After reading this article you will learn about:- 1. Introduction to Hearing 2. Hearing Mechanism 3. The Human Ear 4. Threshold 5. Pitch 6. Loudness Levels 7. Differential Sensitivity 8. Speech Spectrum 9. Psychological and Linguistic Factors 10. Effect of Noise 11. Evaluating the Exposure to High Levels of Noise 12. Reduction at the Ear.
Contents:
- Introduction to Hearing
- Hearing Mechanism
- The Human Ear
- Threshold of Hearing
- Pitch of Hearing
- Loudness Levels of Hearing
- Differential Sensitivity of Hearing
- Speech Spectrum
- Psychological and Linguistic Factors of Hearing
- Effect of Noise on Hearing
- Evaluating the Exposure to High Levels of Noise
- Reduction at the Ear
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1. Introduction to Hearing:
The hearing mechanism in human beings (i.e., the human ear) is the final recipient of sound and noise produced by various sources. The acoustic engineers concerned with noise control must know, therefore, the range of frequencies and sound pressures to which this hearing mechanism responds.
They should also know the manner in which speech sounds and music must be presented to the listeners if they (i.e., the listeners) are to gain a satisfactory amount of information and pleasure from the audio signals. Some of the basic characteristics of hearing are covered in the following sections.
The physicists or engineers may approach the study of hearing as if they were considering a system whose behaviour was well understood. In the physical sciences we are guided in our choice of the quantities to be studied by the knowledge gained from theory and related experiments.
Moreover, it is generally possible (in the physical sciences) to hold constant all but a few of the independent variables, and to measure the behaviour of the system as a few of these variables are systematically varied. Also, in many physical systems, it is possible to superimpose the results of varying quantities individually to obtain the behaviour under conditions where several quantities are changing simultaneously.
The perception of sound, however, depends not only on the physical mechanism of hearing but also on certain psychological factors. In psychological studies also, we can measure (or at least rate) quantities such as just perceptible excitations, just noticeable differences, onset of pain, increased nervous activity, personal likes and dislikes, etc. However, the situation in psychology is somewhat different from that in physics.
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In the first place, there is very little theoretical body of knowledge in psychology to guide us as to what constitutes a valid measurement, or even as to which variables are important and which are ancillary. Secondly, it is very difficult in psychological measurements to hold part of the independent variables constant, and the principle of superposition does not hold good in general.
Experimenters in psychoacoustics soon find that responses to a stimulus presented to a listener under one set of conditions are not the same as the responses obtained to the same stimulus after the listener has acquired a different mental attitude towards the experiment, the experimenter, or something else.
The reader should bear in mind, therefore, that the data presented in the following sections were obtained by particular experimenters using particular stimuli, presented to listeners with particular mental biases, under particular ambient conditions.
It may so happen that other experimenters ostensibly repeating the same experiment may obtain substantially different results unless great care is taken to repeat all factors involved in the original experiment.
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2. Hearing Mechanism:
The sensation of hearing which results from a sensation of the hearing mechanism in human beings. The hearing mechanism comprises the mechanical parts of the ear, the auditory nervous system (including the brain), and the indicator of a response that the whole man represents.
The ear is so remarkable that no small physical instrument possesses properties any more remarkable than it. On one hand, the human ear can withstand the most intense sounds produced in nature, which have sound pressures of 103 to 104 dynes/cm2.
On the other hand, it also responds to sound pressures, at some frequencies, which are as small as 10-4 dynes/cm2. These very small sound pressures produce a displacement of the eardrum that is of the order of 10-11 m for frequencies around 1,000 Hz. This distance, in fact, is less than one-tenth the diameter of a hydrogen molecule.
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One should keep in mind, however, that this fantastic hearing mechanism is more than an extremely sensitive microphone. In fact, it functions also as an analyser of sound with considerable selectivity. For example, sounds of particular frequencies can be detected even in the presence of interfering background noises. Thus the hearing mechanism operates as though it were a set of contiguous “filter” binds.
