After reading this article this article you will learn about:- 1. Meaning of Audiometry 2. Indications for Audiometry 3. Required Equipment 4. Reference Audiograms.
Meaning of Audiometry:
Audiometry means the determination of monaural hearing threshold levels for pure tones by air conduction. Audiometry is a component of a hearing conservation programme in industrial and other situations, and is carried out when indicated by such a programme. Audiometry alone cannot prevent occupational hearing loss.
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On the other hand, audiometry is necessary for the following purposes:
(1) To determine the status of hearing of the workforce or other group of people;
(2) To monitor the effectiveness of noise control measures adapted by an industry or other agency; and
(3) To identity individual workers are adversely affected by noise.
It is hardly necessary to mention that, in addition to audiometry, a hearing conservation programme must also include the following:
(1) A well-defined policy;
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(2) A detailed noise survey;
(3) Dosimetry (determination of noise exposure);
(4) Reduction of excessive noise (by engineering as well as administrative methods);
(5) Provision of hearing protection;
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(6) Education of those likely to be exposed to excessive noise; and
(7) Monitoring on an on-going basis by:
(a) Noise surveys,
(b) Noise dose measurements, and
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(c) Audiometry.
Indications for Audiometry:
Audiometry may be classified into three broad categories, viz.:
(1) Reference or baseline audiometry;
(2) Monitoring or periodic audiometry; and
(3) Consultant audiometry.
Reference or baseline audiometry should be performed on all persons likely to receive a daily noise dose in excess of 80 dB(A) Leq (equivalent continuous sound level) for an eight-hour workday.
On the other hand, monitoring or periodic audiometry should be performed on all persons who are likely to receive a daily noise dose in excess of 85 dB (A) Leq (for an eight-hour workday). Another audiogram in this case should be taken within 90 days of initial exposure for comparison with the reference audiogram.
If no significant change is found in either the audiometric pattern or the noise exposure, monitoring audiometry should be repeated every one or two years. Consultant audiometry is usually performed in specialist clinics when reference or monitoring audiograms are found to be abnormal, or when an assessment has to be made for compensation a worker (exposed to excessive noise) for hearing loss.
Required Equipment for Audiometry:
In order to obtain satisfactory results and to conform to legal and other requirements, the equipment’s used in audiometry have to be selected with great care. It is desirable, for example, that the audiometer should be a discrete frequency, pulsed-tone, air conduction type, and (whether automatic or manual) must conform with international (or at least national) standards.
Whenever practicable, an automatic (i.e., self-recording) audio-meter, preferably controlled by a microprocessor, should be used. There are many reasons why an automatic audiometer should get a preference over a manual one.
Some of these are:
(1) Virtual elimination of systematic error due to the operator,
(2) The unambiguous nature of the subject’s responses;
(3) The provision of a permanent record (without the possibility of recording errors); and
(4) A visible indication of the quality of the subject’s test performance.
Of course, manual audiometers do have the advantage of low price. With standardisation, manual audiometers can give quite satisfactory (though less precise results.
Where the cost of the audiometer is likely to be a significant fraction of the total budget for workers’ hearing conservation programme (e.g., in a developing country like India), it is better to provide a man audiometer than no audiometer at all.
Automatic audiometry is ideal; but if the budget limited one should consider carefully the option of providing a manual audiometer and spend the funds thus saved on noise control measures.
When hearing threshold levels are to be tested down to 0 dB, accurate results are only possible when the background noise levels are sufficiently low. In many circumstances, the necessary low noise levels will only be achieved by the use of a sound isolating booth.
In order to get accurate and satisfactory results, regular calibration of audiometers is very essential. Where daily calibration is live tests of calibration (hearing levels and frequencies) should be done at least annually.
It is important that new audiometers are checked at more frequent intervals until the stability of calibration is established. Regular subjective checks (e.g testing a person with known normal hearing) may disclose any gross discrepancy which might occur between formal calibrations.
Finally the operator should be fully trained and competent in basic audiometry. He should, moreover, have access to the advice of an otologist (ear specialist) and a physician in occupational health.
Some Requirements:
Recent exposure to loud noise may temporarily elevate the hearing threshold levels in an industrial worker or others. These “temporary threshold shifts” are minimised if loud noises (those in excess of 80 dB (A)) are avoided (by the person to be tested) in the sixteen hours prior to reference audiometry For monitoring audiometry, however, insistence on 16 hours away from loud noises may not be practicable.
Even in such cases, there should be a period of quite, prior to audiometry, of not less than seven hours. This may be achieved quite adequately by the use of an appropriate hearing protection while at work. Prior to audiometry, it is desirable to have health screening of the person concerned by the audiometrician or other trained person.
This screening should include the following:
(1) The medical and occupational history (particularly in relation to any ear disease and exposure to noise in the past); and
(2) A thorough examination of the ear, nose and throat.
Audiometry techniques will, of course, vary depending on the type of audiometer used (e.g., manual or automatic); but audiometry should be performed in a standard way, for each person, according to the manufacturer’s instructions, and in conformity with international and/or national standards. The test frequencies employed for audiometry should include 0.5, 1, 2, 3, 4 and 6 kHz.
Reference Audiograms:
The results of audiometry may have medical implications. For example, many cases of hearing impairment or possible ear disease will be detected by an initial audiometry performed as part of a hearing conservation programme.
