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Chapter 9 noise in industry
Applied Ergonomics 1970, 1.4,217-222
Chapter 9 Noise in industry S o u n d and its m e a s u r e m e n t are explained. Also t h e e f f e c t s o f noise on h e a l t h and e f f i c i e n c y are discussed, w i t h w a y s o f controlling them.
What noise does in industry Noise is important in industry for three main reasons: people do not like it, it damages their hearing, and it has a bad effect on their working efficiency. These three effects are not necessarily related to one another. For instance, a noise which is very annoying may not be loud enough to damage hearing, or make people work less well. On the other hand, there is the rather alarming possibility that noises which do not annoy people at all may quite often damage their hearing or impair their efficiency. It is important therefore to distinguish the various effects from one another and not to assume that all is well just because the workers in a noisy factory seem to be contented. Equally, of course, a barrage of complaints about noise may tell one very little about the sounds that are going on in a factory, but perhaps rather more about the general level of morale. It is important therefore to have some idea of the effects which noise may have on people at work.
Sound and its measurement The vibration of a violin string, or a piece of machinery, produces a rapid rise and fall in the pressure of the surrounding air. These changes in pressure travel through the air in the form of waves and, if they strike the ear of a human being, he may hear a sound depending on the amplitude of the wave and how rapidly the source is vibrating. The power involved in sound waves is tiny; it may be less than a hundredth of that needed to run a domestic electric light bulb. All the same the effects on people listening to the sound may be serious. In order to talk about these effects one needs to introduce two technical terms used in measurements of sound. The first is hertz (Hz), the unit used to measure the frequency of a sound. In the simple case when waves are arriving at regular intervals at the ear, the frequency is the number of waves arriving each second; when the number is small one hears a low note and when it is large one hears a high-pitched note. It is only for frequencies between about 20 and 15 000 Hz that one hears a sound; much less power is needed to produce an audible sound at, say, 3 000 Hz than is needed at higher or, especially, at lower frequencies. Of course, many sounds in nature do not produce a completely regular series of waves; but it is possible to regard most of the complicated waves as being made up of a number of simple waves, each at a different frequency, and all added together. The noise of a machine contains low, intermediate and high frequencies in varying amounts, and one can say how much noise there is at each frequency.
Figs 9. la and 9.1 b
Both these men
are working in noisy conditions. Though one of them is obviously
annoyed about it, this does not necessarily mean that the other man is not suffering just as much, or more, from the damaging effects of excess noise; one cannot assume that all is well merely because he apl0ears contented.
Applied Ergonomics September 1970
217
The second technical term is the decibel (dB), which is the unit used to measure the intensity of a sound. The loudest sounds we may meet have an intensity of more than a million million times the intensity of the faintest sound we can hear. A scale of decibels takes this into account. It is a logarithmic scale and ensures that proportional changes in the intensity shall be covered by the same number of units: thus a tenfold increase, whether from 1 to 10, 10 to 100 or 100 to l 000, is represented by a change of 10 dB. A very wide range of intensity is covered by 130 dB, as shown in the following table. The dB level of typical sounds is also indicated (but this can be only a rough guide). Intensity
Equivalent decibels
10 000 000 000 000 1 000 000 000 000 1O0 000 000 000 l 0 000 000 000 1 000 000 000 100 000 000 10 000 000 l 000 000 100 000 10 000 1 000 100 10 1
130 120 110 100 90 80 70 60 5O 40 30 20 10 0
Typical sounds 135 dB Hydraulic press at 0-914 m (3 ft) 105 dB Jet taking offat 180 m (200 yds) 95 dB Automatic lathe at close range 75 dB Office machines between desks 65 dB Speech at 0-914 or 1-219 m (3 or 4 ft)
20 dB Whisper at 1"219 m (4 ft) 0 dB Threshold of hearing at 1 000 Hz
A useful rule in measuring intensity is that doubling the intensity corresponds to approximately 3 dB. Therefore: A 20-fold change = 10 + 3 = 13 dB. A 200-fold change = 20 + 3 = 23 dB.