Even more remarkable than its analysing capability is the ability of the auditory system to judge loudness, pitch, and musical quality of sound – the functions that are performed in some manner in association with the brain.
3. The Human Ear:
A simplified form of the human ear is shown in Fig. 1. The sound enters the ear through the auditory canal (also known as “external ear”). This canal has a diameter of about 0.7 cm and a length of approximately 2.7 cm.
The eardrum is a thin membrane terminating this canal. The eardrum has an area of about 0.8 cm2. Three small bones (or ossicles) known as hammer, anvil and stirrup, and joined together, are located in the middle ear.
The first of these small bones (i.e., the hammer) connects to the main eardrum, while the third (i.e., the stirrup) connects with a second membrane called the oval window. This oval window forms the entrance to the inner ear for the normal passage of sound.
The oval window is located at one end of the cochlea. The cochlea is a hollow, snail-shaped member formed from bone and filled with a colourless liquid. It is spiral-shaped with a length of about 3.5 cm.
The cross-sectional area of cochlea is approximately 4 mm2 at the stirrup end, decreasing to roughly 1 mm2 at the far end. The cochlea is divided in the middle by the Cochlear partition, which extends along the length of the cochlea. This partition is partly bony and partly a gelatinous membrane called the basilar membrane.
Surprisingly, the basilar membrane is smaller at the larger end of the cochlea. On the surface of the basilar membrane, there are approximately 25,000 nerve endings of the main auditory nerve. The liquid-filled chamber is further divided into two parts by a very thin membrane known as Reissner’s membrane.
In addition to the oval window, which communicates with the liquid-filled chamber above the cochlear partition, there is a round window which communicates with the lower liquid-filled chamber, and which acts as a pressure release. The area of the oval window is roughly 3 mm2, and that of the round window is about 2 mm2.
The volume of air contained in the middle ear is about 2 cm3. The mechanical advantage of the system of three small bones (the ossicles) in transferring sound vibrations from the eardrum to the oval window is about 1.3 to 1.
However, the effective ratio of areas of the eardrum and of the oval window times this mechanical advantage provides an increase in sound pressure from the eardrum to the liquid of the cochlea by a factor of about 15. This transformation is advantageous to the transfer of the vibrations of air to cochlear fluid.
A motion of the oval window produced by a motion of the eardrum sets up one or more waves that travel along the membrane and through the liquid, with the result that for each frequency there is a point of maximum excitation on the membrane.
The end of the membrane nearest the oval window “resonates” at the higher frequencies, while that at the far end of the spiral “resonates” at the lower frequencies. Thus the basilar membrane may be considered as a wide-band mechanical filter which partially separates a complex sound into its components.
The result is that a particular group of nerves is excited more vigorously by a particular frequency than by other frequencies.
4. Threshold of Hearing:
A threshold of hearing (also known as “threshold of audibility”) for a specified signal is the minimum effective sound pressure of that signal that is capable of evoking an auditory sensation (in the absence of any noise) in a specified fraction of trials. The threshold of hearing is usually expressed in decibels with respect to 0.0002 microbar as the reference pressure.
The question is sometimes asked as to how low or how high a frequency a person can hear. Tests have revealed that listeners appear to hear sounds even at very low frequencies, provided it is sufficiently loud. The lower limit for audibility is often cited as 20 Hz, and the sound with frequencies lower than this is known as infrasound.
If a listener with acute hearing is seated in an anechoic (i.e., echo-free) chamber facing a source of sound at a distance greater than 1 m, he can hear a pure tone of frequency 20 Hz if the sound pressure level is about 20 dB or more. On the other hand, a sound pressure level of 135 dB or more would be required to produce audibility at 2 Hz in a listener under the same conditions.
