THE ASSESSMENT OF OCCUPATIONAL NOISE EXPOSURE

Transcription

1 Ann. occup. Hy%. Vol. 16, pp Pergamon Pren Printed in Great Britain THE ASSESSMENT OF OCCUPATIONAL NOISE EXPOSURE A. M. MARTIN Hearing Conservation Unit, Institute of Sound and Vibration Research, University of Southampton Abstract A brief survey is given of three standard methods for the assessment of occupational noise exposure currently available in the U.K. Equipment and measurement techniques are described for the determination of equivalent continuous sound level for industrial noise. It is accepted that the "equal energy principle" provides a simple and unified system for the assessment of damage risk and the conclusion is drawn that the noise dosemeter provides a relatively simple method of noise measurement. INTRODUCTION THE PAST decade has been a period of changing social attitudes towards occupational deafness. These changes have culminated in the rapidly growing awareness of the problems involved in the measurement of noise and the prevention of noise-induced hearing loss. This situation has been brought about by two major contributory factors. The establishment and acceptance, in the majority of European countries, of a single system for assessing the hazard to hearing from industrial noise; and several civil court cases in the U.K. in which employees have sued employers for damages for noise-induced hearing loss (COLES and MARTIN, 1973). The result has been the recognition of the need for a simple system of noise measurement and assessment which is applicable to all types of industrial noise. This paper gives a brief survey of the standard methods for the assessment of noise exposure currently available. It also considers some of the equipment available to the industrial hygiene laboratory and discusses some of the problems involved in the measurement of noise. THE ASSESSMENT OF NOISE A great deal of time has been spent over the past three decades in an attempt to formulate methods for quantifying the relationship between noise exposure and hearing loss. Initially much of the work was based on laboratory experiments involving short-term hearing losses such as temporary threshold shift. However, the relationship between temporary hearing loss and persistent hearing loss caused by long-term exposure to noise in 'industry has since been doubted (see for example MARTIN, 1970; WARD, 1970; WALKER, 1970). In the last few years data have become available that have enabled persistent hearing losses to be correlated directly to long-term noise exposure. Much of the impetus for this work has been derived from an industrial survey carried out jointly by the Medical Research Council and National Physical Laboratory and reported by BURNS and ROBINSON (1970). This work has since formed the basis for a method of assessing the hazard to hearing from industrial noise. 353

2 354 A. M. MARTIN The equal energy principle BURNS and ROBINSON (1970) have shown for steady state noise that a relationship exists between A-weighted sound energy and persistent hearing loss. This is known as the "equal energy principle" and assumes that equal amounts of A-weighted sound energy cause equal amounts of damage to hearing. Subsequent research by ATHERLEY and MARTIN (1971) has shown that this principle may also be extended to include industrial impact noise. More recently RICE and MARTIN (1973) have concluded that the equal energy principle may also be applied to the assessment of risk to hearing from high-level transient noises such as gunfire. Consequently a single and relatively simple principle may now be utilized as the basis for the assessment of the majority of occupational noises. A-weighted sound energy and the energy principle may be considered to be the unifying factors in what up to the present time has been a somewhat confused area of knowledge. The equal energy concept is embodied in an equivalent-continuous sound level, L e q, which may be defined as the level of continuous noise, in db(a), which in the course of a working day would cause the same sound energy to be received as that due to actual noise over a typical day. The parameter Leq may also be considered as a measure of noise dose, a concept which is familiar to many other aspects of occupational medicine but relatively new to the field of noise. A-weighted sound energy received during noise exposure may be deduced from the product of the noise level in db(a) and the duration of exposure. A doubling of sound energy represents an increase in noise level of 3 db. Thus, for example, exposure to 90 db(a) for a given period is equivalent in terms of sound energy, and therefore hazard, to an exposure to 93 db(a) for half that period and so on. Table 1 gives values of sound level and exposure duration which, when considered together, represent an L e q of 90 db(a) for 8 hr. These figures are considered at the present time to be maximum permissible limits for exposure to noise, and form the basis of the current standard documents available in this country for the assessment of industrial noise. TABLE 1. VALUES OF SOUND LEVEL AND EXPOSURE DURATION CORRESPONDING TO AN L eq OF 90 bb(a) FOR 8 HR Sound level [db(a)] Permissible duration of exposure per day 8hr min 15 7* 225 sec * n less than 1 sec