0
0
It should also be noted that the smallest change appreciated by the ear is about 1 dB whether the change is in a faint sound or in a loud sound. This approximate relationship to the performance of the ear makes the decibel a convenient unit. It is necessary to remember that dB figures are usually quoted with reference to an arbitrary zero - actually 0"00002 newtons/m 2 (0.0002 dynes/cm 2 ) - which is approximately the faintest sound we can hear at 1 000 Hz.
0 Effects on the ear
Fig 9.2 One of the several laboratory experiments designed to show the effects of noise on the efficiency of various kinds of work. In this case, the task is to touch with a stylus the metal contacts corresponding to the particular light which is on. A man doing this task under noisy conditions makes mistakes.
218
Most people realize that a noise can harm their hearing if it is sufficiently loud, but the usually think in terms of some quite exceptionally violent sound, producing an effect as dramatic as the rupture of an ear drum. In fact, deafness can be produced in a much more insidious way, by continual exposure to noise which might well be regarded as acceptable in ordinary industrial life. The deafness in this case is not due to any effect on the ear drum, but results from damage to the delicate mechanism which converts the sound energy into impulses travelling up the nerves to the brain. There are several reasons why this damage may not be particularly obvious to the person who is suffering from it. Firstly, it often develops slowly. It may take ten years of exposure to a noise for eight hours a day before the effects become large enough to be serious. By that time the victim may well have forgotten what it was like to hear as well as he did in his youth: or if he does notice any difficulty in hearing he may put it down to his age. Deafness is a quite common accompaniment of age, but people who work in loud noise show a more marked impairment. Secondly, the kind of deafness which is produced by noise is one which may make it hard to hear faint sounds, but leaves the loudness of ordinary sounds more or less unimpaired, although it may distort them. The result is that the person who is being deafened hears conversation at normal loudness, and although he may think that people do not speak clearly, he does not suspect that he is getting deaf.
Applied Ergonomics September 1970
Yet another reason why people fail to notice that noise is making them deaf is that the effects may not be the same for all frequencies; if the noise is concentrated at only one frequency, deafness also will tend to be concentrated at one frequency. This is unusual because noises normally contain several frequencies, but the deafening effect will not be equally great over all frequencies. The sounds to which the ear becomes especially insensitive tend to be slightly higher in pitch than the noise to which it has been exposed, but the two are related. Measurements of the hearing acuity of large groups of men who have worked in noise all their lives reveal that hearing loss does occur. If the noise is one with a fairly wide range of frequencies, hearing loss often first occurs to sounds in the region of 4 000 Hz. The part of the ear which deals with such sounds seems to be especially vulnerable. This may suggest that noises which contain energy at a slightly lower frequency, say, about 2 000 Hz, are the most serious ones. Such noises will particularly tend to produce deafness in this vulnerable region. On the other hand, the vulnerable region is not the one in which most of the sounds of speech occur. They are somewhat lower in frequency - below 3 000 Hz - and to ensure good hearing at this frequency one must also take precautions against noises containing a considerable amount of energy at frequencies below 1 500 Hz. As a rough guide, the following table gives a minimum level in decibels for each octave. If this level is exceeded one should immediately suspect that the noise may be producing deafness and start the necessary action.
Fig 9.3 Another experimental task being performed under noise conditions. The man has to match the movement of one pointer by controlling the movement of the other through a lever on the right. The experiment takes place in a soundproofed room, and a variety of sounds are transmitted through the loudspeaker overhead.
Octave band specified as centre frequency Hz
Sound pressure level dB*
63 125 250 500 1 000 2 000 4 000
97 91 87 84 82 80 79
8 000
78
*Above a zero level of 0"00002 newtons/rn 2. From: W. Burns, 'Noise and Man', John Murray, 1968.
Untrt,oted treoted rocm$ t'oom$
Effects on work
Fig 9.4 In a study of work in a photographic factory, breakages in film attributed to human error were counted before and after acoustic treatment of the factory. The number of errors dropped markedly in the treated room.
8CklB
9OdB
IOOdB
Fig 9.5 Errors in a laboratory task when high and low frequency noise are compared at three intensities.
