This article throws light upon the top two ways for controlling noise. The ways are: 1. Engineering Control 2. Medical Control.
Way # 1. Engineering Control:
The methods of controlling noise for a particular situation may depend on many factors. No simple set of rules, therefore, can be established to control noise. In order to make maximum progress in the direction of noise control, one person might be appointed in an organisation to co-ordinate the work on noise control.
One way to reduce noise is to control it at the source. An example of this would be to make sure that all bolts, screws, etc. are tight so that they will not rattle excessively. Another method of noise control is substitution.
An example of substitution is to replace metal table-tops with wooden ones as a working surface for metal parts. Isolation is a third method of noise control. An example of isolation is placing a particularly noisy operation in a separate room away from other operations, thus exposing only a few people to the noise instead of many.
An important method of isolating noise is to construct a barrier (or wall) between the source of noise and persons likely to be exposed to it. The effectiveness of an isolating barrier depends on the materials used for the barrier, and the frequency or type of noise being isolated. For example, a heavy concrete or brick wall will stop more sound than a plywood wall. The heavier the wall, the less noise will get through it.
The amount of sound stopped by a barrier is known as “transmission loss”. Frequency is also a factor in transmission loss. High-frequency sound is blocked more easily than a low-frequency one. Since high-frequency noises are more damaging to the ear than those containing low frequencies, and since high frequencies are blocked more easily with massive barriers, isolation is an effective method of noise control.
A fourth method of noise control is through the use of resilient mountings. If heavy machines are firmly bolted to concrete or wood floors, the floors are often transformed into huge sounding boards that not only amplify the noise, but also spread it throughout the building. The use of rubber or other resilient mountings will usually reduce both noise and vibration.
The use of resilient floor coverings is advisable to further reduce the noise level. Familiar examples of resilient mounting are electric refrigerators, and the rubber mounting of automobile engines. In a plant, every machine will be a problem from the view-point of noise, and thus the services of a technical expert in the area of noise control are highly desirable.
A fifth method of noise control is the use of sound-absorbent materials. Hard surfaces (such as brick and plaster walls) reflect sound and produce reverberation. If certain facing materials of known acoustical properties are applied to ceilings and walls, air-borne noise will be partially absorbed. The more energy that is thus absorbed, the less that will be reflected back into the room to add to the original noise intensity.
There are prefabricated acoustical files, acoustical plasters, sprayed-on compositions, and blankets which have been fabricated from very porous material (such as glass wool control of noise by acoustical treatment is highly technical, and assistance should be sought from experts in this field).
It should be remembered, moreover, that acoustical treatment can do nothing to lessen the direct noise of machines. It is highly efficient, however, in decreasing indirect or reflected noise.
Way # 2. Medical Control:
Most early noise-induced hearing losses pass unnoticed unless they are detected by appropriate hearing tests. A suitable hearing test for this purpose is the pure-tone air- conduction threshold test.
Here the term “air conduction” describes the path by which the test sounds reach the ear, this type of test is generally referred to as an audiometric examination, and it tests the hearing acuity. It is performed with an instrument known as an “audiometer”.
In the audiometric examination, test sounds are generated by earphones and conducted through the air in the ear canal to the eardrum. The recommended test frequencies for an audiometric examination are 0.5,1,2,3,4 and 6 kHz.
The record of test results is called a threshold audiogram. It is a permanent record of an audiometric test. Audiometric examinations should be given to all new employees before they begin work. An audiogram giving various levels of hearing loss is shown in Fig. 4. An audiogram with a hearing loss is shown in Fig. 5.
There are three types of hearing loss:
(a) Middle ear or conductive loss;
(b) Nerve deafness; and
(c) Acoustic trauma.
An audiogram of the middle ear or conductive loss is shown in Fig. 6. This type of hearing loss is caused by an obstruction of some kind in the middle ear. The person suffering from this type of hearing loss is not likely to be bothered much by Kid noises. He will, therefore, be less subject to fatigue caused by loud noises.
Nerve deafness, on the other hand, results from damage to the inner ear. An audiogram showing the nerve deafness is given in Fig. 7.
The acoustic trauma typically results from exposure to high levels of noise. An example of acoustic trauma is given in the audiogram shown in Fig. 8. Note the sharp dip in this audiogram in the frequency range 3-4 kHz, which is characteristic of acoustic trauma.
As the noise exposure continues, the notch in the audiogram (see Fig. 8) becomes wider, and eventually the person suffering from acoustic trauma can notice his loss of hearing.
A classification of various degrees of hearing loss is given in Table 1. Persons with certain types of hearing loss, such as nerve deafness, may be susceptible to more acoustic damage. Such persons should not be considered for work in areas of high noise level.
On the other hand, those with pure conductive deafness are not prone to additional loss, and would be excellent in such areas. It follows, therefore, that both the type and degree of hearing loss should be taken into account for the purpose of workers’ placement.
In addition to helping the placement of an employee on the job, the audiogram examination also provides a good permanent record. The audiogram should become a permanent part of the medical record, should be kept confidential, and should not be removed from permanent files. Such an audiogram will serve as a base upon which to evaluate subsequent changes in the hearing acuity of the workers concerned.
It is important to remember in this connection that the work done in this area by the audiometric technician must be acceptable to medical and legal experts who will use the data.
It is necessary, therefore, that the audiometric technician must be competent and well trained. Similarly, the audiometer used for such examinations must meet the specifications laid down by international or national organisations dealing with standards (e.g., Bureau of Indian Standards, New Delhi).
The accuracy of an audiometer should be checked regularly, at least once a month. This check, known as a “biological calibration”, consists of making audiograms for 3 or 4 young persons who have no history of previous ear disease or hearing loss.
The testing condition and techniques used in the biological calibration must conform to current standards. For example, at each test frequency the average threshold of the young persons should be within 5 dB of the audiometer zero; if not, the acoustical calibration of the audiometer is probably incorrect.
If the audiometer is not in correct acoustical calibration, it should be returned to the manufacturer for service and adjustment. In any case, an audiometer should be returned to the manufacturer for service once a year.
A hearing-test room should be located in as quiet a place as possible, preferably within practical access but away from outside walls, elevators, heating and plumbing noises, etc. If the highest background noise levels do not exceed the values listed in Table 2, test-room noises will not affect test results.
Acoustical treatment of audiometry rooms may prove to be a difficult task and it should be undertaken only with the help of an acoustical consultant. Prefabricated audiometry booths are usually more satisfactory. They may also be cheaper than either reconstructed rooms or sound-treated rooms.
Medical control of hearing conservation must include competent supervision by a physician; otherwise it cannot be considered adequate. It should include the supervision of the audiometric facility to be certain that the audiogram data are accurate and reliable.