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Application of the probe microphone method to measure attenuation of hearing protectors against high impulse sound levels
Applied Acoustics 27 (1989) 13-25
Application of the Probe Microphone Method to Measure Attenuation of Hearing Protectors Against High Impulse Sound Levels Chang-Chun Liu, Jussi Pekkarinen & Jukka Starck* Institute of Occupational Health, Laajaniityntie 1, SF-01620 Vantaa, Finland (Received 4 July 1988; revised version received 6 October 1988; accepted 28 October 1988)
A BS TRA C T The purpose of the present study was to develop a probe microphone method to measure the attenuation of earplugs, earmuffs and their combination against high impulse sound levels from military firearms. The field measurements were made on three earmuffs and one earplug. A probe microphone was supplied with a replaceable soft tube (outer diameter 1.5 mm) to transmit the sound pressure from the ear canal to the microphone. The tube was led between the skin and the cushion ring of the earmuff and through the earplug to the ear canal. A formula was developed to estimate the potential effect of tube length on the results. Subjective laboratory tests of earplugs showed good agreement with the probe microphone method below the 3"15kHz octave band. The field measurements showed that earplugs may provide attenuation equal to that of large-volume earmuffs against low-frequency impulses from large-calibre weapons. It was also shown that a prominent increase of attenuation can be achieved by using earmuffs and earplugs simultaneously.
INTRODUCTION
The use of hearing protectors has been increasing in industrial workplaces, and also when shooting with sport or military firearms. In many countries men participate in military service before entering working life. The frequency content of shooting impulses seems crucially important in the * To whom correspondence should be addressed. 13
Applied Acoustics 0003-682X/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
14
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
attenuation of hearing protectors, in the genesis of immediate hearing damage and in the risk of developing later occupational hearing loss. 1-3 The equal energy principle can be applied to the risk assessment of noiseinduced permanent hearing loss when the peak levels of industrial noise do not exceed 140 dB. 4'5 The American Conference of Governmental Industrial Hygienists has issued the recommendation that no industrial impulses in excess of 140dB peak level should be permitted. 6 It is known from previous studies that hand-held weapons produce about 150-160 dB, and anti-tank bazookas over 180dB peak levels, l'v's Allegedly, as the calibre increases, the peak level increases and the spectral maximum decreases. Laboratory tests have shown that earplugs are more effective than earmuffs against low-frequency noise. 9"1° However, laboratory tests have been carried out for steady state noise in 125-8000Hz one-third octave bands, the lowest octave already being disturbed by the occlusion effect.ix Thus the frequency range in the subjective tests has not covered the low frequencies present in shots from large-calibre weapons. Laboratory tests are justified in hearing protector design and in maintaining product quality control. The workplace results, when compared to laboratory test attenuations, indicate that 50% of the workers tested are receiving less than one-half of the potential attenuation of the earplugs. 12 In an industrial noise survey, earmuffs were found to decrease impulsiveness, in addition to the equivalent level. ~3 According to the damage risk criterion for shooting impulses, the longduration impulses corresponding to the low-frequency content of impulses cause a higher risk for hearing loss than high-frequency impulses, s The effect of low-frequency impulses has been studied in animal experiments. 2'3 Contrary to the previous damage risk criterion, the low-frequency impulses from cannons or howitzers caused relatively less damage than the impulses from rifles, when the same peak level was assumed for both weapons? However, the peak sound pressure level of a cannon is even higher than that of rifles, by 30 dB and, accordingly, cannon impulses are more harmful to hearing than rifle impulses. However, the effect of hearing protectors was not considered. 3 The miniature microphone has previously been used to measure the attenuation of soft foam plugs. However, the microphone and associated wires affected the fit of the earplug, and it was impossible to achieve a proper standard insertion? A probe microphone has been used with a metal tube inside the earplug to measure the attenuation of hand-held weapon impulses. 14 The purpose of the present study was to develop a probe microphone method to measure earplug attentuation in field conditions, and to measure
Attenuation of hearing protectors
15
the combined attenuation of earplugs and earmuffs against high-level impulses.
MATERIALS AND METHODS
Probe microphone Noise measurements were taken simultaneously outside and inside the hearing protectors. Noise inside the ear canal was measured with a 1/4 inch probe microphone (B&K 4135/WA 03787), which was originally designed for the measurement of hearing aids (Fig. 1). A normal 1/4 inch condenser microphone (B & K 4136) was used to measure the ambient noise outside the hearing protectors: it was mounted on a tripod located about 20 cm from the ear of the test person, at a grazing incidence in relation to the sound source. The dynamic ranges were 52-172dB for the probe microphone and 59-183 dB for the normal 1/4 inch microphone. The sound pressure was transmitted to the probe microphone through a replaceable soft tube made of silicon, with an outer diameter of 1"5 m m and inside diameter of 0.8 mm. The tube was pushed through the earplug, which was made of soft expanding plastic foam (E.A.R.). A small hole was pierced
Fig. 1.
The location and shielding of the probe microphone placed inside the aluminium cylinder; length 130 ram, diameter 30 mm, wall thickness 3 ram.
16
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
through the centre of the earplug without taking away any material. For earmuffs the tube was led between the skin and cushion ring. In the measurements taken for test persons, the length of the tube was 100 mm. The attenuation of the hearing protectors against the firearm impulses was defined as the difference between the peak sound pressure levels of the impulses outside and inside the hearing protectors. The unweighted peak levels were considered to comprise the most appropriate estimate for the risk of immediate damage in the inner ear.