Some laboratory experiments have shown that noise lowers the efficiency of working, while other experiments have failed to do so. This is probably not a contradiction, but merely means that some types of work are easily affected while others are not. If, for example, one has to press a button when a light comes on, and one knows when this is likely to happen, one can probably do it just as well under noisy conditions as in quiet ones. Furthermore, if one happens to be feeling rather sleepy and nothing very much happens in the job to keep one awake, the noise may actually prevent drowsiness and so make one's reactions faster. The bad effects of noise come rather when one has to work under fairly stimulating conditions; if, for example, one has to pay attention to a large number of lights flashing rapidly in a random order so that one cannot relax for a second. (Fig 9.2) A number of experiments in the laboratory have shown that people make mistakes in their work when noise is applied. On the average, they do not seem to work any slower, although some individuals may do so. The main effect is on the accuracy of the work. Experimental studies in factories tend to confirm these findings: the effects of loud noise are shown in the increased wastage of material due to human error, or in the undue delays before stopped machines are noticed, rather than in a reduced average rate of work of the man. This kind of effect on work may be of no
Applied Ergonomics September 1970
219
......
'7
Fig 9.6a A typical sound level meter used in a working situation. economic importance in some jobs, but of great importance in others. It is also suspected, although nobody has yet proved it directly, that, if people are less accurate when working under noisy conditions, they may also be liable to make the kind of mistake which produces accidents. An important point to remember is that the effects of noise which have been proved have only appeared when the noise was very loud: in all the cases which have shown positive results, the sound pressure level of the noise has been greater than 90 dB above the usual zero. In other words, it is the sort of noise in which one just cannot make oneself heard no matter how loud one shouts. The level of 90 dB is not too far from the level which has to be regarded as a threat to hearing; and therefore, if precautions are taken to prevent deafness, they will also tend to prevent effects on working efficiency. Nevertheless one must remember that short spells of work under noisy conditions may reduce the efficiency of work even though they may not lead to permanent deafness.
Y
CALIBRATION D~RECTION SOUND USVEL
METER
R.S.J4S~ TYPE 1480G SERIAL No, Z7 OAW| INSTRUMENTSLTO MADIE JN ENGLAND
This level - 90 dB - is not at all unusual in industry, but it is very much higher than the level which people often complain about in offices and other places away from the factory floor. An obvious reason why a lower level is needed in offices is that conversation is difficult unless the noise is reduced well below 90 dB; indeed, 60 dB would be a more reasonable figure. If the work involves speech, as office work usually must, the tolerable limit of noise is bound to be much lower. T h e a n n o y i n g effects of noise
|
PUSH
FAST IN
ON/OFF
OUT
stow
a/~TTERy
_. i c.,c.
,?o
WEIGHTED'~ --4
li
,o + 60
On the whole, the louder a noise is, the more people complain about it. Even quite faint sounds, however, may annoy some people, and there are large differences between individuals in the kinds of noise which they find most objectionable. This makes it impossible to lay down firm rules about this aspect of noise, but, other things being equal, most people find high-pitched noises more annoying than low-pitched ones, and interrupted or sudden unexpected noises more annoying than steady prolonged ones. Sounds whose sources are unknown are als, especially irritating; and people often complain much more about a noise when they feel that it is unnecessary and due to thoughtlessness. This means that explanations and apologies may sometimes do more than anything else to reduce the annoyance caused by noise.
~@
l ]
F~LTE~ J
-B
J * ,so
so
~ ze
4e
jo
W h a t to do a b o u t noise -.,~_ SOuNo tEVEt
tN
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~ oux
:
OUTPUT
i1
Fig 9.6b A close up of the 1400 G sound-level meter illustrated in Fig 9.6a.
