Document y5nRbXjM1y6dyrb43RygbkgX

40--Noise and Hearing Conservation ess is such that the work task can be confined to a limited area, isolation of workeis in a separate noise insulated room provides ef fective control This control method has been observed in chemical processing, powei, and metal products plants with noise reduction in side the rooms of as much as 30 decibels Machine insulation. Machines mounted di rectly on floors and walls vibrate them, re sulting in sound radiation The proper use of machine mountings insulates the machines and reduces the transmission of sound vibra tions to the floors and walls Control of noise by absorption. Noise pro duced at a sound source travels out m all di rections When the sound waves encounter other machinery or walls, they are reflected Thus, the total noise exposure within the room is equal to the sum of the direct and reflected noise The application of sound absorption material to walls can reduce the noise ex posure m the 100m However, this method has limited industrial application as the ab sorbing material has no effect upon the direct noise from the source Substitution of less-noisy machines. This approach to noise control may have limited application However, there are certain areas in which substitution has potential application These include "squeeze" type equipment in the place of drop hammers, welding in place of riveting, and chemical cleaning of metal in stead of high speed polishing and grinding Reduction of exposure time. Experience has shown that limiting the total daily ex posure reduces the noise hazard This prin ciple is illustrated m the Threshold Limit Val ues listed earlier Personal protection against noise. There are many operations in industry that cannot be quieted by engineering methods Ear plugs or muffs worn by the worker can provide ef fective protection in such cases Numerous scientific reports have shown that such devices can provide 25 to 40 dB of protection if prop erly worn As with the usual safety protective device, ear protectors may cause a degree of discomfort to the wearer These devices are, however, used successfully m many industries where adequate attention is given to program development and employee education Be sure to check the current OSHA rules and regulations regarding the attenuation characteristics of hearing protective devices before selecting them Hearing Evaluation Hearing evaluation, an important part of a hearing conservation program, involves audiometnc testing Such testing should be done at tune of hiring and may be done at periodic intervals, at the time of job change, or upon leaving employment While there may be need for all of these evaluations m an in dustrial hearing conservation program, the preplacement test is the most important An audiometer is required for assessing an in dividual's hearing ability Depending on the type of test signal used, audiometers may be called "pure tone audiometers" or "speech audiometers " Manual and automatic models are available Pure tone audiometers are standard instru ments for industrial use For hearing tests likely to be required in industry, the test fre quencies of 500, 1000, 2000, 3000, 4000, and 6000 Hz are needed Audiometric testing facilities In industrial situations, medical facilities may be near noisy industrial areas and back ground noise levels m these facilities may be relatively high If noise interferes with the audiometer signal, the subject will appear to be "partially deaf " To obtain reliable audiometnc tests, it is necessary to maintain the background noise level in the test area below certain values This may be accomplished by the design and construction of special rooms, or by the use of a prefabricated, commercially available audiometric test booth Audiometers must be calibrated periodically to ensure accurate test results The calibra tion must include both the electronic circuits and the earphones, and calibration should be repeated at intervals not longer than one year and more frequently when erratic operation or erroneous test results are suspected Rough daily calibrations can be made by testing the hearing of persons with known patterns 1252