Laboratory testing of the probe microphone measurements The effect of tube length on the results was tested using white noise (93+ l dB, 5 0 H z - 1 2 . b k H z ) as a test signal. The frequency response function between the two microphones was measured with a narrow-band two-channel digital signal analyser (HP 5420A) (Fig. 2). The probe microphone was placed inside the aluminium cover filled with absorption foam (Fig. 1). The attenuation of the microphone cover, with 100mm of tube blocked against ambient noise, was measured with a onethird octave analyser (B&K 2131) (Fig. 2). In the laboratory a _ 0 . b d B measurement accuracy was achieved for sound level meters and analysers. The standard laboratory test for subjective measurements was carried out with ten subjects to compare the attenuation of earplugs with and without the tube.15 A tube (length 20 mm) blocked at both ends was pushed through the earplug. The hearing of the subjects, aged 25 to 45 years, was tested prior to the measurements, and only those with normal hearing were accepted. In the test, the earplugs were inserted to about half of their length into the ear canal. The test persons fitted the device by themselves, under the supervision of the operator. The same ten subjects used for the subjective measurements participated in the measurements of noise reduction by probe microphone, and of the frequency response function of the open ear. During the measurement of the frequency response function, 2 0 m m of the 100mm probe tube was placed in the ear canal, near to the eardrum. White noise (100___0"3 dB, IO
[~]
M 2 IO
[-~
M 1
Aor B Fig. 2. Block diagram of the equipment in laboratory measurements. M1, B & K 4136 l/4inch condenser microphone for outside measurements; M2, B&K4135, probe microphone; a t, B&K 2209 sound level meter; a2, B&K 2204 sound level meter; A, HP5420A for narrow-band frequency response function analyses; B, B&K 2131 for the analyses of attenuation in one-third octave bands.
Attenuation of hearing protectors
17
20 Hz-20 kHz) was generated (B&K 1405) and amplified (B&K 2706) to the loudspeaker located in front of the subject, at a grazing incidence to the ear. The measurements were recorded by a one-third octave analyser (B&K 2131) from 125Hz to 10kHz. Field measurements
The attenuations of the earplug (E.A.R.) and the three earmuffs worn commonly by Finnish army conscripts were measured for cannon shots. The earmuffs selected were Silenta Mil (small cup volume, 67 cm 3, total weight 137 g), Silenta Ergo (medium cup volume, 145 cm 3, 194 g) and Silenta Super (large cup volume, 165 cm 3, 258 g), since they represent different acoustical attenuations. The distance between the weapon and the subject was 10m. When the measurement was made the subject was sitting facing the cannon. The normal 1/4 inch microphone and probe microphone were located as in the laboratory tests in relation to the test person and the direction of the sound source. The field measurements were taken by a digital multi-channel sampling unit (HP 6944), controlled by a microcomputer (HP 200) (Fig. 3). Digital sampling was preferred for the wide dynamic range that is necessary in accurate shot impulse measurements. Altogether, 30 cannon shots were sampled simultaneously from two channels during a time period of 164ms, with a rate of 50000 samples per second from each channel corresponding to the time resolution of 20/as. The amplitude resolution was 12 bits, corresponding to 72dB. 16 The
DISPLAY
i I b)
ELI SCANNER
I II
i
ICONVERTERI:
J t--" HP 6 9 4 4 multichannel
MICRO i COMPUTER
Jl I Ii[
HP 9 9 2 0 /
sampling unit PLOTTER
Fig. 3. The microcomputer system for noise measurements in the field conditions: (a) microphone B&K 4136, outside; (b) probe microphone B&K 4135, inside.
18
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
impulses were sampled with linear weighting, because A-weighting was considered inappropriate for high-level impulses causing immediate hearing impairment. 3 RESULTS The difference of peak levels between the channels was 1.1 _+0.4dB in five repetition measurements of cannon shots when both microphones were attached side by side on a tripod. Frequency response functions between the normal microphone and the probe microphone with and without the tube (lengths of 40-160 mm) were analysed with white noise in the laboratory (Fig. 4). When the probe microphone had no tube, the frequency response function was within __+2dB below 3-15kHz. The influence of the different tube lengths on the attenuation of noise was estimated by the frequency response function. The graphical estimates were calculated statistically using the following linear regression for logarithmic conversion (Fig. 4): y = 10(0.56× l o g f -
2.31)
X 0"1 X L + 0.000 296 x f - - 0"49
(1)
where Y is the attenuation (dB), L is the length of the tube (mm), a n d f i s the frequency (Hz). F r e q u e n c y r e s p o n s e function (dB) lO 8
6 4 2 o
-2 \-
-4 --6 --8-
-10 100
I 1000
'
lq-a 10 0 0 0 F r e q u e n c y (Hz)
Fig. 4. Frequency response function between normal microphone (B & K 4136) and probe microphone (B & K 4135) without tube (P~), with tube lengths of 40mm (¢~..,~,), 100mm (~zx-~t), 160mm (/'~_~,), according to the measurements in the laboratory (broken line), and according to the developed formula derived from the measurements eqn (1) (curved line).
Attenuation of hearing protectors
19
Standard deviation (dB) 105- ...........
-F:--
010~ I0
--¢
20 30.
a
,,t-, '<
I t
40.
1
[:-----
50.
60
125
250
5(~0
I'K
2'K 3.1'5K 4K 6.3K 8K Frequency (Hz)
Fig. 5. The average and standard deviation for the standard test and for the probe microphone measurement, N = 1 0 . - - , Probe microphone measurement; - . - , standard test (REAT) with normal earplug; - - ' , standard test (REAT), earplug with a hole and closed tube.
The frequency response function was used to evaluate the attenuation of the cover with a 100mm blocked tube, and it was evaluated at 25-33 dB, depending on frequency. This represents the methodological limit for the maximal attenuation of earplugs which can be measured. TABLE 1 Average Peak Sound Pressure Level and Standard Deviations Outside (L~k out) and Inside (L~a k in) the Hearing Protectors, and the Attenuation o f the Peaks by Hearing Protectors when Shooting with a 105 mm Cannon; N = number of impulses
Hearing protectors
N
L~, k out (dB)
L ~ in (dB)
Attenuation peak (dB)
Earmuffs Large-volume = Middle-volume = Small-volume =
5 5 5
154"3 ___1"8 154.5 -t- 1-6 153"0 ___ 1.2
136-9 ___2.4 139.3 + 2.9 145"3 __+0"7
17.4 _ 1'8 15.2 _ 1.4 7"7 _ 1.1
Earplug E.A.R.