The upper limit of hearing is quite variable from person to person. It has been found that young people can hear up to 20 kHz if the tone is fairly intense. Middle-aged people, on the other hand, usually hear up to 12-16 kHz; but the level at which the tone is presented to them is again important.
The threshold of hearing varies with a great many factors. In the first place, it is different from person to person. Secondly, the threshold of audibility varies from day to day and from hour to hour, even for the same person.
It is well known, for example, that after an exposure to even a moderate noise level, there is a temporary shift of hearing threshold upward. One of the principal factors affecting the threshold of hearing is the age of the listener. Tables 1 and 2 show the results of studies of the progressive loss of aural sensitivity (i.e., upward shift of hearing threshold), for men and women, respectively, with increasing age.
Threshold of Tolerance:
At the other extreme of the hearing range, we are interested in the maximum sound levels which the ear can stand without discomfort, tickle, or pain. Listeners wearing earphones report that they begin to experience discomfort when a pure tone reaches sound pressure levels greater than 110 db. A tickling sensation is aroused in the ear when the levels are greater than 130 db.
Definite pain may occur when the sound pressure levels exceed 140 db. These values, moreover, appear to be nearly independent of frequency in the range between 50 Hz and 8 kHz. On the other hand, listeners who are exposed to high sound levels daily can stand about 10 dB more. These results are summarised in Table 3.
Similar results have been reported for noises which have a continuous spectrum (such as the white noise or random noise). In the case of a continuous wide-band frequency spectrum, however, the threshold is reached when the energy in any one critical bandwidth at some point along the frequency scale reaches the levels given in Table 3.
5. Pitch of Hearing:
Pitch is an important aspect of auditory sensation. In terms of pitch sounds may be ordered on a scale extending from “low” to “high”, such as a musical scale Pitch, however, is a subjective quality. It is mainly a function of the frequency of sound; but pitch is also dependent on the sound pressure level and the composition of sound.
Frequency, of course, is a physical quantity. Two tones of the same frequency, but with different sound pressure levels, will sound different in pitch. Thus, a 200-Hz tone of one level, for example, will sound as though it had a different frequency from a 200-Hz tone of another sound pressure level. It has also been found that a listener does not usually consider an octave in frequency as a doubling of pitch.
In order to measure pitch, an experiment is set up in which an observer is given two separate tone generators (oscillators). These generators are so arranged that they may be connected alternately to a loudspeaker or to earphones. The observer is asked to adjust the frequency of one of the oscillators until it seems to have twice the pitch of the other.
This procedure is repeated for a number of settings of the reference oscillator extending over the frequency range. The observer is also asked to increase and to decrease the pitch by factors other than two. From these data, a scale of pitch is developed. It should be noted here that we know very little about the pitch of complex sounds.
6. Loudness Levels of Hearing:
When we hear a sound, we often make a qualitative judgement of its “loudness.” A crash of thunder, for example, is said to be “extremely loud”, while the singing of a person in the distance may seem to be “not very loud”. Such qualitative expressions regarding the loudness of sound have been made quantitative for some kinds of sound.
The simplest way to judge quantitatively the loudness of a sound is to compare it with some standard sound. This standard sound has been chosen to be a 1 -kHz tone. The loudness level of any other sound is defined as the sound pressure level of a 1-kHz tone that sounds as loud as the sound in question. The unit of loudness level is known as phon.
For example, if a 1-kHz tone with a sound pressure level of 70 dB (re’ 0.002 microbar) sounds as loud as a certain square wave (regardless of the sound pressure level of the square wave), the square wave is said to have a loudness of 70 phones.
Extensive measurements have been carried out to determine the loudness levels of pure tones and narrow bands of noise as a function of frequency and sound pressure levels. The results of such measurements give equal-loudness contours (plots of sound pressure levels as functions of frequency for different levels of loudness).
The equal-loudness curves are especially useful in the design of radio receivers. In concert halls, music is usually played at fairly high levels. This music, when reproduced in our homes via the radio or record player, is often listened to at greatly reduced levels. Such music appears to have lost its bass quality because the human ear discriminates against low-frequency tones when the loudness levels are low.