220
If the noise in some work-place seems rather loud to ordinary listening, so that speech becomes difficult, the first thing to do is to measure the sound level. This is is done by a meter, Fig 9.6b, which consists of a calibrated microphone capable of converting the sound into an electrical signal whose strength can then be read from a dial. Prices of such meters vary, but a not uncommon figure is £100. In addition, it is important to) be able to analyse the sound into its different frequencies and determine how much of the energy is in each part of the frequency range; and the equipment for doing this will add to the cost. Obviously
Applied Ergonomics September 1970
where the use of the equipment is likely to be infrequent, it is a good idea to share or borrow it, or to engage the services of a consultant who has such equipment at his disposal. The National Physical Laboratory can also advise on problems which arise in measuring the noise, and in appropriate cases will carry out detailed investigations and make recommendations. If the level of noise is too high; the best cure is to stop it at source, by paying attention to silencing, maintenance, and the mounting of vibrating machinery on isolating mounts to stop transmission of the sound away from the machine. When all this has been done, however, there are bound to be some sources of noise which are still to loud. Where possible they should be kept away from the workers - insulating walls should be put in between the noise and the man. Where this cannot be done, it may be worth while to increase the amount of soft absorbent surfaces in the room. Many factories have hard walls, floors and ceilings, which reflect the noise back and forth and so make it unnecessarily loud. With the common types of absorbent materials mistreatment can only reduce the noise to a limited extent. The reduction may be worthwhile if the noise is near the critical level, but treatment at source is preferable. In some cases the Building Research Station may be able to advise on methods of noise reduction. Even then, there may remain intractable cases where the noise is still up in the 100 dB region or so, above the danger levels akeady mentioned. The only answer then is for the workers to use individual ear protectors. The most widely known of these are, of course, ear plugs, and if properly fitted these can do a great deal of good. They are certainly a great improvement over the use of cotton wool and substances of that sort, which are not very effective in reducing the amount of noise reaching the ear. It is, however, difficult under industrial conditions to get ear plugs to fit each individual properly, and they are not so much use if they do not fit well. Another way of protecting the ears is to wear muffs, like a pair of headphones but without the phones themselves. Whichever kind of ear defender is used, the most that can be expected is that they will only reduce the noise by about 30 dB and at the high frequencies around 1 000 Hz. There is, therefore, a limit to the amount of noise to which people should be exposed, even with these protectors. Furthermore, if the noise is so high as to warrant protection, each man should have his hearing tested when he first starts work under these conditions, and should be tested again at intervals of six months or a year from then onwards. The reason for this is that some people are much more susceptible than others, while a few will manage to work in noise above the critical level without becoming deaf. Unfortunately, there is no way of predicting which individuals will fall into either group, and therefore one wants to look for deterioration of hearing as soon as it begins to appear. This question of noise as a hazard in industry is recognized as one of national importance. Recently the Ministry of Pensions and National Insurance sponsored a large-scale research project into certain aspects of occupational deafness, which was undertaken jointly by the Medical Research Council and the National Physical Laboratory (Burns and Robinson, 1970). The purpose of this study was to monitor hearing of people exposed to industrial noise, in order to establish the relation between noise and damage to hearing, and to make recommendations concerning hearing-conversation measures.
Figs 9.7a and 9.7b The exterior and interior of an enclosure for a high speed diesel generator.
Applied Ergonomics September 1970
221
The biggest problem, as anyone with industrial experience will realize, is how to make sure that people, who may be exposed to noise, realize the dangers and take the necessary trouble to protect themselves. Some people, who do not mind noise, tend to assert that it is quite harmless, and to neglect common sense precautions. The effects of noise above a certain level are real, and every precaution should be takn not only to minimize the risk of permanent deafness, but also to prevent needless inefficiency and annoyance.
Further reading
Acknowledgements Illustrations in chapter 9 were provided by the following sources: Figs 9.6a and 9.6b Dawe Instruments Ltd Figs 9.7a and 9.7b Burgess Products Co. Ltd
222
Anon 1960 'Noise in factories'. Factory Building Studies No. 6 HMSO. Beranek, L. L. 1960 'Noise reduction'. McGraw-Hill. Broadbent, D. E. 1957 Effects of noise on behaviour. In 'Handbook of noise control', edited by C. M. Harris. New York: McGraw-Hill. Broadbent, D. E. and Little, E. A. J. 1960 Occupational Psychology, 34.2, 133-140. Effects of noise reduction in a work situation. BS Code of Practice, CP3 1960 'Sound insulation and noise reduction' British Standards Institution. Burns, W., and Littler, T. S. 1960 Noise. In 'Modern trends in occupational health'. London: Butterworth. Bums, W. 1968 'Noise and man'. John Murray. Bums, W., and Robinson, D. W. 1970 'Hearing and noise in industry'. London: HMSO. Parkin, P. H., and Humphries, H. R. 1962 'Acoustics, noise and buildings'. Faber and Faber. Woodhead, M. M. 1960 The Manager, May. Research on industrial noise.