3 The assessment of occupational noise exposure 355 Standard methods of assessment In Britain at the present time there are three authoritative documents available that describe methods for the assessment of hazard to hearing from occupational noise exposure based upon an energy concept (WALKER and MARTIN, in Press): 1. The Code of Practice for Reducing the Exposure of Employed Persons to Noise, prepared in 1971 by the Industrial Health Advisory Committee's Sub-Committee on Noise, and published in 1972 by HMSO on behalf of the Department of Employment. 2. The British Occupational Hygiene Society (BOHS) Hygiene Standard for Wide-Band Noise, published in The International Organisation for Standardisation (ISO) Recommendation R1999 Assessment of Occupational Noise Exposure for Hearing Conservation Purposes, published in The Code of Practice is perhaps the most suitable document for application to the industrial situation in the United Kingdom at present. However, it gives only a single demarkation between acceptable exposures and those which are unacceptable, whereas the ISO and BOHS standards provide more detailed information regarding the percentage of persons who would be expected to exceed specified hearing losses after given noise exposure. The Code specifies a limit for exposure to noise of an L eq of 90 db(a) and describes methods of measurement which can be used to determine whether the limit is exceeded. It makes the assumption that steady-state, fluctuating, intermittent and impulse noise may all be described in terms of Leq. Furthermore, it recommends overriding limits for the unprotected ear, these being a sound pressure level of 135 db for steady-state noise and 150 db for impulse noise. The Code is important in Britain because it specifies safe conditions of work. Although it is as yet only advisory in nature, H.M. Factory Inspectorate relies upon its recommendations to define the auditory safety of a working environment. The BOHS and ISO documents both set down values for noise exposure that will result in no more than a specified proportion of the exposed population suffering no more than a specified amount of hearing loss. The number of persons who will suffer these hearing losses is expressed as a percentage of the total exposed population since it is impossible at the present time to predict accurately, prior to the noise exposure, the effect that noise will have upon an individual. The British Occupational Hygiene Society's document refers solely to steadystate noise and is intended to restrict occupational exposure to noise so that handicap does not occur in more than 1 per cent of persons exposed during their working lifetime. This objective is considered to be achieved if the noise-induced impairment to hearing at the end of a 30 yr working lifetime does not exceed levels of 40 db calculated as an average of audiometric frequencies, 0-5, 1, 2, 3 and 6 khz. To meet this criterion noise immission should not exceed 105 db(a), that is noise levels should not exceed an L eq of 90 db(a). The BOHS Hygiene Standards also states that it must not be used for impulse noise or if there are significant pure tones present in the noise (the BOHS is at present considering a hygiene standard for impulse noise). ISO recommendation R1999 does not quantify absolute levels which must not be exceeded. Rather it gives a practical relationship between occupational noise exposure, expressed in terms of noise level and duration within a normal working week of

4 356 A. M. MARTIN 40 hr, and the risk of increase in percentage of persons in specified age groups that may be expected to show hearing impairment as a result of specified amounts of occupational noise exposure. Hearing is considered to be impaired for conversational speech if the arithmetic average of hearing levels for the frequencies 0-5, 1 and 2 khz is 25 db or more. The Recommendation does not set down actual limits for habitual noise exposure. Nevertheless, a level of 90 db(a) is mentioned as one commonly found in those countries having laws or regulations prohibiting hazardous noise exposure. Impulse noises of less than 1 second duration or single high-level transients such as gunfire are excluded. Although the definitions of handicap and impairment are different in the BOHS and ISO documents, they may be assumed to be equivalent in terms of hazard to hearing. That is, the same noise level assessed by the two methods will probably result in the prediction of the same hearing losses. However, they do not employ the same units of exposure time nor do they cover precisely the same range of exposures, as shown in Table 2. The BOHS document assumes habitual exposure of 8 hrs per day, 5 days per week, 48 weeks per year for a working lifetime of 30 years, and allows for certain durations which differ from this total of 1920 hr per year. The lower limit to these adjustments is not clearly stated but the standard does provide means for the assessment of regular exposures down to at least 30 min duration per day, i.e. a total of 120 hr per year. The upper limit of exposure duration is 12 hr per day, i.e. 60 hr per week or a total of 2880 hr per year. The ISO Recommendation describes noise exposure in terms of a 40 hr week for 50 weeks of the year for a variable time of up to 45 yr, again with some provision for shorter durations. The minimum exposure duration is 10 min per week for a maximum permissible sound level of 114 db(a). No special allowance is made for exposures which do not occur regularly throughout the year or for exposures greater than 40 hr per week. Table 2 compares these specifications of exposure time in terms of the maximum permissible sound level, in db(a). The Code of Practices does not directly specify maximum or minimum limits for exposure duration. TABLE 2. COMPARISON OF BOHS AND ISO SPECIFI- CATIONS OF EXPOSURE TIME BOHS hr/day Maximum permissible sound level db(a) ISO Duration week 40 hr min