To give the illusion that the music is being reproduced with the same tonal content as it had in the concert hall, radio designers install compensating electronic networks which change the response of the audio system in the radio receiver automatically when the volume control is adjusted, in the manner opposite to the way the human ear changes its response as the signal level is varied.
Such compensation is known as “frequency weighting”. The electrical circuitry that modifies the frequency response is called the “weighting network” or the “tone control.”
Loudness:
The concept of loudness level, as discussed above, is very useful; but it does not give the whole story. From the equal-loudness contours, for example, it is not possible to say how much louder one sound is than another. In order to answer this question, several extensive sets of measurements have been made to determine a scale of loudness.
Loudness may be defined as the intensive attribute of an auditory sensation, in terms of which sound may be ordered on a scale extending from “soft” to “loud.” Loudness is mainly a function of the sound pressure; but, in addition, it is also dependent on the frequency and wave form of the sound concerned.
The unit of loudness is known as sone. By definition, a pure tone of 1 kHz frequency, and 40 dB above a normal listener’s threshold, produces a loudness of 1 sone. In the measurements that led to a loudness scale, listeners were asked to change the loudness by factors of 2,10,0.5 and 0.1. From these data, a curve of loudness as a function of loudness level can be constructed, as shown in Fig. 2.
For higher sound pressure levels, as seen from Fig. 2, 10 dB change in the loudness level corresponds roughly to a doubling of the loudness in sones. At lower levels, however, a change of 10 dB corresponds to a change in loudness amounting to a factor of about 3. At the very lowest levels, on the other hand, 10 dB change in the loudness level corresponds to a change in loudness by a factors of almost 20.
The curve given in Fig. 2 is useful in the determination of the loudness level of a complex sound, given the loudness level of its components (or the spectrum level). This curve is often known as the “transfer function”.
7. Differential Sensitivity of Hearing:
A person is able to detect a change in sound pressure level of about 1 dB for any tone between 50 Hz and 10 kHz if the level of the tone is greater than 50 dB above the threshold for that particular tone.
Under ideal laboratory listening conditions, with signals supplied by an earphone, changes in the sound pressure level of as little as 0.3 dB can be detected by the ear in the middle-frequency range. For sound pressure levels less than 40 dB, however, changes in the level of 1 to 3 dB are necessary in order to be perceptible.
Similarly, for frequencies above 1 kHz and sound pressure levels in excess of 40 dB, the minimum perceptible change in frequency which the human ear can detect is of the order of about 0.3 per cent.
At frequencies below 1 kHz, and for the same range of sound pressure levels, the ear can detect a change in frequency of as little as about 3 Hz. At low pressure levels (and particularly at low frequencies), however, the minimum perceptible change in frequency may be many times these values.
8. Speech Spectrum:
The subject of speech communication is too large to be treated here and, moreover, beyond the scope of this work. However, a few the simpler attributes of speech signals will be discussed in this and the following sections.
Speech is a succession of utterances that produces a sound wave whose frequencies and amplitudes change rapidly with time. The human voice has, on the average a power spectrum that peaks (for men) at about 500 Hz, and an octave- band spectrum that drops off above 1 kHz at a rate of about 8 dB per octave. Moreover the human voice is directional at high frequencies.
Each syllable of speech lasts about 1/8 sec., and the average interval between syllables is approximately 0.1 sec. Some sounds (the vowels, for example) are produced at the vocal cords. Other sounds are produced by the noises of air movement through the mouth and over the tongue and lips. The frequency spectrum of either type of sound is shaped by the resonant cavities formed by the throat, mouth, teeth and lips.
The vowel sounds are not as critical to speech intelligibility as the consonant sounds. It is unfortunate that the consonant sounds are so weak and, consequently, are easily masked by noise. In some languages (in Hebrew, for examples) no vowels are written, only consonants.