5
152.2 + 2"4
134.7 + 3"0
17'5 + 1"1
Earplug E.A.I~. and earmuff with Large volume Small volume
5 5
154-5 _+ 0"4 154"6 _ 0"6
131" 1 ___2"7 128"8 _ 1"1
23-4 __+2"5 25'8 _ 0"9
= Measured with a previous method using two ( B & K 4136) microphones. 7
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
20
The difference between the attenuation achieved by the subjective test and that measured by the probe microphone method was below 3 dB in the 125 Hz to 2 kHz frequency range, and more than 10 dB above 2 kHz (Fig. 5). The results for the attenuation obtained with the subjective test and with the probe microphone method for the same subjects also show that the effect of the tube on the earplug attenuation was negligible below the 3.15 kHz octave band (Fig. 5). The peak levels inside and outside were averaged over five measurements for each protector and each combination of protectors (Table 1). The attenuation of the earplug against impulses from a 105 mm cannon (17"5 dB) was as good as the large-volume earmuff (17.4 dB) and the medium-volume earmuff (15.2 dB), and much better than the small-volume earmuff (7.7 dB) Sound :)ressure (Pa) 1100
550
-550
-1100 0
40
81
122
163 Time (ms)
Sound pressure (Pa) 1100
550
V ' "T'V"
,.v-v
......
-550
-1100
40
81
122
163 Time (ms)
Fig. 6. The sound pressure measured from 105 mm cannon shot outside ( (- • - • - • - ) (a) an earplug, and (b) an earplug and earmuff.
) and inside
Attenuation of hearing protectors
21
(Table 1). The combination of either the large-volume earmuff or the smallvolume earmuff with the earplug caused similar attenuation (Table 1). The duration of the first impulse peak is 3.6 ms for the cannon, which corresponds to a frequency of about 140 Hz (Fig. 6). The waveforms outside hearing protectors also show higher frequencies. However, inside the hearing protectors the curves of the impulses were smooth and the highfrequency noise had disappeared. The earmuffs, earplugs and their combinations attenuated more high-frequency noise than low-frequency noise.
DISCUSSION The tube length influences attenuation only at high frequencies, and its effect can be calculated by the developed formula (eqn (1)). The effect of a 100 mm tube was within _+3"5 dB below 2000 Hz. The difference between the attenuation of a normal earplug and an earplug with a closed microphone tube inside it measured by the standard test procedure was within 3"0dB at any frequency from 125Hz to 8kHz (Fig. 5). This indicates that the hole for the tube in the earplug was tight, and that the effect of leakage caused by the tube on the attenuation was estimated to be negligible. Inside the earmuff the tube was led between the skin and cushion ring. The comparison in the laboratory showed that, below 500Hz, the standard test yielded attenuation values that were 1.0-3-0dB higher than those measured by the probe microphone method. This was probably caused by amplified physiological noise due to the occlusion effect.11 The results suggest that, below the 3.15 kHz octave band, approximately the same attenuation can be obtained by the probe microphone method and the subjective test. The real-ear attenuation measured by the probe microphone method was generally in close agreement with the data of previous studies.~ 7,~8 Above 3-15 kHz the attenuation of the earplug measured by the subjective procedure was 10dB more than that of the probe microphone method (Fig. 5). This is probably due to the absorption of high-frequency noise by the 100mm tube used in the field measurements, in addition to the noise isolation of the tube against test noise. Comparing the results from a study using instrumented cadaver ears with those from the probe microphone method in laboratory conditions, close agreement below 2kHz was generally found for constant noise, a9 The cannon has frequencies that are mainly between 50 and 500 Hz, and the
22
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
frequency response function of the open ear is negligible in this frequency range, a9 Thus it has not been considered in the present study. The first pressure wave with a high positive peak followed by a negative pulse determines the wavelength and, further, the main frequency component of the impulse. This can be verified both theoretically and by simulating with an F F T analyser. For cannon impulses, this duration is less than 10ms, but for rifles it is less than 1 ms. The corresponding main frequencies are approximately 100 Hz for cannons and 1000 Hz for rifles. The attenuation of hearing protectors in subjective laboratory tests is quite different for continuous noise with these frequencies. We found that the attenuation for impulses was primarily in line with that for continuous noise. Cannon impulses were definitely less attenuated than rifle impulses in previous studies. 1 However, the transient behaviour of hearing protectors is not necessarily linear due to the leakages and extremely high pressures. We did not study the non-linear effects even though those could also have been present. In our study the emphasis was more on methodological testing than on investigating the properties of weapon types, the earmuff models and the individual differences in fitting the hearing protector to the head. These questions will need a considerably larger number of measurements, and it is suggested that this task should be carried out in future with the method presented here. The duration of the first positive sound pressure peak outside and inside the earplug corresponds to about 140 Hz (Fig. 6). In the present study, the attenuation of the earplug (17"5 dB) f o r c a n n o n shot was in agreement with that derived in the laboratory with the subjective method at 125 Hz (Fig. 5) and with that measured with instrumented cadaver ears at 125 Hz. 19 The field measurements showed the frequency dependence of the attenuation, which is in line with the laboratory test results using a subjective method.7'15 In a previous study the attenuation of the E.A.R. earplug with a metal tube was measured with a probe microphone method for hand-held weapon impulses. TM In contrast to our study, in that study the impulses were Aweighted. The only reason presented for the use of A-weighting was to avoid overloading of the tape recorder. There is no evidence to date for the use of A-weighting when measuring high-level impulse noise for risk assessment purposes. 2'3 The noise reduction of the E.A.R. earplug against impulses for cannon, measured in the field, was 10dB less than those measured for rifles in the study using instrumented cadaver ears. ~9 The reason for this difference is probably that the earplug offers more attenuation at high frequencies than at low frequencies. The field measurements showed that the attenuation of the earplug
Attenuation of hearing protectors
23
(17"5 dB) was the same as that of large-volume earmuff and about 10dB higher than that of the small-volume earmuff for cannon shots measured with a previous method. 1 The combination of earplug and large-volume earmuff generated approximately 6dB more attenuation than either the earplug or earmuff alone for cannon shot (Table 1). The attenuation of the combination of the earplug and the small-volume earmuff was 8 dB more than that generated by the earplug alone, and 18dB more than that generated by the small-volume earmuff alone (Table 1). Combining either the large-volume earmuffs or small-volume earmuffs with the earplug yields about the same overall attenuation, although the difference between the attenuation generated by the large-volume earmuff and by the small-volume earmuff alone was large (9.7dB). This result suggests that, for the earplug chosen for the present study, the choice of earmufftype was less important for their combined attenuation. This view is in agreement with a previous study. 2° When a standard insertion foam plug was combined with a small-volume earmuff, an additional 10 dB was gained in the laboratory, according to a previous study, as compared to the attenuation of the E.A.R. earplug alone. 9 The increases in total attenuation caused by the addition of the earmuffs are similar in these studies.