5 The assessment of occupational noise exposure 357 METHODS OF MEASUREMENT OF L eq Whichever standard specification of damage risk is employed, the fundamental measure of an individual's noise exposure is Leq. This in turn entails the measurement of both the sound level and duration of the noise. This section describes briefly the instrumentation necessary for the measurement of different types of noise and the techniques of measurement to be applied in practice. Measurement techniques are categorised in terms of the necessary instrumentation as opposed to the types of noise measured. This approach is designed to provide a brief guide for the measurement of noise for those laboratories in industry which already possess certain types of commercially available measurement equipment and to indicate possible future requirements. A block diagram summarizing the basic approach to the problem and the essential equipment required is given in Fig. 1. The ability of different types of equipment to measure both steady state and impact noise will also be considered. MOOPHONE AMPLIFIER DOSEMETER SLM STORAGE OSCILLOSCOPE TAPE RECORDER FIG. 1. Block diagram of basic alternative equipment systems available for the measurement of industrial noise. The sound level meter The measurement of L e q for steady-state noise may be made simply with a precision sound level meter (SLM) conforming to the specification recommended by the International Electrotechnical Commission (IEC 179, 1965) or British Standard (BS4197, 1967) and a timepiece. The meter should be set to the A-weighting network and "slow" dynamic characteristic. Ideally, measurements of the sound level and exposure time should be made in close proximity to the ears of the personnel exposed to the noise but in reverberant conditions small displacements will not make significant differences to the measured levels. When the noise is steady in level and unbroken duration, a few measurements of the level should provide a reliable value of Leq. Short-term fluctuations in sound level of no more than 10 db, can be dealt with approximately by "eye-averaging" the meter reading. However, if the sound level varies by more than 10 db or varies more slowly with time, such as the noise produced by many cyclic industrial processes, the individual values of A-weighted sound level and "on" time should be recorded for each particular sound level. These components may then be summed to give a composite value for Leq using one of the standard techniques, such as those described in the BOHS Hygiene Standard (1971) or the Code of Practice (1971).