Applied Ergonomics September 1970
Chapter 9 Noise in industry S o u n d and its m e a s u r e m e n t are explained. Also t h e e f f e c t s o f noise on h e a l t h and e f f i c i e n c y are discussed, w i t h w a y s o f controlling them.
What noise does in industry Noise is important in industry for three main reasons: people do not like it, it damages their hearing, and it has a bad effect on their working efficiency. These three effects are not necessarily related to one another. For instance, a noise which is very annoying may not be loud enough to damage hearing, or make people work less well. On the other hand, there is the rather alarming possibility that noises which do not annoy people at all may quite often damage their hearing or impair their efficiency. It is important therefore to distinguish the various effects from one another and not to assume that all is well just because the workers in a noisy factory seem to be contented. Equally, of course, a barrage of complaints about noise may tell one very little about the sounds that are going on in a factory, but perhaps rather more about the general level of morale. It is important therefore to have some idea of the effects which noise may have on people at work.
Sound and its measurement The vibration of a violin string, or a piece of machinery, produces a rapid rise and fall in the pressure of the surrounding air. These changes in pressure travel through the air in the form of waves and, if they strike the ear of a human being, he may hear a sound depending on the amplitude of the wave and how rapidly the source is vibrating. The power involved in sound waves is tiny; it may be less than a hundredth of that needed to run a domestic electric light bulb. All the same the effects on people listening to the sound may be serious. In order to talk about these effects one needs to introduce two technical terms used in measurements of sound. The first is hertz (Hz), the unit used to measure the frequency of a sound. In the simple case when waves are arriving at regular intervals at the ear, the frequency is the number of waves arriving each second; when the number is small one hears a low note and when it is large one hears a high-pitched note. It is only for frequencies between about 20 and 15 000 Hz that one hears a sound; much less power is needed to produce an audible sound at, say, 3 000 Hz than is needed at higher or, especially, at lower frequencies. Of course, many sounds in nature do not produce a completely regular series of waves; but it is possible to regard most of the complicated waves as being made up of a number of simple waves, each at a different frequency, and all added together. The noise of a machine contains low, intermediate and high frequencies in varying amounts, and one can say how much noise there is at each frequency.
Figs 9. la and 9.1 b
Both these men
are working in noisy conditions. Though one of them is obviously
annoyed about it, this does not necessarily mean that the other man is not suffering just as much, or more, from the damaging effects of excess noise; one cannot assume that all is well merely because he apl0ears contented.
Applied Ergonomics September 1970
217
The second technical term is the decibel (dB), which is the unit used to measure the intensity of a sound. The loudest sounds we may meet have an intensity of more than a million million times the intensity of the faintest sound we can hear. A scale of decibels takes this into account. It is a logarithmic scale and ensures that proportional changes in the intensity shall be covered by the same number of units: thus a tenfold increase, whether from 1 to 10, 10 to 100 or 100 to l 000, is represented by a change of 10 dB. A very wide range of intensity is covered by 130 dB, as shown in the following table. The dB level of typical sounds is also indicated (but this can be only a rough guide). Intensity
Equivalent decibels
10 000 000 000 000 1 000 000 000 000 1O0 000 000 000 l 0 000 000 000 1 000 000 000 100 000 000 10 000 000 l 000 000 100 000 10 000 1 000 100 10 1
130 120 110 100 90 80 70 60 5O 40 30 20 10 0
Typical sounds 135 dB Hydraulic press at 0-914 m (3 ft) 105 dB Jet taking offat 180 m (200 yds) 95 dB Automatic lathe at close range 75 dB Office machines between desks 65 dB Speech at 0-914 or 1-219 m (3 or 4 ft)
20 dB Whisper at 1"219 m (4 ft) 0 dB Threshold of hearing at 1 000 Hz
A useful rule in measuring intensity is that doubling the intensity corresponds to approximately 3 dB. Therefore: A 20-fold change = 10 + 3 = 13 dB. A 200-fold change = 20 + 3 = 23 dB.