CONCLUSIONS The present measurement technique provides an estimate of hearing protector attenuation at low frequencies which has been impossible to achieve for earplugs with previously available methods. The probe microphone method is practical for measuring the high-level low-frequency impulses generated by large-calibre weapons in the field, but it is not suitable for the measurement of hand-held weapons such as titles, due to the attenuation of high frequencies. The attenuation of the peak pressure of cannon shot impulses was equal for earplugs and for large-volume earmuffs. The exposure limit for occupational impulse noise is 140dB peak level. 6 This limit is always exceeded in shooting with large-calibre weapons without hearing protectors. The simultaneous use of earmuffs and earplugs increased the attenuation by at least 6dB compared to their attenuation alone, but the total attenuation was less than the sum of the individual values. Even the combined protection is not always sufficient to attenuate peak levels below 140dB. However, the risk of heating loss was clearly reduced by using combined protection, and should be recommended for shooting with largecalibre weapons and for continuous noise dominated by frequencies below
24
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
500Hz. For the earplug chosen, either large-volume or small-volume earmuffs increased attenuation remarkably. The attenuation of combined hearing protectors against low-frequency impulses cannot be estimated on their individual test results in the laboratory. The tube microphone method gives reliable results in the low-frequency range, which is typically poorly attenuated by hearing protectors.
REFERENCES 1. Ylikoski, J., Pekkarinen, J. & Starck, J., The efficiency of earmuffs against impulse noise from firearms. Scand. Audiol., 16 (1987) 85-8. 2. Price, G. R., Relative hazard of weapons impulses. J. Acoust. Soc. Am., 73(2) (1983) 556-66. 3. Price, G. R., Hazard from intense low-frequency acoustic impulses. J. Acoust. Soc. Am., 80(4) (1986) 1076-86. 4. ANSI S 3.28, Draft American National Standard, Methods for the evaluation of the potential effect on human hearing of sounds with peak A-weighted sound pressure levels above 120 decibels and peak C-weighted sound pressure levels below 140 decibels. Acoustical Society of America, 1986. 5. ISO/DIS 1999.2, Draft International Standard, Acoustics~etermination of occupational noise exposure and estimation of noise-induced hearing impairment. International Organization for Standardization, Geneva, 1985. 6. TLVs, Threshold limit values and biological exposure indices for 1987-1988, American Conference of Governmental Industrial Hygienists, 1987, pp. 100-2. 7. Starck, J., Pekkarinen, J. & Aatola, S, Attenuation of earmuffs against low frequency noise. J. Low Freq. Noise Vibr., 6 (1987) 167-74. 8. Pfander, F., Bongartz, H., Brinkmann, H. & Kietz, H., Danger of auditory impairment from impulse noise: a comparative study of the CHABA damagerisk criteria and those of the Federal Republic of Germany. J. Acoust. Soc. Am., 67(27) (Feb. 1980) 628-33. 9. Berger, E. H., Laboratory attenuation of earmuffs and earplugs both singly and in combination. Am. Indust. Hyg. Assoc. J., 44(5) (1983) 321-9. 10. Berger, E. H., Methods of measuring the attenuation of hearing protection devices. J. Acoust. Soc. Am., 79(6) (1986) 1655-87. 11. Berger, E. H., Influence of physiological noise and the occlusion effect on the measurement of real-ear attenuation at threshold. J. Acoust. Soc. Am., 74(1) (1983) 81-94. 12. Lempert, B. L. & Edwards, R. G., Field investigations of noise reduction afforded by insert-type hearing protectors. Am. lndust. Hyg. Assoc. J., 44(12) (1983) 894-902. 13. Pekkarinen, J., Industrial impulse noise, crest factor and the effect of earmuffs. Am. Indust. Hyg. Assoc. J., 48(10) (1987) 861-6. 14. Shenoda, F. B. & Ising, H., Sound attenuation of ear protectors under conditions of impulsive noise: field measurements. Appl. Acoust., 23 (1988) 297-307. 15. ISO 4869 (1981). Acoustics--measurement of sound attenuation of hearing protectors--subjective method. 4 pp.
Attenuation of hearing protectors
25
16. Pekkarinen, J. & Starck, J., Digital high-speed sampling of combined exposure to noise and vibration. Scand. J. Work Environ. Health, 12 (1986) 327-31. 17. Internal Report no. 21, Nordic Round Robin Test on Hearing Protector Measurements, Vol. I by Torben Poulsen, Nordtest project 336-82. The Acoustics Laboratory, Technical University of Denmark, 1984, p. 84. 18. Royster, L. H., An evaluation of the effectiveness of two different insert types of ear protection in preventing TTS in an industrial environment. Am. lndust. Hyg. Assoc. J., 41 (1980) 161-9. 19. Martin, A. M., An investigation of the relationships between the attenuation of earplugs and earmuffs with incident sound level for steady-state and impulse sound using instrumented cadaver ears. ISVR Tech. Rep. No. 95, 1977, pp. 1-37. 20. Rawlinson, R. D., Wheeler, P. D. & Custard, G., The acoustical attenuation of some combinations of earplugs and earmuffs. Ann. Occup. Hyg., 31(3) (1987) 299-309.
