings, carpet, furnishings.
Sound Barrier – Walls, sound absorbing curtains, glass, enclosures.
Isolation – Of workers or sound sources.
Vibration Damping – Isolation, noise dampers including on ductwork.
Mufflers or Silencers – Especially high pressure air noise.
Equipment Maintenance and Lubrication – Especially bearings.
The field of noise control engineering is vast with new noise sources and methods of control continuously being developed.
If you need assistance in selecting possible engineering solutions to excessive worker noise exposure, let us know at email@example.com or 800-344-4414.
The following are some recommended references:
Salmon, V., Mills, J.S., Industrial Noise Manual, NIOSH, Contract No. HSM 99-73-82, June 1975.
Hedeen, R.A., Compliance of Materials for Noise Control, NIOSH, Contract No. 210-77-00-63, May 1980.
Berger, E.H., Ward, W.D., Morrill, J.C., Royster, L.H. Noise and Hearing Conservation Manual, 4th Edition, American Industrial Hygiene Association, 1986, ISBN No. 0932627-21-8.
OSHA Technical Manual, Section III, Chapter 5, www.osha.gov/dts/osta/otm/new_noise/.
Goelzer, B., Hansen, C.H., Sehrndt, G.A., Occupational Exposure to Noise: Evaluation, Prevention and Control, WHO, www.who.int/occupational_health/publications/occupnoise/en.
NIOSH, Occupational Noise Exposure, Publication No. 98-126, June 1998.
Harris, C.M., Noise Control in Buildings: A Practical Guide for Architects and Engineers, ISBN-13: 978-0070268876, 1994.
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