0
0
It should also be noted that the smallest change appreciated by the ear is about 1 dB whether the change is in a faint sound or in a loud sound. This approximate relationship to the performance of the ear makes the decibel a convenient unit. It is necessary to remember that dB figures are usually quoted with reference to an arbitrary zero - actually 0"00002 newtons/m 2 (0.0002 dynes/cm 2 ) - which is approximately the faintest sound we can hear at 1 000 Hz.
0 Effects on the ear
Fig 9.2 One of the several laboratory experiments designed to show the effects of noise on the efficiency of various kinds of work. In this case, the task is to touch with a stylus the metal contacts corresponding to the particular light which is on. A man doing this task under noisy conditions makes mistakes.
218
Most people realize that a noise can harm their hearing if it is sufficiently loud, but the usually think in terms of some quite exceptionally violent sound, producing an effect as dramatic as the rupture of an ear drum. In fact, deafness can be produced in a much more insidious way, by continual exposure to noise which might well be regarded as acceptable in ordinary industrial life. The deafness in this case is not due to any effect on the ear drum, but results from damage to the delicate mechanism which converts the sound energy into impulses travelling up the nerves to the brain. There are several reasons why this damage may not be particularly obvious to the person who is suffering from it. Firstly, it often develops slowly. It may take ten years of exposure to a noise for eight hours a day before the effects become large enough to be serious. By that time the victim may well have forgotten what it was like to hear as well as he did in his youth: or if he does notice any difficulty in hearing he may put it down to his age. Deafness is a quite common accompaniment of age, but people who work in loud noise show a more marked impairment. Secondly, the kind of deafness which is produced by noise is one which may make it hard to hear faint sounds, but leaves the loudness of ordinary sounds more or less unimpaired, although it may distort them. The result is that the person who is being deafened hears conversation at normal loudness, and although he may think that people do not speak clearly, he does not suspect that he is getting deaf.
Applied Ergonomics September 1970
Yet another reason why people fail to notice that noise is making them deaf is that the effects may not be the same for all frequencies; if the noise is concentrated at only one frequency, deafness also will tend to be concentrated at one frequency. This is unusual because noises normally contain several frequencies, but the deafening effect will not be equally great over all frequencies. The sounds to which the ear becomes especially insensitive tend to be slightly higher in pitch than the noise to which it has been exposed, but the two are related. Measurements of the hearing acuity of large groups of men who have worked in noise all their lives reveal that hearing loss does occur. If the noise is one with a fairly wide range of frequencies, hearing loss often first occurs to sounds in the region of 4 000 Hz. The part of the ear which deals with such sounds seems to be especially vulnerable. This may suggest that noises which contain energy at a slightly lower frequency, say, about 2 000 Hz, are the most serious ones. Such noises will particularly tend to produce deafness in this vulnerable region. On the other hand, the vulnerable region is not the one in which most of the sounds of speech occur. They are somewhat lower in frequency - below 3 000 Hz - and to ensure good hearing at this frequency one must also take precautions against noises containing a considerable amount of energy at frequencies below 1 500 Hz. As a rough guide, the following table gives a minimum level in decibels for each octave. If this level is exceeded one should immediately suspect that the noise may be producing deafness and start the necessary action.
Fig 9.3 Another experimental task being performed under noise conditions. The man has to match the movement of one pointer by controlling the movement of the other through a lever on the right. The experiment takes place in a soundproofed room, and a variety of sounds are transmitted through the loudspeaker overhead.
Octave band specified as centre frequency Hz
Sound pressure level dB*
63 125 250 500 1 000 2 000 4 000
97 91 87 84 82 80 79
8 000
78
*Above a zero level of 0"00002 newtons/rn 2. From: W. Burns, 'Noise and Man', John Murray, 1968.
Untrt,oted treoted rocm$ t'oom$
Effects on work
Fig 9.4 In a study of work in a photographic factory, breakages in film attributed to human error were counted before and after acoustic treatment of the factory. The number of errors dropped markedly in the treated room.
8CklB
9OdB
IOOdB
Fig 9.5 Errors in a laboratory task when high and low frequency noise are compared at three intensities.