Application of the Probe Microphone Method to Measure Attenuation of Hearing Protectors Against High Impulse Sound Levels Chang-Chun Liu, Jussi Pekkarinen & Jukka Starck* Institute of Occupational Health, Laajaniityntie 1, SF-01620 Vantaa, Finland (Received 4 July 1988; revised version received 6 October 1988; accepted 28 October 1988)
A BS TRA C T The purpose of the present study was to develop a probe microphone method to measure the attenuation of earplugs, earmuffs and their combination against high impulse sound levels from military firearms. The field measurements were made on three earmuffs and one earplug. A probe microphone was supplied with a replaceable soft tube (outer diameter 1.5 mm) to transmit the sound pressure from the ear canal to the microphone. The tube was led between the skin and the cushion ring of the earmuff and through the earplug to the ear canal. A formula was developed to estimate the potential effect of tube length on the results. Subjective laboratory tests of earplugs showed good agreement with the probe microphone method below the 3"15kHz octave band. The field measurements showed that earplugs may provide attenuation equal to that of large-volume earmuffs against low-frequency impulses from large-calibre weapons. It was also shown that a prominent increase of attenuation can be achieved by using earmuffs and earplugs simultaneously.
INTRODUCTION
The use of hearing protectors has been increasing in industrial workplaces, and also when shooting with sport or military firearms. In many countries men participate in military service before entering working life. The frequency content of shooting impulses seems crucially important in the * To whom correspondence should be addressed. 13
Applied Acoustics 0003-682X/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
14
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
attenuation of hearing protectors, in the genesis of immediate hearing damage and in the risk of developing later occupational hearing loss. 1-3 The equal energy principle can be applied to the risk assessment of noiseinduced permanent hearing loss when the peak levels of industrial noise do not exceed 140 dB. 4'5 The American Conference of Governmental Industrial Hygienists has issued the recommendation that no industrial impulses in excess of 140dB peak level should be permitted. 6 It is known from previous studies that hand-held weapons produce about 150-160 dB, and anti-tank bazookas over 180dB peak levels, l'v's Allegedly, as the calibre increases, the peak level increases and the spectral maximum decreases. Laboratory tests have shown that earplugs are more effective than earmuffs against low-frequency noise. 9"1° However, laboratory tests have been carried out for steady state noise in 125-8000Hz one-third octave bands, the lowest octave already being disturbed by the occlusion effect.ix Thus the frequency range in the subjective tests has not covered the low frequencies present in shots from large-calibre weapons. Laboratory tests are justified in hearing protector design and in maintaining product quality control. The workplace results, when compared to laboratory test attenuations, indicate that 50% of the workers tested are receiving less than one-half of the potential attenuation of the earplugs. 12 In an industrial noise survey, earmuffs were found to decrease impulsiveness, in addition to the equivalent level. ~3 According to the damage risk criterion for shooting impulses, the longduration impulses corresponding to the low-frequency content of impulses cause a higher risk for hearing loss than high-frequency impulses, s The effect of low-frequency impulses has been studied in animal experiments. 2'3 Contrary to the previous damage risk criterion, the low-frequency impulses from cannons or howitzers caused relatively less damage than the impulses from rifles, when the same peak level was assumed for both weapons? However, the peak sound pressure level of a cannon is even higher than that of rifles, by 30 dB and, accordingly, cannon impulses are more harmful to hearing than rifle impulses. However, the effect of hearing protectors was not considered. 3 The miniature microphone has previously been used to measure the attenuation of soft foam plugs. However, the microphone and associated wires affected the fit of the earplug, and it was impossible to achieve a proper standard insertion? A probe microphone has been used with a metal tube inside the earplug to measure the attenuation of hand-held weapon impulses. 14 The purpose of the present study was to develop a probe microphone method to measure earplug attentuation in field conditions, and to measure
Attenuation of hearing protectors
15
the combined attenuation of earplugs and earmuffs against high-level impulses.
MATERIALS AND METHODS
Probe microphone Noise measurements were taken simultaneously outside and inside the hearing protectors. Noise inside the ear canal was measured with a 1/4 inch probe microphone (B&K 4135/WA 03787), which was originally designed for the measurement of hearing aids (Fig. 1). A normal 1/4 inch condenser microphone (B & K 4136) was used to measure the ambient noise outside the hearing protectors: it was mounted on a tripod located about 20 cm from the ear of the test person, at a grazing incidence in relation to the sound source. The dynamic ranges were 52-172dB for the probe microphone and 59-183 dB for the normal 1/4 inch microphone. The sound pressure was transmitted to the probe microphone through a replaceable soft tube made of silicon, with an outer diameter of 1"5 m m and inside diameter of 0.8 mm. The tube was pushed through the earplug, which was made of soft expanding plastic foam (E.A.R.). A small hole was pierced
Fig. 1.
The location and shielding of the probe microphone placed inside the aluminium cylinder; length 130 ram, diameter 30 mm, wall thickness 3 ram.
16
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
through the centre of the earplug without taking away any material. For earmuffs the tube was led between the skin and cushion ring. In the measurements taken for test persons, the length of the tube was 100 mm. The attenuation of the hearing protectors against the firearm impulses was defined as the difference between the peak sound pressure levels of the impulses outside and inside the hearing protectors. The unweighted peak levels were considered to comprise the most appropriate estimate for the risk of immediate damage in the inner ear.
