Document 10rxEjDDoYbKvLE6NVYML3g8o

INDUSTRIAL NOISE Variability in the test environment is caused mainly by changing ambient noise levels in the testing area Unless test areas are specially treated to reduce ambient noise levels below certain specified values, the measured thresholds of persons with normal hearing may be masked The usual ambient noises m test environments are predominantly low frequency sounds, and most of the masking is at 1,000 c and below It is usually easy to detect masked low fre quency thresholds on audiograms made m poorly sound-treated environments These audiograms show normal or near normal hearing at 1,000 c and hearing losses as large as 10 db at 250 c and at 500 c Thresholds at 1,000 c may show the effect of masking if the ambient levels are mod erately high. Unless hearing tests are made under extremely bad conditions there is very little, if any, masking effect seen at 2,000 c end higher frequencies. Ambient noise in the testing environment is perhaps the most frequent environmental cause of variations in audiograms, but it is by no means the only cause. The subject must be comfortable so that be will not be come restless and squirm or fidget Hu midity, temperature, barometric pressure, and the size and d&sor of the test room are all factors that influence comfort and it is important to pay attention to these details in order to provide optimum conditions for producing good audiograms. Variability caused directly by the sub ject may be caused by his past experience with hearing tests, to intelligence, educa tion, and physiological condition, or to his hearing and attitude towards the test. In this latter category for example, subjective tinnitus (a purely subjective sensation of noise, such as ringing m the ears) is some times loud enough to mask the signal tone and produce spurious records The attitude of the subject toward the test is of extreme importance Screening audiometry Screening audio metry has only limited use in industry It can be used successfully for followup audio metry since only a change in hearing status s sought. Usually screening is accom plished by testing at several frequencies with a level of 15-20 db. The subject is passed if he hears the rone and failed if not. Those who fail are retested with threshold audiometry. Automatic audiometry The need for test ing die hearing of large numbers of persons has stimulated an interest in automatic audiometry, which allows the subject to op erate the only manual control of the test instrument required during the test When the test is started the mtensity of the sig nal tone increases automatically. The sub ject has been instructed to press a button when he first hears the signal. This button operates a switch which reverses the direc tion of the signal intensity, thus decreasing the signal strength. When the subject no longer hears the signal he releases the hutton and the signal again increases m in tensity The procedure is continued for a set interval during which the subject may "cross" his own threshold as many as ten or fifteen times When the proper time has elapsed the audiometer output automatically shifts to the next frequency and the proce dure is repeated The test continues until an arbitrarily chosen number of frequencies has been tested A record of the test may be made with any of several devices which record the continuous change in output level of the signal as it crosses and recrosses the subject's threshold level. Automatic audio meters have been variously adapted to record the test on specially designed cards, on IBM cards, by automatic typewriter, or by an adding machine In principle all auto matic audiometers are much the same the intensity of the output signal is regulated through a switch which is controlled by the subject Its validity bas been well estab lished Variability caused by the tester is de pendent to a large extent upon his motiva tion. A tester may be extremely well trained, but if he is careless, the test results may be completely invalid A minimum amount of training in giving hearing tests is absolutely essential, especially actual practice training with an audiometer Care ful testing is important also The usefulness of automatic audiometry depends upon the conditions under which it is to be used. As far as the subject is con cerned, automatic audiometry does not save time as most automatic instruments run for six to ten minutes to complete a test, about the time required to complete a test with a mannnl instrument As far as the tester is concerned, however, the instrument makes