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A noise exposure duration indicator
J.
Sound
VS.
(1966)
3 (I),
1-3
A NOISE EXPOSURE
DURATION
INDICATOR-f
R. A. STONG AND KEITH K. NEELY Sensory Capacities Section, Human Factors Wing, Defence Research Medical Laboratories, Toronto, Canada (Received 30 May 1965)
A small lightweight noise exposure duration indicator is described, which records the cumulative time for which one of five pre-set sound pressure levels of noise is exceeded.
The amount and type of hearing loss suffered by an individual exposed to noise depends, in part, upon the intensity of the noise and the duration of exposure to it. The development of adequate criteria and procedures for hearing protection requires the correlation of observed hearing losses, temporary or permanent, with a quantitative knowledge of the conditions of exposure responsible for them (I, 2, 3). This note describes an instrument which has been developed to record the accumulated time during which the ambient noise level exceeds a predetermined value. Various techniques have been used in obtaining an estimate of the cumulative sound energy to which an individual has been exposed. These have consisted of(i) the estimation of an individual’s accumulated exposure time from observations of his work activities (4, s), (ii) the use of interviews and questionnaires (6), (iii) the use of a stop-watch and sound level meter to measure the total durations of exposure to noise above specified sound pressure levels (7), (iv) the analysis, in terms of overall and spectral levels and their duration, of data transmitted from the individual by portable FM telemetry devices (S), and (v) the use of a device which utilizes electrochemical storage cells to provide a measure of total integrated noise energy to which it is exposed during a given interval of time (9, 10). In 1962, work was begun at Defence Research Medical Laboratories to develop a simple device, capable of measuring the duration of exposure to noise exceeding fixed intensity levels, which would require a minimum of technical training and supervision in use. The device is essentially an acoustically operated switch which controls an electrochemical integrator to measure cumulative exposure time(s). In this instance a recording could be made of the cumulative time for which the noise level exceeded one of five pre-set values. As indicated in Figure I, it is composed of a microphone, attenuator, a three-stage amplifier, rectifier and electronic switch. The microphone used in the prototype device was a dynamic type with low impedance and a response sensitivity of + 5 dB from 150 to 18,000 c/s. The attenuator was of the Ladder Type, with five sections each providing IO dB of attenuation. Pre-set sound pressure levels of activation, i.e., 85, 95, 105, I 15 and 125 dB (re 0*0002 microbar), could be selected using a rotary switch. The last stage of the amplifier was operated as a squaring amplifier, to prevent the switch from remaining energized due to energy stored in the averaging circuit after the noise level had decayed below the triggering threshold. The rectifier, a half-wave voltage doubler, provided a t Defence Research Medical Laboratories, I
I
Technical
Note No. 564.
2
R. A. STONG AND K. K. NEELY
d.c. voltage to activate a Schmitt trigger which controlled the current to the indicator. An electrochemical integrator was used with a ballast resistor to obtain a measure of the total time during which the switch was energized, i.e., during which the SPL of the noise exceeded a pre-set level. The integrated time display unit had 85 divisions on an 85 mm scale, corresponding to 850 min (IO min per division) of exposure time. The device was I in. deep, 6 in. long and 3.5 in. wide, including a microphone whose diameter was I in. and length 4.5 in., and weighed 470 g. It was relatively easy to calibrate and required a minimum of maintenance and repair. Calibration was carried out with white noise, generated electrically and reproduced in a reverberant room. The SPL of the noise was increased until the lower triggering point
Figure I. A noise exposure duration indicator.
was reached (i.e., the noise peaks were equal to the average triggering level) and this level was recorded. The intensity of the noise was then increased to a level at which the device was continuously energized and this level recorded. The difference between these two levels represents the triggering accuracy of the device when measuring white noise which had a peak factor of approximately 12 dB. Calibration at 85,g5, 105, I 15 and 122 dB levels (maximum white noise SPL available) was accomplished in this manner. The tests revealed no indication that the triggering response would be non-linear at 125 dB. Neglecting microphone response, a recording accuracy within + 3 dB of the pre-set levels was attained. A calibration check using pink noise (equal energy per octave band) at the 85 and g5 dB levels between 150 and g6oo c/s gave essentially the same results as white noise. Pure tones also were used to test the accuracy of the unit ; an accuracy of & I dB was obtained at frequencies between 180 and 6000 c/s. No attempt was made to equalize the microphone response which rolled off below zoo c/s.