Laboratory testing of the probe microphone measurements The effect of tube length on the results was tested using white noise (93+ l dB, 5 0 H z - 1 2 . b k H z ) as a test signal. The frequency response function between the two microphones was measured with a narrow-band two-channel digital signal analyser (HP 5420A) (Fig. 2). The probe microphone was placed inside the aluminium cover filled with absorption foam (Fig. 1). The attenuation of the microphone cover, with 100mm of tube blocked against ambient noise, was measured with a onethird octave analyser (B&K 2131) (Fig. 2). In the laboratory a _ 0 . b d B measurement accuracy was achieved for sound level meters and analysers. The standard laboratory test for subjective measurements was carried out with ten subjects to compare the attenuation of earplugs with and without the tube.15 A tube (length 20 mm) blocked at both ends was pushed through the earplug. The hearing of the subjects, aged 25 to 45 years, was tested prior to the measurements, and only those with normal hearing were accepted. In the test, the earplugs were inserted to about half of their length into the ear canal. The test persons fitted the device by themselves, under the supervision of the operator. The same ten subjects used for the subjective measurements participated in the measurements of noise reduction by probe microphone, and of the frequency response function of the open ear. During the measurement of the frequency response function, 2 0 m m of the 100mm probe tube was placed in the ear canal, near to the eardrum. White noise (100___0"3 dB, IO
[~]
M 2 IO
[-~
M 1
Aor B Fig. 2. Block diagram of the equipment in laboratory measurements. M1, B & K 4136 l/4inch condenser microphone for outside measurements; M2, B&K4135, probe microphone; a t, B&K 2209 sound level meter; a2, B&K 2204 sound level meter; A, HP5420A for narrow-band frequency response function analyses; B, B&K 2131 for the analyses of attenuation in one-third octave bands.
Attenuation of hearing protectors
17
20 Hz-20 kHz) was generated (B&K 1405) and amplified (B&K 2706) to the loudspeaker located in front of the subject, at a grazing incidence to the ear. The measurements were recorded by a one-third octave analyser (B&K 2131) from 125Hz to 10kHz. Field measurements
The attenuations of the earplug (E.A.R.) and the three earmuffs worn commonly by Finnish army conscripts were measured for cannon shots. The earmuffs selected were Silenta Mil (small cup volume, 67 cm 3, total weight 137 g), Silenta Ergo (medium cup volume, 145 cm 3, 194 g) and Silenta Super (large cup volume, 165 cm 3, 258 g), since they represent different acoustical attenuations. The distance between the weapon and the subject was 10m. When the measurement was made the subject was sitting facing the cannon. The normal 1/4 inch microphone and probe microphone were located as in the laboratory tests in relation to the test person and the direction of the sound source. The field measurements were taken by a digital multi-channel sampling unit (HP 6944), controlled by a microcomputer (HP 200) (Fig. 3). Digital sampling was preferred for the wide dynamic range that is necessary in accurate shot impulse measurements. Altogether, 30 cannon shots were sampled simultaneously from two channels during a time period of 164ms, with a rate of 50000 samples per second from each channel corresponding to the time resolution of 20/as. The amplitude resolution was 12 bits, corresponding to 72dB. 16 The
DISPLAY
i I b)
ELI SCANNER
I II
i
ICONVERTERI:
J t--" HP 6 9 4 4 multichannel
MICRO i COMPUTER
Jl I Ii[
HP 9 9 2 0 /
sampling unit PLOTTER
Fig. 3. The microcomputer system for noise measurements in the field conditions: (a) microphone B&K 4136, outside; (b) probe microphone B&K 4135, inside.
18
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
impulses were sampled with linear weighting, because A-weighting was considered inappropriate for high-level impulses causing immediate hearing impairment. 3 RESULTS The difference of peak levels between the channels was 1.1 _+0.4dB in five repetition measurements of cannon shots when both microphones were attached side by side on a tripod. Frequency response functions between the normal microphone and the probe microphone with and without the tube (lengths of 40-160 mm) were analysed with white noise in the laboratory (Fig. 4). When the probe microphone had no tube, the frequency response function was within __+2dB below 3-15kHz. The influence of the different tube lengths on the attenuation of noise was estimated by the frequency response function. The graphical estimates were calculated statistically using the following linear regression for logarithmic conversion (Fig. 4): y = 10(0.56× l o g f -
2.31)
X 0"1 X L + 0.000 296 x f - - 0"49
(1)
where Y is the attenuation (dB), L is the length of the tube (mm), a n d f i s the frequency (Hz). F r e q u e n c y r e s p o n s e function (dB) lO 8
6 4 2 o
-2 \-
-4 --6 --8-
-10 100
I 1000
'
lq-a 10 0 0 0 F r e q u e n c y (Hz)
Fig. 4. Frequency response function between normal microphone (B & K 4136) and probe microphone (B & K 4135) without tube (P~), with tube lengths of 40mm (¢~..,~,), 100mm (~zx-~t), 160mm (/'~_~,), according to the measurements in the laboratory (broken line), and according to the developed formula derived from the measurements eqn (1) (curved line).
Attenuation of hearing protectors
19
Standard deviation (dB) 105- ...........
-F:--
010~ I0
--¢
20 30.
a
,,t-, '<
I t
40.
1
[:-----
50.
60
125
250
5(~0
I'K
2'K 3.1'5K 4K 6.3K 8K Frequency (Hz)
Fig. 5. The average and standard deviation for the standard test and for the probe microphone measurement, N = 1 0 . - - , Probe microphone measurement; - . - , standard test (REAT) with normal earplug; - - ' , standard test (REAT), earplug with a hole and closed tube.
The frequency response function was used to evaluate the attenuation of the cover with a 100mm blocked tube, and it was evaluated at 25-33 dB, depending on frequency. This represents the methodological limit for the maximal attenuation of earplugs which can be measured. TABLE 1 Average Peak Sound Pressure Level and Standard Deviations Outside (L~k out) and Inside (L~a k in) the Hearing Protectors, and the Attenuation o f the Peaks by Hearing Protectors when Shooting with a 105 mm Cannon; N = number of impulses
Hearing protectors
N
L~, k out (dB)
L ~ in (dB)
Attenuation peak (dB)
Earmuffs Large-volume = Middle-volume = Small-volume =
5 5 5
154"3 ___1"8 154.5 -t- 1-6 153"0 ___ 1.2
136-9 ___2.4 139.3 + 2.9 145"3 __+0"7
17.4 _ 1'8 15.2 _ 1.4 7"7 _ 1.1
Earplug E.A.R.