Some laboratory experiments have shown that noise lowers the efficiency of working, while other experiments have failed to do so. This is probably not a contradiction, but merely means that some types of work are easily affected while others are not. If, for example, one has to press a button when a light comes on, and one knows when this is likely to happen, one can probably do it just as well under noisy conditions as in quiet ones. Furthermore, if one happens to be feeling rather sleepy and nothing very much happens in the job to keep one awake, the noise may actually prevent drowsiness and so make one's reactions faster. The bad effects of noise come rather when one has to work under fairly stimulating conditions; if, for example, one has to pay attention to a large number of lights flashing rapidly in a random order so that one cannot relax for a second. (Fig 9.2) A number of experiments in the laboratory have shown that people make mistakes in their work when noise is applied. On the average, they do not seem to work any slower, although some individuals may do so. The main effect is on the accuracy of the work. Experimental studies in factories tend to confirm these findings: the effects of loud noise are shown in the increased wastage of material due to human error, or in the undue delays before stopped machines are noticed, rather than in a reduced average rate of work of the man. This kind of effect on work may be of no
Applied Ergonomics September 1970
219
......
'7
Fig 9.6a A typical sound level meter used in a working situation. economic importance in some jobs, but of great importance in others. It is also suspected, although nobody has yet proved it directly, that, if people are less accurate when working under noisy conditions, they may also be liable to make the kind of mistake which produces accidents. An important point to remember is that the effects of noise which have been proved have only appeared when the noise was very loud: in all the cases which have shown positive results, the sound pressure level of the noise has been greater than 90 dB above the usual zero. In other words, it is the sort of noise in which one just cannot make oneself heard no matter how loud one shouts. The level of 90 dB is not too far from the level which has to be regarded as a threat to hearing; and therefore, if precautions are taken to prevent deafness, they will also tend to prevent effects on working efficiency. Nevertheless one must remember that short spells of work under noisy conditions may reduce the efficiency of work even though they may not lead to permanent deafness.
Y
CALIBRATION D~RECTION SOUND USVEL
METER
R.S.J4S~ TYPE 1480G SERIAL No, Z7 OAW| INSTRUMENTSLTO MADIE JN ENGLAND
This level - 90 dB - is not at all unusual in industry, but it is very much higher than the level which people often complain about in offices and other places away from the factory floor. An obvious reason why a lower level is needed in offices is that conversation is difficult unless the noise is reduced well below 90 dB; indeed, 60 dB would be a more reasonable figure. If the work involves speech, as office work usually must, the tolerable limit of noise is bound to be much lower. T h e a n n o y i n g effects of noise
|
PUSH
FAST IN
ON/OFF
OUT
stow
a/~TTERy
_. i c.,c.
,?o
WEIGHTED'~ --4
li
,o + 60
On the whole, the louder a noise is, the more people complain about it. Even quite faint sounds, however, may annoy some people, and there are large differences between individuals in the kinds of noise which they find most objectionable. This makes it impossible to lay down firm rules about this aspect of noise, but, other things being equal, most people find high-pitched noises more annoying than low-pitched ones, and interrupted or sudden unexpected noises more annoying than steady prolonged ones. Sounds whose sources are unknown are als, especially irritating; and people often complain much more about a noise when they feel that it is unnecessary and due to thoughtlessness. This means that explanations and apologies may sometimes do more than anything else to reduce the annoyance caused by noise.
~@
l ]
F~LTE~ J
-B
J * ,so
so
~ ze
4e
jo
W h a t to do a b o u t noise -.,~_ SOuNo tEVEt
tN
nl~
~ oux
:
OUTPUT
i1
Fig 9.6b A close up of the 1400 G sound-level meter illustrated in Fig 9.6a.