When the long-time average speech spectrum is plotted in terms of its spectrum level (root-mean-square sound pressure in 1-Hz bands) as measured one metre in front of the talker, it appears as shown in Fig. 3 by the curve marked “average level of speech”.
Tests indicate that the useful dynamic range of speech in each frequency band appears to be about 30 db. Of this number, the root-mean- square peaks lie about 12 dB above the average speech level, while the weakest syllables lie about 1 dB below it.
Speech Intelligibility:
In attempting to estimate speech intelligibility, one must also take into account the properties of hearing mechanism. Tests have shown that the proper frequency scale to use as a base for calculations is one that is nearly proportion to the pitch scale.
In addition, one must also bring into such calculations the threshold of hearing and the “overload point” of the average hearing mechanism. Here the phrase overload point means the sound pressure level at each frequency above which the hearing mechanism no longer seems to respond to the stimulus.
All the factors discussed in the preceding paragraph are combined into one graph shown in Fig. 3, which shows as spectrum levels the following:
(a) The threshold of hearing for continuous-spectrum sounds;
(b) The peak, average and minimum levels of speech for a raised man’s voice measured at a distance of one metre directly in front of him; and
(c) The “overload spectrum level” for the ear.
If the spectrum levels of speech at the listener’s ear are such that the central shaded region of Fig. 3 lies above the threshold of hearing of the listener and above the ambient noise, but below the overload line, all syllables of the speech will be audible to the listener and the speech intelligibility will be nearly perfect.
This corresponds to an articulation index of 100 per cent. On the other hand, if the ambient noise covers part of the shaded speech region or if part of this region falls below the threshold of hearing or above the over load level for the hearing mechanisms, all syllables of speech will not be audible to the listener. In this case, the articulation index will be less than unity (i.e., less than 100 per cent).
The percentage articulation index may be defined as the ratio (times 100) of the speech area not covered over by the items mentioned in the preceding sentence (viz. the ambient noise, the threshold of hearing, and the region of overload) to the total speech area as shown in Fig. 3.
9. Psychological and Linguistic Factors of Hearing:
The prediction of sentence or word intelligibility is a very difficult task because it depends on many factors other than the noise level, reverberation time of the room, and the level of the signals. Experiments have shown, for example, that the score in the articulation index test increases with the signal-to-noise ratio.
Similarly, an improvement in articulation is shown as the number of sounds per word is increased. Different talkers and different listeners, however, yield different scores in such tests. Learning is also a very important factor in the result of an articulation tests.
It is obvious from the above discussion that speech-intelligibility tests should be conducted only after careful planning and, moreover, the statistical nature of the results should be fully appreciated. The involvement of various factors mentioned above indicates that absolute predictions of articulation test scores are not possible.
One can say, however, that if the calculated articulation index exceeds 60 per cent, a speech communication system is probably satisfactory. On the other hand, if the articulation index is less than 30%, the system is probably unsatisfactory. Between 30 and 60 per cent, the speech communication system should be viewed with suspicion and, if possible, detailed articulation tests should be performed.
10. Effect of Noise on Hearing:
Although there is still much to be learned about the relationship between noise exposure and the resulting hearing loss, some information on this aspect of noise pollution has already been accumulated through research and experimentation. It is known, for example, that many noise exposures can produce permanent hearing loss.
We know also that noise-induced hearing loss may be permanent or temporary or both, and a person will recover from a temporary hearing loss after he is away from the noise atmosphere for a period of time. The permanent hearing loss resulting from long-term exposure to noise, on the other hand, cannot be replaced.
The reason is that such hearing loss is due to the physical destruction of certain structures in the ear. Moreover, the amount of hearing loss produced by a given noise exposure usually varies from one individual to another.
