Let:t:er t:o t:he ed JEor Exposure critieria for industrial ultrasound Dear Sir, Some time ago, I proposed an exposure criterion for the prediction o f 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 Grigoreva (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 o f sound levels measured in one-third-actave bands, my criterion allowing maximum levels of 75 dB in one-third-actave 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 onethird-actave band centred on 20 kHz are 17.6 kHz to 22.5 kHz. The lower end of this frequency range is within the upper end o f the audible frequency range o f a certain proportion o f 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 my attention 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 o f 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.

the octave band centred on 16 kHz, or one-third-octave bands centred on frequencies up to and including 20 kHz, or narrow bands centred on frequencies up to 22.5 kHz and the permitted level is 110 dB i n : octave bands centred on frequencies of 32 kHz and above, or one-third-octave bands centred on frequencies of 25 kHz and above, or 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. Yours IV. I. Acton,

Senior Consultant, Institute of Sound and Vibration Research, Wolfson Unit, The University, Southampton SO9 5NM.

References Aeton, W. 1. A criterion for the prediction of auditory and subjective effects due to air-borne noise from ultrasonic sources, Annals of Occupational Hygiene 11 (19681 227. Acton, W. 1. The effects of industrial air-borne ultrasound on humans, Ultrasonics 12 (1974) 124 Acton, W. L and Carson, M. B. Auditory and subjective effects of airborne noise from industrial ultrasonic sources, British Journal of Medicine 24 (1967) 297 Grigoreva, V. M. Ultrasound and the question of occupational hazards, Maschinstreochiya 8 (1966) 32 abstract in Ultrasonics 4 (19661 214 Parrack, H. O. (19691 Letter dated 15 December 1969 quoted by Michael, P. L. et. al. - An evaluation of industrial acoustic radiation above 10 kltz, Pennsylvania State University Report (February 19741 prepared for Physical Agents Branch of National Institute for Occupational Safety and Health.

New book Books listed below are those recently received by the editorial office or those mentioned in advance information from the publishers. Their inclusion here does not prevent them from being reviewed in a later issue.

Ultrasonoscopic differential diagnosis in obstetrics and gynaecology R. O. Meudt and IV~. Hinse/mann

Springer-Verlag (1975) cloth, US $40.20, pp 138

Atlas of ultrasonic diagnosis in obstetrics and gynaecoiogy K-H. Sch/ensker

Georg Thieme Publishers, Stuttgart (19751 pp 158

The proposed revised criterion may be summarised as follows:-

J. W. Tucker and V. W. Rampton

The permitted level is 75 dB i n : -

North Holland Publishing Co (1972), US $ 54.25, pp 148

42

Microwave ultrasonics in solid state physics

ULTRASONICS. JANUARY 1976