Am. occup. Hyg. VoL 18, pp. 267-268. Pergamon Pren 1973. Printed in Great Britain
LETTER TO THE EDITOR EXPOSURE CRITERIA FOR INDUSTRIAL ULTRASOUND Sir, time ago, the writer proposed an exposure criterion for the prediction of auditory and subjective effects due to airborne noise from ultrasonic sources (ACTON, 1968). This was in agreement with another criterion, in so far as it went, due to GRIGOR'EVA (1966) and published in a Russian journal, even though the latter had not been to hand at the time of writing. A slightly later criterion due to PARRACK (1969) was in agreement, to within 5 dB, with the earlier criteria. The subject of exposure criteria was covered in a review of the effects of industrial airborne ultrasound on humans published in 1974 (ACTON, 1974). All these criteria were expressed in terms of sound levels measured in one-thirdoctave bands, the present writer's criterion allowing maximum levels of 75 dB in one-third-octave bands centred on frequencies up to 16 kHz, and maximum levels of 110 dB in bands centred on 20 kHz and above. Subsequent experience has shown that the criterion has generally worked quite satisfactorily. The 5 dB difference between the American and the other proposals probably reflected slightly different national attitudes at that time in specifying the proportion of an exposed population which could be acceptably placed at risk. However, one shortcoming in these criteria has come to light, which was due to the way the original proposals were worded. The nominal frequency limits of the one-third-octave band centred on 20 kHz are 17-6-22-5 kHz. The lower end of this frequency range is within the upper end of the audible frequency range of a certain proportion of the population, and particularly the young females who often operate industrial ultrasonic devices. At sound levels in the region of 110 dB, these frequencies can cause severe auditory and subjective effects such as persisting headaches, nausea, tinnitus and an unpleasant sensation of " fullness " or pressure in the ears, although permanent hearing loss is unlikely (ACTON and CARSON, 1967). It has been brought to the attention of the writer that representatives of certain sections of the ultrasonic machinery supply industry are using this shortcoming of the criteria to justify the use of these lower frequencies, which may occur accidentally as a result of production tolerances, or perhaps be utilised deliberately for mechanical reasons. It is proposed, therefore, that the criterion should be revised so that the 75 dB level is extended to include the one-third-octave band centred on 20 kHz. This will allow a 10% manufacturing tolerance on the design frequency of ultrasonic devices with a reasonable certainty that production items will not cause undesirable physiological and subjective effects. Where narrower band analysis methods are used, the step should occur at a frequency of 22-5 kHz instead of 20 kHz as assumed previously. No corrections should be made to measured levels to allow for narrower (or wider, e.g. octave) band measurements, as pure-tone sound is usually involved. SOME
Downloaded from http://annhyg.oxfordjournals.org/ at University of Exeter on August 15, 2015
267
268
Letter to the Editor
W. I. ACTON Wolf son Unit for Noise and Vibration Control, Institute ofSound and Vibration Research, The University, Southampton SO9 SNH
REFERENCES ACTON, W. I. (1968) A criterion for the prediction of auditory and subjective effects due to air-borne noise from ultrasonic sources. Ann. occup. Hyg. 11, 227. ACTON, W. I. (1974) The effects of industrial air-borne ultrasound on humans. Ultrasonics 12, 124. ACTON, W. I. and CARSON, M. B. (1967) Auditory and subjective effects of airborne noise from industrial ultrasonic sources. Br. J. ind. Med. 24, 297. GRIOOR'EVA, V. M. (1966) Ultrasound and the question of occupational hazards. Maschinstreochiya 8, 32. Abstract in Ultrasonics, 1966, 4, 214. PARRACK, H. O. (1969) An evaluation of industrial acoustic radiation above 10 kHz. Letter. (Quoted by MICHAEL, P. L. et al. (1974) Pennsylvania State University Report. Physical Agents Branch, National Institute for Occupational Safety and Health, Cincinnati (Ohio), U.S.A.)
Downloaded from http://annhyg.oxfordjournals.org/ at University of Exeter on August 15, 2015
The proposed revised criterion may be summarised as follows: the permitted level is 75 dB in the octave band centred on 16 kHz, or in one-third-octave bands centred on frequencies up to and including 20 kHz, or in narrow bands centred on frequencies up to 22.5 kHz; the permitted level is HOdB in octave bands centred on frequencies of 32 kHz and above, or in one-third-octave bands centred on frequencies of 25 kHz and above, or in narrower bands centred on frequencies of 22-5 kHz and above. A word of caution should perhaps be repeated that the linear frequency response of the microphone fitted to many commercially available sound level meters does not extend to the octave or one-third-octave bands centred on 16 kHz and above, even though these filters may be supplied with the instrument. The manufacturer's specification should be consulted on this point.