Even if two persons are exposed to the same noise, such exposure may produce considerable hearing loss in one person, but none in the other. It is also known that noise-induced hearing loss first affects man’s hearing of sounds higher in frequency than those necessary for speech communication, i.e., above the frequency of 2 kHz.
It should be noted in this connection that different types of noises have different effects on the hearing mechanism. It is generally believed that noise of high intensity is more injurious to hearing when it is of high frequency than when the frequency is lower. For example, the horn of a diesel locomotive produces a sound of low frequency. The air siren, on the other hand, produces a high-frequency sound.
Noise-induced hearing loss depends upon noise levels as well as exposure time. It follows, therefore, that any attempt to assess the need for hearing conservation must take into account both of these factors. For example, a man exposed to high-intensity noise for hours a day will be more likely to incur hearing damage than a man exposed to the same noise for only one hour per day.
Unfortunately, the limits for injurious noise levels are still vague and uncertain, and the entire problem of hearing loss due to industrial noise requires much additional research and evaluation. We can, however, specify certain noise levels that indicate when it is advisable to initiate a hearing conservation programme, and also a programme for noise control, in an industry.
For example, when the noise is distributed more or less evenly throughout the eight octave bands (or frequency ranges) mentioned earlier, the noise exposure is regular (many hours each working day throughout the week), and if the noise level at 300-600 Hz or at 600-1,200 Hz is 85 dB or more, then it is time to introduce a programme of noise control.
Permissible daily exposure times for various noise levels are shown in Table 4. Protection against the effects of noise exposure should be provided to industrial workers when the sound levels exceed those shown in Table 4, when measured on the A-weighted scale of a standard sound level meter at slow response.
When noise levels are determined by octave-band analysis, the equivalent A-weighted sound level should be determined. This equivalent sound level, which may differ from the actual sound level of the noise (as measured by a sound level meter), should be used to determine the exposure limits from Table 4.
When industrial workers or other employees are subjected to noise levels exceeding the permissible limits shown in Table 4, feasible administrative or engineering controls should be utilised.
If such controls fail to reduce noise levels to within the limits of Table 4, personal hearing protective equipment should be provided and used to reduce the sound levels accordingly. In all cases where the noise levels exceed the limits shown in this table, a continuing effective hearing conservation programme should be administered.
11. Evaluating the Exposure to High Levels of Noise:
The initiation of a hearing conservation programme should be considered whenever the employees exposed to high levels of noise have:
(a) Difficulty in communicating by speech while they are in the presence of noise;
(b) Head noises or ringing in ears after working in the presence of noise for several hours; and
(c) A loss of hearing that has the effect of muffling speech and certain other sounds after several hours of exposure to the noise. (This hearing loss is temporary, and usually disappears in a few hours.)
Before initiating the necessary control measures, the first step to be taken is to survey the plant concerned in an effort to evaluate the potential noise hazard.
An approximate idea of the noise intensity may be obtained by walking through the plant with another person having normal hearing, and trying to carry on a conversation with him. When it is difficult or impossible to understand a loud voice at a distance of about 50 cm, a noise hazard may be assumed to exist.
The next step should be to determine which machines are producing excessive noise. It should also be determined which noises are necessary and which are unnecessary. Necessary noises are those which are an inevitable result productive processes, such as a noise produced at a machine’s point of operation. For example, the impact noise of a punch press is a necessary noise.
The unnecessary noises, on the other hand, are those which are generally due to faulty maintenance, worn machine parts, or other causes which can be remedied without involving any fundamental change in operations. The rattle of a loose guard is an example of unnecessary noise.
As an important aid in calculating the noise, reference should be made to available technical data on the average noise levels of the common tools of production in use by all the ordinary types of industry. It should also be determined how many employees are engaged in operations involving excessive, necessary noise; and how many others in the adjoining areas are exposed to it.
For example, four punch presses may be directly exposing four operators to excessive noise produced by these machines in operation.