6 358 A. M. MARTIN In many practical situations, noise levels fluctuate considerably as various industrial processes, possibly operating in the same area, generate noise with different temporal patterns. It may be necessary in such cases to carry out work-study procedures with a SLM to establish a reliable estimate of L eq. The situation is further complicated by the requirement to obtain a measure of the noise dose received by an individual. If an individual remains in the same location during the working day, then his noise dose may be assessed relatively simply. However, if he moves around from quiet to relatively noisy surroundings in a random fashion both during the day and from day to day, such as maintenance workers for example, the measurement of Leq with a SLM for that individual becomes an extremely complicated procedure. The need to monitor a number of individuals' noise doses probably makes the use of a SLM impractical in such circumstances. The measurement of Le Q for impulse noise is not easily achieved with a SLM. This is because measures are required of both sound level and time and it is not usually possible to measure both the peak sound level and the duration of an impulse with a SLM and a simple clock or watch. There is available a precision impulse-sound level meter (ISLM), conforming to the German Standard DIN (1968) which incorporates an "impulse" mode feature. In this mode the ISLM integrates sound energy over a period of 35 msec and is intended to give a measure of the subjective loudness of impulse sounds. It does not give a measure of the true peak value of an impulse in this mode (MARTIN et ai, 1973). However, some SLMs also incorporate a true "peak" reading mode and hence do provide an accurate value for the peak sound level in this case. It is important to distinguish between "impulse" and "peak" reading SLMs when measuring impulse noise. Although it is possible to obtain a measure of the peak sound level of an impulse with certain types of SLM, this is not enough information to enable a reliable estimate of oq to be made. There is still the question of the impulse duration. Research has been carried out into the relationship between measurements of impulse noise with an ISLM complying with DIN in the "impulse" mode and the Leq for that noise deduced by other methods. HEMPSTOCK, et ai (1972) investigated this relationship for repetitive impulses. They calculated adjustments to be made to the "impulse hold" meter reading of an ISLM for various impulse noise durations and repetition rates to enable Leq to be estimated. More recently WALKER et al. (1973) compared the Leq of single short-duration impulses obtained with oscilloscopic techniques with measurements made with an ISLM in the "impulse hold" mode. Their results suggest that there is a linear relationship between Leq and the ISLM readings. Further work is in progress to evaluate the accuracy of the method. These findings are important as they indicate the possibility of obtaining an approximate estimate of Leq for impulse noise with an ISLM. Thus, those laboratories in industry which already possess such instruments may be able to assess certain types of impulse noise by this method in the future. The noise dosemeter This is a relatively new development in the noise field which promises to simplify the measurement of L eq considerably. The dosemeter measures both A-weighted sound level and duration simultaneously and hence can provide a direct measure of

7 The assessment of occupational noise exposure 359 Leq. [n principle, it should be able to deal with any type of noise, both steady and impulsive, having any type of temporal pattern and therefore should provide a relatively simple measure of Le Q in any industrial situation. Care should be taken to ensure that the dosemeter used complies with the energy concept and not the standard American specification of 5 db per doubling of exposure time which is incorporated in the Walsh-Healey Act. The dosemeter relies upon long-term integration techniques for the measurement of Z-eq. That is, it sums over a period of time all the A-weighted sound energy incident upon it irrespective of its temporal characteristics. It does not possess limiting dynamic meter characteristics such as "fast" or "slow", and therefore does not in this respect modify the incoming signal. Consequently it should be equally capable of measuring Leq for steady-state noises or for impulses having fast rise times. The dosemeter is available in two forms: a relatively large static device and a small personal monitoring instrument worn by the exposed person. The larger instrument has a slightly better technical specification than the personal device and may be used to measure the Leq of noise generated by a particular machine or in a particular area. However, it suffers from a similar drawback to the SLM in that it does not easily provide a measure of the noise dose received by an individual if that individual moves around a factory from day to day in random fashion. It is in this type of situation that a personal noise dosemeter has an advantage. Being worn in a pocket, it monitors directly all the sound energy reaching the ears of an individual, wherever he may be, and thus greatly facilitates the measurement of total personal noise dose. The measurement of Leq of steady state noises, even with complex variations in sound level and temporal pattern, is a relatively simple matter with a dosemeter. However, care should be exercised to ensure that the duration of the measurement period is sufficient to provide a representative measure of a complete day's exposure. Similarly, in the case of many type of impulse noise, L eq may be measured simply and directly. However, certain types of industrial noise contain both steady-state and impulse noise where the peak sound levels of the impulses are considerably greater than the background noise levels, although both may be hazardous. In this situation care should be taken that the peaks of the impulses do not overload the dosemeter, thereby introducing errors. A dynamic range of at least 60 db is necessary for a dosemeter to give reliable results in such circumstances. Tape recorders The use of a precision tape recorder in the field can greatly facilitate the determination of Led f r industrial noise. Tape recordings allow noises having complex sound level and temporal characteristics to be analysed in the laboratory using a dosemeter or other analytical equipment such as a graphic level recorder, statistical distribution analyzer or oscilloscope. The measurement of steady state noise by this technique is straightforward provided that the usual safeguards and standards are maintained. In the case of impulse noise, even greater care is needed. WALKER and BEHAR (1971) have shown that the measurement of impulse noise by this method introduces an error of about 1 db in the peak values of the impulses. This relatively small error occurs because, as the noise is A-weighted, the limited frequency response of such instruments has