In such cases, the person concerned should be informed of the findings. In addition to this, a decision should be made as to the need to refer the case to a physician for further diagnosis and possible treatment .Such a referral should be made if any of the criteria listed in Table 1 are met. These criteria have been recommended by the National Acoustics Laboratories of Australia.
Abnormal hearing levels in the low-frequency range (2 kHz and below) occur only in the most advanced cases of noise-induced deafness when a considerable hearing loss will already exist at the higher frequencies.
Thus low frequencies are not particularly useful for the purpose of detecting noise –induced changes in hearing, and for monitoring the effectiveness of a hearing conation programme. On the other hand, abnormal low-frequency toeholds are often due to conditions amenable to medical treatment, so a medical referral may be beneficial.
Hearing losses at high frequencies (3 kHz and above) can be caused by many factors such as:
(1) Exposure to excessive noise;
(2) Viral infections;
(3) Certain forms of medication;
(4) Blows to the head; and
(5) Hereditary disorders;
Since the damage from these causes is irreparable, and because most of these conditions (apart from noise induced deafness) also cause changes in hearing at low frequencies, it may not be necessary to refer for medical treatment those persons who only exhibit high frequency hearing loss. They may however, be recommended for a hearing aid or other rehabilitation.
Rehabilitation and/or hearing aid should be considered for all person who show severe irreversible hearing loss, and perceive it to be serious enough to receive help. As a general rule, those whose total hearing loss in each ear the frequencies of 0.5,1 and 2 kHz equals or exceeds 75 dB should be referred for further assessment.
In such cases, the person concerned should be informed of the findings. In addition to this, a decision should be made as to the need to refer the case to a physician for further diagnosis and possible treatment. Such a referral should be made if any of the criteria listed in Table 1 are met. These criteria have been recommended by the National Acoustics Laboratories of Australia.
Abnormal hearing levels in the low-frequency range (2 kHz and below) occur only in the most advanced cases of noise-induced deafness, when a considerable hearing loss will already exist at the higher frequencies.
Thus low frequencies are not particularly useful for the purpose of detecting noise-induced changes in hearing, and for monitoring the effectiveness of a hearing conservation programme. On the other hand, abnormal low-frequency thresholds are often due to conditions amenable to medical treatment, so a medical referral may be beneficial.
Hearing losses at high frequencies (3 kHz and above) can be caused by many factors such as:
(1) Exposure to excessive noise;
(2) Viral infections;
(3) Certain forms of medication;
(4) Blows to the head;
(5) Hereditary disorders; and
(6) Vascular disease.
Since the damage from these causes is irreparable, and because most of these conditions (apart from noise induced deafness) also cause changes in hearing at low frequencies, it may not be necessary to refer for medical treatment those persons who only exhibit high frequency hearing loss. They may, however, be recommended for a hearing aid or other rehabilitation.
Rehabilitation and/or hearing aid should be considered for all persons who show severe irreversible hearing loss, and perceive it to be serious enough to receive help. As a general rule, those whose total hearing loss in each ear at the frequencies of 0.5,1 and 2 kHz equals or exceeds 75 dB should be referred for further assessment.
Monitoring Audiograms:
When the comparison between a monitoring audiogram and the reference audiogram indicates a significant change in hearing threshold, one needs criteria for taking action. Deteriorating group hearing levels may be taken as a warning that a hearing conservation programme is failing. However, the arrest of the hearing deterioration requires the identification and treatment of the affected individuals.
An individual exhibiting a significant hearing loss may not be adequately protected, or he may be unusually susceptible to excessive noise (and in need of special protection or quiter work). The early stages of noise-induced hearing loss are usually manifested as a deterioration of threshold levels in the frequency range 3-6 kHz.
However, other frequencies should also be included in the criteria for action . For example, changes in hearing at low frequencies (2 kHz and below) may exceed those at higher frequencies (3 kHz and above) in cases of advanced noise-induced hearing loss. On the other hand, changes in hearing at lower frequencies may also be indicative of other pathology.
Action Programme:
Whenever there is a difference of +15 dB or more at any test frequency (from 0.5 to 6 kHz) between the monitoring and reference thresholds, the following action should be taken:
(a) In the first place, make sure that no transient condition exists that could account for the difference. Such conditions include, for example, cold, earache and recent exposure to excessive noise. If such a condition is found to exist, action should be deferred and the person concerned should be retested as soon as possible after the remission of symptoms.
(b) If no such transient condition exists, remove and replace the earphones, repeat the audiometric test, and, at each frequency, average the results with those of the audiograms already obtained. If the average hearing level (at each frequency) does not differ by 15 dB or more from the corresponding hearing level in the reference audiogram, no further action is necessary.
If at any frequency the average of the two audiograms differs by 15 dB or more from the corresponding hearing level of the reference audiogram, the action outlined in the following steps is recommended.
(c) The person concerned should be advised of the test results.
(d) The person’s noise exposure history since the last audiogram should be carefully checked.
(e) Check also the suitability and condition of the person’s protective equipment, the technique of fitting, and the frequency and extent of its use.
(f) Any corrective action, which appears necessary in the light of foregoing tests, should be taken.
(g) If any of the medical referral criteria listed in Table 1 are exceeded, or if any change in the hearing level since the most recent audiogram exceeds 20 dB (and is of known cause), suggest the person to seek medical advice.
(h) The person concerned should be retested in six months.
(i) All audiometry records should be carefully filed and retained throughout the person’s employment.
The above programme of action is based on the recommendations of the National Acoustics Laboratories, Australia.