In addition, 20 additional employees in the department, who themselves are not working on noisy operations, may also be indirectly exposed to the punch press noise. When a thorough survey has thus been conducted, and information such as that mentioned in the preceding paragraph been obtained, appropriate control measures can be more readily and accurately determined.
Proper control measures, fall into the following two categories:
(a) Medical Controls; and
(b) Engineering Controls.
These two types of controls must work together, since both are essential to a hearing conservation programme.
To be effective, a hearing conservation programme should include the following:
(a) A noise exposure analysis (evaluation of exposure);
(b) Provision for control of noise (engineering control); and
(c) Measurement of hearing acuity of employees (Medical control).
12. Reduction at the Ear:
In some situations, even after sound control measures have been applied, the noise level may still be too high because of the nature of the operation. The operators of punch presses, drop forge hammers, and chipping hammers, for example, cannot be protected by the noise-control measures.
It is necessary, in such cases, that the operators wear properly designed and fitted ear protectors which will reduce the intensity of sound reaching the hearing mechanism. This is as necessary for the protection of ears as is the use of safety goggles for the protection of eyes.
The amount of protection offered by a good hearing protection device varies somewhat with design; but the device may be considered to reduce the level of the noise reaching the bearing mechanism by about 25-30 db.
In most cases, a reduction of 25-30 dB will reduce the noise to a safe level. However, ear protection is still not understood too well by most people, and a great deal of education is necessary to make hearing protection programmes effective.
The principal purpose of all ear-protective equipment is to reduce the intensity of harmful noise before it reaches the inner ear. Without this protection, the energy which is produced as a result of noise gradually destroys the delicate nerve endings which pick up sound vibrations.
Experience in many industrial plants has proved almost conclusively that properly-fitted ear protectors, if worn consistently, will prevent hearing damage from hazardous noise levels.
In one plant, for example, where a large number of workers were exposed to the same noise levels, one group used hearing protectors, and the remainder did not use them. Audiometric measurements indicated later that the protected group had relatively unchanged hearing ability, whereas there was noticeable lessening of hearing ability in the unprotected group.
As a general rule, whenever workers are exposed regularly for many hours a day to noise levels above 85 dB (A) in the frequency range of 300-600 Hz or 600-1,200 Hz, the use of hearing protection devices is recommended.
Doctors are especially careful to advise workers with beginning or advanced hearing loss to wear such protective devices, and thus prevent further deterioration of hearing due to noise. A worker fitted properly with effective ear protection can work in almost any noisy environment with no danger to his hearing.
The responsibility for hearing protection rests with management. In most industrial plants, one individual should be given the task of following through with the details of hearing protection programme. There are many kinds of ear protective devices in the market.
They can be divided into two general classes:
(a) Insert-type protectors (earplugs); and
(b) Ear muffs and helmets.
Most of such protective devices seem to result in about the same amount of noise reduction, which is 20-35 decibels. When both helmet (or muff) and insert-type protectors are used, there is an additional 3-5 dB total reduction of the sound energy.
Insert-type ear protectors are made of cotton (impregnated with wax), rubber, or neoprene. In general, the material and shape of the insert-type devices have little to do with their effectiveness, except as they affect the personal likes or dislikes of the wearer. Contrary to popular opinion, dry cotton affords little or no protection for excessive noise.
Personal choice, as well as the degree of high-intensity noise, should govern the choice of muff, helmet, or insert-type ear protection, or a combination of them. To be fully effective, insert type protectors must be properly fitted, and must be used at all times of noise exposure.
Employees will be far more willing to wear hearing protectors when they feel that they have been given ample opportunity to select a type and size best suited to them. The best way is to show several types of protection to the prospective wearer, explain their features, allow him to examine their construction and feel their softness, and emphasize that whatever device he chooses will be personally fitted.
Supervisors and foremen can do a great deal in the course of the normal routine to promote hearing protection among employees. The all-important thing is that every employee who is sold on hearing protection is bound to spread some of his enthusiasm among his fellow workers.