220
If the noise in some work-place seems rather loud to ordinary listening, so that speech becomes difficult, the first thing to do is to measure the sound level. This is is done by a meter, Fig 9.6b, which consists of a calibrated microphone capable of converting the sound into an electrical signal whose strength can then be read from a dial. Prices of such meters vary, but a not uncommon figure is £100. In addition, it is important to) be able to analyse the sound into its different frequencies and determine how much of the energy is in each part of the frequency range; and the equipment for doing this will add to the cost. Obviously
Applied Ergonomics September 1970
where the use of the equipment is likely to be infrequent, it is a good idea to share or borrow it, or to engage the services of a consultant who has such equipment at his disposal. The National Physical Laboratory can also advise on problems which arise in measuring the noise, and in appropriate cases will carry out detailed investigations and make recommendations. If the level of noise is too high; the best cure is to stop it at source, by paying attention to silencing, maintenance, and the mounting of vibrating machinery on isolating mounts to stop transmission of the sound away from the machine. When all this has been done, however, there are bound to be some sources of noise which are still to loud. Where possible they should be kept away from the workers - insulating walls should be put in between the noise and the man. Where this cannot be done, it may be worth while to increase the amount of soft absorbent surfaces in the room. Many factories have hard walls, floors and ceilings, which reflect the noise back and forth and so make it unnecessarily loud. With the common types of absorbent materials mistreatment can only reduce the noise to a limited extent. The reduction may be worthwhile if the noise is near the critical level, but treatment at source is preferable. In some cases the Building Research Station may be able to advise on methods of noise reduction. Even then, there may remain intractable cases where the noise is still up in the 100 dB region or so, above the danger levels akeady mentioned. The only answer then is for the workers to use individual ear protectors. The most widely known of these are, of course, ear plugs, and if properly fitted these can do a great deal of good. They are certainly a great improvement over the use of cotton wool and substances of that sort, which are not very effective in reducing the amount of noise reaching the ear. It is, however, difficult under industrial conditions to get ear plugs to fit each individual properly, and they are not so much use if they do not fit well. Another way of protecting the ears is to wear muffs, like a pair of headphones but without the phones themselves. Whichever kind of ear defender is used, the most that can be expected is that they will only reduce the noise by about 30 dB and at the high frequencies around 1 000 Hz. There is, therefore, a limit to the amount of noise to which people should be exposed, even with these protectors. Furthermore, if the noise is so high as to warrant protection, each man should have his hearing tested when he first starts work under these conditions, and should be tested again at intervals of six months or a year from then onwards. The reason for this is that some people are much more susceptible than others, while a few will manage to work in noise above the critical level without becoming deaf. Unfortunately, there is no way of predicting which individuals will fall into either group, and therefore one wants to look for deterioration of hearing as soon as it begins to appear. This question of noise as a hazard in industry is recognized as one of national importance. Recently the Ministry of Pensions and National Insurance sponsored a large-scale research project into certain aspects of occupational deafness, which was undertaken jointly by the Medical Research Council and the National Physical Laboratory (Burns and Robinson, 1970). The purpose of this study was to monitor hearing of people exposed to industrial noise, in order to establish the relation between noise and damage to hearing, and to make recommendations concerning hearing-conversation measures.
Figs 9.7a and 9.7b The exterior and interior of an enclosure for a high speed diesel generator.
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The biggest problem, as anyone with industrial experience will realize, is how to make sure that people, who may be exposed to noise, realize the dangers and take the necessary trouble to protect themselves. Some people, who do not mind noise, tend to assert that it is quite harmless, and to neglect common sense precautions. The effects of noise above a certain level are real, and every precaution should be takn not only to minimize the risk of permanent deafness, but also to prevent needless inefficiency and annoyance.
Further reading
Acknowledgements Illustrations in chapter 9 were provided by the following sources: Figs 9.6a and 9.6b Dawe Instruments Ltd Figs 9.7a and 9.7b Burgess Products Co. Ltd
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Anon 1960 'Noise in factories'. Factory Building Studies No. 6 HMSO. Beranek, L. L. 1960 'Noise reduction'. McGraw-Hill. Broadbent, D. E. 1957 Effects of noise on behaviour. In 'Handbook of noise control', edited by C. M. Harris. New York: McGraw-Hill. Broadbent, D. E. and Little, E. A. J. 1960 Occupational Psychology, 34.2, 133-140. Effects of noise reduction in a work situation. BS Code of Practice, CP3 1960 'Sound insulation and noise reduction' British Standards Institution. Burns, W., and Littler, T. S. 1960 Noise. In 'Modern trends in occupational health'. London: Butterworth. Bums, W. 1968 'Noise and man'. John Murray. Bums, W., and Robinson, D. W. 1970 'Hearing and noise in industry'. London: HMSO. Parkin, P. H., and Humphries, H. R. 1962 'Acoustics, noise and buildings'. Faber and Faber. Woodhead, M. M. 1960 The Manager, May. Research on industrial noise.
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