5
152.2 + 2"4
134.7 + 3"0
17'5 + 1"1
Earplug E.A.I~. and earmuff with Large volume Small volume
5 5
154-5 _+ 0"4 154"6 _ 0"6
131" 1 ___2"7 128"8 _ 1"1
23-4 __+2"5 25'8 _ 0"9
= Measured with a previous method using two ( B & K 4136) microphones. 7
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
20
The difference between the attenuation achieved by the subjective test and that measured by the probe microphone method was below 3 dB in the 125 Hz to 2 kHz frequency range, and more than 10 dB above 2 kHz (Fig. 5). The results for the attenuation obtained with the subjective test and with the probe microphone method for the same subjects also show that the effect of the tube on the earplug attenuation was negligible below the 3.15 kHz octave band (Fig. 5). The peak levels inside and outside were averaged over five measurements for each protector and each combination of protectors (Table 1). The attenuation of the earplug against impulses from a 105 mm cannon (17"5 dB) was as good as the large-volume earmuff (17.4 dB) and the medium-volume earmuff (15.2 dB), and much better than the small-volume earmuff (7.7 dB) Sound :)ressure (Pa) 1100
550
-550
-1100 0
40
81
122
163 Time (ms)
Sound pressure (Pa) 1100
550
V ' "T'V"
,.v-v
......
-550
-1100
40
81
122
163 Time (ms)
Fig. 6. The sound pressure measured from 105 mm cannon shot outside ( (- • - • - • - ) (a) an earplug, and (b) an earplug and earmuff.
) and inside
Attenuation of hearing protectors
21
(Table 1). The combination of either the large-volume earmuff or the smallvolume earmuff with the earplug caused similar attenuation (Table 1). The duration of the first impulse peak is 3.6 ms for the cannon, which corresponds to a frequency of about 140 Hz (Fig. 6). The waveforms outside hearing protectors also show higher frequencies. However, inside the hearing protectors the curves of the impulses were smooth and the highfrequency noise had disappeared. The earmuffs, earplugs and their combinations attenuated more high-frequency noise than low-frequency noise.
DISCUSSION The tube length influences attenuation only at high frequencies, and its effect can be calculated by the developed formula (eqn (1)). The effect of a 100 mm tube was within _+3"5 dB below 2000 Hz. The difference between the attenuation of a normal earplug and an earplug with a closed microphone tube inside it measured by the standard test procedure was within 3"0dB at any frequency from 125Hz to 8kHz (Fig. 5). This indicates that the hole for the tube in the earplug was tight, and that the effect of leakage caused by the tube on the attenuation was estimated to be negligible. Inside the earmuff the tube was led between the skin and cushion ring. The comparison in the laboratory showed that, below 500Hz, the standard test yielded attenuation values that were 1.0-3-0dB higher than those measured by the probe microphone method. This was probably caused by amplified physiological noise due to the occlusion effect.11 The results suggest that, below the 3.15 kHz octave band, approximately the same attenuation can be obtained by the probe microphone method and the subjective test. The real-ear attenuation measured by the probe microphone method was generally in close agreement with the data of previous studies.~ 7,~8 Above 3-15 kHz the attenuation of the earplug measured by the subjective procedure was 10dB more than that of the probe microphone method (Fig. 5). This is probably due to the absorption of high-frequency noise by the 100mm tube used in the field measurements, in addition to the noise isolation of the tube against test noise. Comparing the results from a study using instrumented cadaver ears with those from the probe microphone method in laboratory conditions, close agreement below 2kHz was generally found for constant noise, a9 The cannon has frequencies that are mainly between 50 and 500 Hz, and the
22
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
frequency response function of the open ear is negligible in this frequency range, a9 Thus it has not been considered in the present study. The first pressure wave with a high positive peak followed by a negative pulse determines the wavelength and, further, the main frequency component of the impulse. This can be verified both theoretically and by simulating with an F F T analyser. For cannon impulses, this duration is less than 10ms, but for rifles it is less than 1 ms. The corresponding main frequencies are approximately 100 Hz for cannons and 1000 Hz for rifles. The attenuation of hearing protectors in subjective laboratory tests is quite different for continuous noise with these frequencies. We found that the attenuation for impulses was primarily in line with that for continuous noise. Cannon impulses were definitely less attenuated than rifle impulses in previous studies. 1 However, the transient behaviour of hearing protectors is not necessarily linear due to the leakages and extremely high pressures. We did not study the non-linear effects even though those could also have been present. In our study the emphasis was more on methodological testing than on investigating the properties of weapon types, the earmuff models and the individual differences in fitting the hearing protector to the head. These questions will need a considerably larger number of measurements, and it is suggested that this task should be carried out in future with the method presented here. The duration of the first positive sound pressure peak outside and inside the earplug corresponds to about 140 Hz (Fig. 6). In the present study, the attenuation of the earplug (17"5 dB) f o r c a n n o n shot was in agreement with that derived in the laboratory with the subjective method at 125 Hz (Fig. 5) and with that measured with instrumented cadaver ears at 125 Hz. 19 The field measurements showed the frequency dependence of the attenuation, which is in line with the laboratory test results using a subjective method.7'15 In a previous study the attenuation of the E.A.R. earplug with a metal tube was measured with a probe microphone method for hand-held weapon impulses. TM In contrast to our study, in that study the impulses were Aweighted. The only reason presented for the use of A-weighting was to avoid overloading of the tape recorder. There is no evidence to date for the use of A-weighting when measuring high-level impulse noise for risk assessment purposes. 2'3 The noise reduction of the E.A.R. earplug against impulses for cannon, measured in the field, was 10dB less than those measured for rifles in the study using instrumented cadaver ears. ~9 The reason for this difference is probably that the earplug offers more attenuation at high frequencies than at low frequencies. The field measurements showed that the attenuation of the earplug
Attenuation of hearing protectors
23
(17"5 dB) was the same as that of large-volume earmuff and about 10dB higher than that of the small-volume earmuff for cannon shots measured with a previous method. 1 The combination of earplug and large-volume earmuff generated approximately 6dB more attenuation than either the earplug or earmuff alone for cannon shot (Table 1). The attenuation of the combination of the earplug and the small-volume earmuff was 8 dB more than that generated by the earplug alone, and 18dB more than that generated by the small-volume earmuff alone (Table 1). Combining either the large-volume earmuffs or small-volume earmuffs with the earplug yields about the same overall attenuation, although the difference between the attenuation generated by the large-volume earmuff and by the small-volume earmuff alone was large (9.7dB). This result suggests that, for the earplug chosen for the present study, the choice of earmufftype was less important for their combined attenuation. This view is in agreement with a previous study. 2° When a standard insertion foam plug was combined with a small-volume earmuff, an additional 10 dB was gained in the laboratory, according to a previous study, as compared to the attenuation of the E.A.R. earplug alone. 9 The increases in total attenuation caused by the addition of the earmuffs are similar in these studies.