8 360 A.M. MARTIN little effect on the accuracy of the method. Provided that care is taken to ensure that neither microphones nor tape recorders clip the peaks of the impulses and possible signal to noise ratio limitations are noted, a reliable measure of L e q should be obtained. Oscilloscope techniques An oscilloscope may be used for the measurement of Leq for any type of industrial impulse noise. It can be used to advantage for the measurement of those impulse noises not adequately catered for by a dosemeter or where a dosemeter is not available. However, it is not usually convenient for the measurement of steady state noise. The basic requirement is for an oscilloscope preferably of the storage type and camera so that recordings of the pressure-time waveform of the impulse noise can be obtained. The impulse signal may come directly from a SLM or microphone and amplifier, or may be tape recorded and analysed in the laboratory (see Fig. 1). Measurements are made of a number of parameters of the waveform of the noise and L e(l is deduced from these parameters. MARTIN and ATHERLEY (1973) have recently described in this journal a simple method for the measurement of equivalent continuous noise level, L e q, of impulse noise from oscilloscope recordings of the impulse waveform. The noise is divided into three categories, according to repetition rate, and the technique employed varies according to category. The system as a whole is intended to be used in conjunction with the BOHS "Hygiene Standard for wide band noise" (1971). Oscilloscopic techniques, like the SLM, become rather complex when attempting to assess the individual noise dose of a person who moves around a factory. A number of measurements of the waveform may be required, together with work study procedures, if the noise is generated by several different machines or by a varying cyclic manufacturing process, However, it does provide an accurate alternative to the dosemeter, as it is the only other method of measuring Z. e q for industrial impulse noise available at the present time. Calibration The calibration of all measurement equipment is essential for reliable results. All instrumentation should be calibrated prior to, and again immediately after, making the measurements and this should be checked at regular intervals during its use. Equipment such as the SLM and dosemeter can be calibrated relatively simply following the manufacturer's instructions with the standard acoustic source normally provided. Recording a calibration signal on tape requires due regard to be paid to the problems of recording level, overloading and the possible maximum peak sound levels of the noise to be recorded. The use of a random acoustic source in this respect may add problems of peak clipping owing to the relatively high crest factors involved and therefore a non-random source is preferable. The calibration of an ocilloscope using a standard non-random source such as a pistonphone is essential, as the signal waveform itself is used to calibrate the graticule. The oscilloscope graticule should be calibrated in terms of the linear sound pressure units: N/m 2 (Pascalls).

9 The assessment of occupational noise exposure 361 DISCUSSION The majority of the apparatus described here should be available in any adequately equipped acoustics or industrial hygiene laboratory. Measurements of the noise may be made with some or all of this equipment as illustrated in Fig. 1, depending upon the type and complexity of the noise in question. The main requirement is that a reliable estimate is obtained of the noise dose, I eq, received by each individual member of the workforce. If the total workforce remains in one location in a noise with constant sound level and temporal characteristics, the task of evaluating Lea f r each worker is relatively simple. However, as variations in the sound level and temporal pattern become more complex and random, and as individual workers tend to move from one noise environment to another, the determination of Leq becomes increasingly difficult. The more variable the noise, the greater the number of measurements and time required to obtain a reliable result. Measurements made with a personal noise dosemeter may require less effort, although in the extreme case of random movements around a factory, an individual may be required to wear a dosemeter for a number of shifts before a representative measure of his noise exposure can be arrived at. If the noise exposure cannot be reliably determined either due to lack of equipment or the complex nature of the noise, then, according to the Code of Practice, any entry to a place where the sound level is 90 db(a) or more should be treated as hazardous. Thus a SLM alone may be used to establish those areas and machines where the noise level exceeds 90 db(a), see for example Table 1, but more detailed measurements are required to determine e q- This latter course of action is particularly desirable where the noise exposure is borderline or consists predominantly of impulses. Having established an individual's noise dose, this may be compared with the maximum permissible limit for Z. eq of 90 db(a) recommended by the BOHS Hygiene Standard and the Code of Practice. However it should be stressed that this is not a "safe" limit or desirable limit as, according to ISO Recommendation R1999 (1971), 18 per cent of the exposed group will receive "... hearing impairment for conversational speech..." if exposed to the noise for a working lifetime of 30 yr. Leq should be reduced to 80 db(a) or less for the percentage risk of hearing impairment to become negligible. The current figure of 90 db(a) is chosen merely as a compromise between a morally desirable limit and practical and economic requirements. When the noise dose exceeds the recommended limit, noise control measures should be undertaken. Basically these may take the form of reducing the noise at source, placing either machine or man in an acoustic enclosure or the wearing of personal hearing protection. Essentially, the problem should be approached in that order and in particular, the use of hearing protectors should be considered as an interim measure while more permanent engineering noise control is being carried out. The Code of Practice and ISO R1999 describe methods for the calculation of L e q resulting from noise abatement measures. CONCLUSIONS The recent publication of the Department of Employment Code of Practice (1972), the BOHS Hygiene Standard (1971) and ISO Recommendation R1999 (1971) have at long last brought a measure of uniformity and agreement to the problem of