REFERENCES I. K. K. NEELY 1959 Med. 5’er.J. (Can.) 15,235. Hearing conservation for the armed forces. z. K. K. NEELY 1962 Proceedings 4th Int’l Cong. Acoust. (Copenhagen) paper H-42. The implementation of a hearing conservation program. (Published by Pergamon Press.) 3. K. K. NEELY 1964 Proceedings CLC 3rd Ann. Conf. Occupational Health and Safety (North Bay, Canada). Noise : effects and control. 4. G. W. KAMPERMAN1958 Noise Control 4, 22. Measurement of high-intensity noise. 5. L. L. KOPRA, C. BRIDGESand M. SIEGELMAN1957 Sch. Aviat. Med., Randolph AFB, Report No. 57-73. Hearing acuity of air force flight-line personnel. 6. L. L. KOPRAand L. E. STRICKLAND 1961 Sch. Aerospace Med. Brooks AFB, Report No. 61-117. Noise exposure of B-52 and KC-135 aircraft maintenance personnel.
A
NOISE EXPOSURE DURATION
INDICATOR
3
7. W. B. SNOW 1959 Noise Control 5, 13. Sound-level surveys for hearing conservation. 8. J. R. COX 1959 Noise control 5, 54. The noise cumulator. 9. R. W. BENSON, G. COOK, S. H. PEARSALLand J. WOODS 1964 Aerospace Medical Research Laboratories, Wright-Patterson AFB, Tech. Dot. Report No. AMRL-TDR-64-4. Experimental investigation of a prototype noise exposure meter. IO. N. N. ESTERand J. J. MOORE 1962 Aerospace Medical Research Laboratories, Wright-Patterson AFB, Tech. Dot. Rept. No. MRL-TDR-62-56. Development of a noise exposure meter.
Sound
VS.
(1966)
3 (I),
1-3
A NOISE EXPOSURE
DURATION
INDICATOR-f
R. A. STONG AND KEITH K. NEELY Sensory Capacities Section, Human Factors Wing, Defence Research Medical Laboratories, Toronto, Canada (Received 30 May 1965)
A small lightweight noise exposure duration indicator is described, which records the cumulative time for which one of five pre-set sound pressure levels of noise is exceeded.
The amount and type of hearing loss suffered by an individual exposed to noise depends, in part, upon the intensity of the noise and the duration of exposure to it. The development of adequate criteria and procedures for hearing protection requires the correlation of observed hearing losses, temporary or permanent, with a quantitative knowledge of the conditions of exposure responsible for them (I, 2, 3). This note describes an instrument which has been developed to record the accumulated time during which the ambient noise level exceeds a predetermined value. Various techniques have been used in obtaining an estimate of the cumulative sound energy to which an individual has been exposed. These have consisted of(i) the estimation of an individual’s accumulated exposure time from observations of his work activities (4, s), (ii) the use of interviews and questionnaires (6), (iii) the use of a stop-watch and sound level meter to measure the total durations of exposure to noise above specified sound pressure levels (7), (iv) the analysis, in terms of overall and spectral levels and their duration, of data transmitted from the individual by portable FM telemetry devices (S), and (v) the use of a device which utilizes electrochemical storage cells to provide a measure of total integrated noise energy to which it is exposed during a given interval of time (9, 10). In 1962, work was begun at Defence Research Medical Laboratories to develop a simple device, capable of measuring the duration of exposure to noise exceeding fixed intensity levels, which would require a minimum of technical training and supervision in use. The device is essentially an acoustically operated switch which controls an electrochemical integrator to measure cumulative exposure time(s). In this instance a recording could be made of the cumulative time for which the noise level exceeded one of five pre-set values. As indicated in Figure I, it is composed of a microphone, attenuator, a three-stage amplifier, rectifier and electronic switch. The microphone used in the prototype device was a dynamic type with low impedance and a response sensitivity of + 5 dB from 150 to 18,000 c/s. The attenuator was of the Ladder Type, with five sections each providing IO dB of attenuation. Pre-set sound pressure levels of activation, i.e., 85, 95, 105, I 15 and 125 dB (re 0*0002 microbar), could be selected using a rotary switch. The last stage of the amplifier was operated as a squaring amplifier, to prevent the switch from remaining energized due to energy stored in the averaging circuit after the noise level had decayed below the triggering threshold. The rectifier, a half-wave voltage doubler, provided a t Defence Research Medical Laboratories, I
I
Technical
Note No. 564.