LETTER TO THE EDITOR EXPOSURE CRITERIA FOR INDUSTRIAL ULTRASOUND Sir, time ago, the writer proposed an exposure criterion for the prediction of auditory and subjective effects due to airborne noise from ultrasonic sources (ACTON, 1968). This was in agreement with another criterion, in so far as it went, due to GRIGOR'EVA (1966) and published in a Russian journal, even though the latter had not been to hand at the time of writing. A slightly later criterion due to PARRACK (1969) was in agreement, to within 5 dB, with the earlier criteria. The subject of exposure criteria was covered in a review of the effects of industrial airborne ultrasound on humans published in 1974 (ACTON, 1974). All these criteria were expressed in terms of sound levels measured in one-thirdoctave bands, the present writer's criterion allowing maximum levels of 75 dB in one-third-octave bands centred on frequencies up to 16 kHz, and maximum levels of 110 dB in bands centred on 20 kHz and above. Subsequent experience has shown that the criterion has generally worked quite satisfactorily. The 5 dB difference between the American and the other proposals probably reflected slightly different national attitudes at that time in specifying the proportion of an exposed population which could be acceptably placed at risk. However, one shortcoming in these criteria has come to light, which was due to the way the original proposals were worded. The nominal frequency limits of the one-third-octave band centred on 20 kHz are 17-6-22-5 kHz. The lower end of this frequency range is within the upper end of the audible frequency range of a certain proportion of the population, and particularly the young females who often operate industrial ultrasonic devices. At sound levels in the region of 110 dB, these frequencies can cause severe auditory and subjective effects such as persisting headaches, nausea, tinnitus and an unpleasant sensation of " fullness " or pressure in the ears, although permanent hearing loss is unlikely (ACTON and CARSON, 1967). It has been brought to the attention of the writer that representatives of certain sections of the ultrasonic machinery supply industry are using this shortcoming of the criteria to justify the use of these lower frequencies, which may occur accidentally as a result of production tolerances, or perhaps be utilised deliberately for mechanical reasons. It is proposed, therefore, that the criterion should be revised so that the 75 dB level is extended to include the one-third-octave band centred on 20 kHz. This will allow a 10% manufacturing tolerance on the design frequency of ultrasonic devices with a reasonable certainty that production items will not cause undesirable physiological and subjective effects. Where narrower band analysis methods are used, the step should occur at a frequency of 22-5 kHz instead of 20 kHz as assumed previously. No corrections should be made to measured levels to allow for narrower (or wider, e.g. octave) band measurements, as pure-tone sound is usually involved. SOME
Downloaded from http://annhyg.oxfordjournals.org/ at University of Exeter on August 15, 2015
267
268
Letter to the Editor
W. I. ACTON Wolf son Unit for Noise and Vibration Control, Institute ofSound and Vibration Research, The University, Southampton SO9 SNH
REFERENCES ACTON, W. I. (1968) A criterion for the prediction of auditory and subjective effects due to air-borne noise from ultrasonic sources. Ann. occup. Hyg. 11, 227. ACTON, W. I. (1974) The effects of industrial air-borne ultrasound on humans. Ultrasonics 12, 124. ACTON, W. I. and CARSON, M. B. (1967) Auditory and subjective effects of airborne noise from industrial ultrasonic sources. Br. J. ind. Med. 24, 297. GRIOOR'EVA, V. M. (1966) Ultrasound and the question of occupational hazards. Maschinstreochiya 8, 32. Abstract in Ultrasonics, 1966, 4, 214. PARRACK, H. O. (1969) An evaluation of industrial acoustic radiation above 10 kHz. Letter. (Quoted by MICHAEL, P. L. et al. (1974) Pennsylvania State University Report. Physical Agents Branch, National Institute for Occupational Safety and Health, Cincinnati (Ohio), U.S.A.)
Downloaded from http://annhyg.oxfordjournals.org/ at University of Exeter on August 15, 2015
The proposed revised criterion may be summarised as follows: the permitted level is 75 dB in the octave band centred on 16 kHz, or in one-third-octave bands centred on frequencies up to and including 20 kHz, or in narrow bands centred on frequencies up to 22.5 kHz; the permitted level is HOdB in octave bands centred on frequencies of 32 kHz and above, or in one-third-octave bands centred on frequencies of 25 kHz and above, or in narrower bands centred on frequencies of 22-5 kHz and above. A word of caution should perhaps be repeated that the linear frequency response of the microphone fitted to many commercially available sound level meters does not extend to the octave or one-third-octave bands centred on 16 kHz and above, even though these filters may be supplied with the instrument. The manufacturer's specification should be consulted on this point.