10 362 A. M. MARTIN noise assessment in the U.K. Their total dependence upon and utilization of the equal-energy concept for hearing damage has provided a unifying theme to what up to the present time has been a somewhat complex subject. Furthermore the acceptance in the Code of Practice of the validity of the energy principle for any type of industrial noise has provided the basis for a single and relatively simple system of noise measurement. The recent introduction of the noise dosemeter should allow reliable estimates of e q or percentage noise dose to be obtained in the future with a minimum of effort and expertise. Furthermore, it should provide the non-expert with the means for the assessment of the hazardous nature of any industrial noise. These two recent advances indicate that the major problems involved in the measurement and assessment of industrial noise have fundamentally been solved. Although there remain a few unusual noises which are not simply measured and to which the energy principle may not apply with scientific exactness, the advantages of a simple and unified system of noise evaluation are paramount. REFERENCES ATHERLEY, G. R. C. and MARTIN, A. M. (1971) Ann. occup. Hyg. 14, BRITISH OCCUPATIONAL HYGIENE SOCIETY (1971) Ann. occup. Hyg. 14, 57. BS4197 (1967) Specification for a Precision Sound Level Meter. British Standards Institution, London. BURNS, W. and ROBINSON, D. W. (1970) Hearing and Noise in Industry, H.M.S.O., London. COLES, R. R. A. and MARTIN, A. M. (1973) /. Sound & Vib. 28, DEPARTMENT OF EMPLOYMENT (1972) Code of Practice for Reducing the Exposure of Employed Persons to Noise. H.M.S.O., London. DIN 45633, Part 2 (1968) Precision Sound Level Meter. Additional Requirements for the Extension of Precision Sound Level Meters to an Impulse Sound Level Meter. Deutscher Normenausschuss, Berlin. HEMPSTOCK, T. I., ELSE, D. and POWELL, J. A. (1972) Appl. Acoust. 5, EC 179 (1965) International Electrotechnical Commission, Geneva. ISO Recommendation R1999 (1971) International Organisation for Standardisation, Geneva. MARTIN, A. M. (1970) PhD Thesis, University of Salford. MARTIN, A. M., ACTON, W. I., LUTMAN, M. and WALKER, J. G. (1973) /. Sound & Vib. 28, MARTIN, A. M. and ATHERLEY, G. R. C. (1973) Ann. occup. Hyg. 16, RICE, C. G. and MARTIN, A. M. (1973) /. Sound & Vib. 28, WARD, D. W. (1970) /. acoust. Soc. Am. 48, WALKER, J. G. (1970) Ann. occup. Hyg. 13, WALKER, J. G., FORREST, M. and BEHAR, A. (1973) Unpublished data, University of Southampton. WALKER, J. G., and BEHAR, A. (1971) /. Sound & Vib. 19, WALKER, J. G. and MARTIN, A. M. (In Press) Hearing conservation, In Noise and Vibration Control for Industrialists (Edited by LONGMORE, D. K. and PETRUSEWICZ, S. A.). Paul Elek, London.