2
R. A. STONG AND K. K. NEELY
d.c. voltage to activate a Schmitt trigger which controlled the current to the indicator. An electrochemical integrator was used with a ballast resistor to obtain a measure of the total time during which the switch was energized, i.e., during which the SPL of the noise exceeded a pre-set level. The integrated time display unit had 85 divisions on an 85 mm scale, corresponding to 850 min (IO min per division) of exposure time. The device was I in. deep, 6 in. long and 3.5 in. wide, including a microphone whose diameter was I in. and length 4.5 in., and weighed 470 g. It was relatively easy to calibrate and required a minimum of maintenance and repair. Calibration was carried out with white noise, generated electrically and reproduced in a reverberant room. The SPL of the noise was increased until the lower triggering point
Figure I. A noise exposure duration indicator.
was reached (i.e., the noise peaks were equal to the average triggering level) and this level was recorded. The intensity of the noise was then increased to a level at which the device was continuously energized and this level recorded. The difference between these two levels represents the triggering accuracy of the device when measuring white noise which had a peak factor of approximately 12 dB. Calibration at 85,g5, 105, I 15 and 122 dB levels (maximum white noise SPL available) was accomplished in this manner. The tests revealed no indication that the triggering response would be non-linear at 125 dB. Neglecting microphone response, a recording accuracy within + 3 dB of the pre-set levels was attained. A calibration check using pink noise (equal energy per octave band) at the 85 and g5 dB levels between 150 and g6oo c/s gave essentially the same results as white noise. Pure tones also were used to test the accuracy of the unit ; an accuracy of & I dB was obtained at frequencies between 180 and 6000 c/s. No attempt was made to equalize the microphone response which rolled off below zoo c/s.
REFERENCES I. K. K. NEELY 1959 Med. 5’er.J. (Can.) 15,235. Hearing conservation for the armed forces. z. K. K. NEELY 1962 Proceedings 4th Int’l Cong. Acoust. (Copenhagen) paper H-42. The implementation of a hearing conservation program. (Published by Pergamon Press.) 3. K. K. NEELY 1964 Proceedings CLC 3rd Ann. Conf. Occupational Health and Safety (North Bay, Canada). Noise : effects and control. 4. G. W. KAMPERMAN1958 Noise Control 4, 22. Measurement of high-intensity noise. 5. L. L. KOPRA, C. BRIDGESand M. SIEGELMAN1957 Sch. Aviat. Med., Randolph AFB, Report No. 57-73. Hearing acuity of air force flight-line personnel. 6. L. L. KOPRAand L. E. STRICKLAND 1961 Sch. Aerospace Med. Brooks AFB, Report No. 61-117. Noise exposure of B-52 and KC-135 aircraft maintenance personnel.
A
NOISE EXPOSURE DURATION
INDICATOR
3
7. W. B. SNOW 1959 Noise Control 5, 13. Sound-level surveys for hearing conservation. 8. J. R. COX 1959 Noise control 5, 54. The noise cumulator. 9. R. W. BENSON, G. COOK, S. H. PEARSALLand J. WOODS 1964 Aerospace Medical Research Laboratories, Wright-Patterson AFB, Tech. Dot. Report No. AMRL-TDR-64-4. Experimental investigation of a prototype noise exposure meter. IO. N. N. ESTERand J. J. MOORE 1962 Aerospace Medical Research Laboratories, Wright-Patterson AFB, Tech. Dot. Rept. No. MRL-TDR-62-56. Development of a noise exposure meter.