CONCLUSIONS The present measurement technique provides an estimate of hearing protector attenuation at low frequencies which has been impossible to achieve for earplugs with previously available methods. The probe microphone method is practical for measuring the high-level low-frequency impulses generated by large-calibre weapons in the field, but it is not suitable for the measurement of hand-held weapons such as titles, due to the attenuation of high frequencies. The attenuation of the peak pressure of cannon shot impulses was equal for earplugs and for large-volume earmuffs. The exposure limit for occupational impulse noise is 140dB peak level. 6 This limit is always exceeded in shooting with large-calibre weapons without hearing protectors. The simultaneous use of earmuffs and earplugs increased the attenuation by at least 6dB compared to their attenuation alone, but the total attenuation was less than the sum of the individual values. Even the combined protection is not always sufficient to attenuate peak levels below 140dB. However, the risk of heating loss was clearly reduced by using combined protection, and should be recommended for shooting with largecalibre weapons and for continuous noise dominated by frequencies below
24
Chang-Chun Liu, Jussi Pekkarinen, Jukka Starck
500Hz. For the earplug chosen, either large-volume or small-volume earmuffs increased attenuation remarkably. The attenuation of combined hearing protectors against low-frequency impulses cannot be estimated on their individual test results in the laboratory. The tube microphone method gives reliable results in the low-frequency range, which is typically poorly attenuated by hearing protectors.
REFERENCES 1. Ylikoski, J., Pekkarinen, J. & Starck, J., The efficiency of earmuffs against impulse noise from firearms. Scand. Audiol., 16 (1987) 85-8. 2. Price, G. R., Relative hazard of weapons impulses. J. Acoust. Soc. Am., 73(2) (1983) 556-66. 3. Price, G. R., Hazard from intense low-frequency acoustic impulses. J. Acoust. Soc. Am., 80(4) (1986) 1076-86. 4. ANSI S 3.28, Draft American National Standard, Methods for the evaluation of the potential effect on human hearing of sounds with peak A-weighted sound pressure levels above 120 decibels and peak C-weighted sound pressure levels below 140 decibels. Acoustical Society of America, 1986. 5. ISO/DIS 1999.2, Draft International Standard, Acoustics~etermination of occupational noise exposure and estimation of noise-induced hearing impairment. International Organization for Standardization, Geneva, 1985. 6. TLVs, Threshold limit values and biological exposure indices for 1987-1988, American Conference of Governmental Industrial Hygienists, 1987, pp. 100-2. 7. Starck, J., Pekkarinen, J. & Aatola, S, Attenuation of earmuffs against low frequency noise. J. Low Freq. Noise Vibr., 6 (1987) 167-74. 8. Pfander, F., Bongartz, H., Brinkmann, H. & Kietz, H., Danger of auditory impairment from impulse noise: a comparative study of the CHABA damagerisk criteria and those of the Federal Republic of Germany. J. Acoust. Soc. Am., 67(27) (Feb. 1980) 628-33. 9. Berger, E. H., Laboratory attenuation of earmuffs and earplugs both singly and in combination. Am. Indust. Hyg. Assoc. J., 44(5) (1983) 321-9. 10. Berger, E. H., Methods of measuring the attenuation of hearing protection devices. J. Acoust. Soc. Am., 79(6) (1986) 1655-87. 11. Berger, E. H., Influence of physiological noise and the occlusion effect on the measurement of real-ear attenuation at threshold. J. Acoust. Soc. Am., 74(1) (1983) 81-94. 12. Lempert, B. L. & Edwards, R. G., Field investigations of noise reduction afforded by insert-type hearing protectors. Am. lndust. Hyg. Assoc. J., 44(12) (1983) 894-902. 13. Pekkarinen, J., Industrial impulse noise, crest factor and the effect of earmuffs. Am. Indust. Hyg. Assoc. J., 48(10) (1987) 861-6. 14. Shenoda, F. B. & Ising, H., Sound attenuation of ear protectors under conditions of impulsive noise: field measurements. Appl. Acoust., 23 (1988) 297-307. 15. ISO 4869 (1981). Acoustics--measurement of sound attenuation of hearing protectors--subjective method. 4 pp.
Attenuation of hearing protectors
25
16. Pekkarinen, J. & Starck, J., Digital high-speed sampling of combined exposure to noise and vibration. Scand. J. Work Environ. Health, 12 (1986) 327-31. 17. Internal Report no. 21, Nordic Round Robin Test on Hearing Protector Measurements, Vol. I by Torben Poulsen, Nordtest project 336-82. The Acoustics Laboratory, Technical University of Denmark, 1984, p. 84. 18. Royster, L. H., An evaluation of the effectiveness of two different insert types of ear protection in preventing TTS in an industrial environment. Am. lndust. Hyg. Assoc. J., 41 (1980) 161-9. 19. Martin, A. M., An investigation of the relationships between the attenuation of earplugs and earmuffs with incident sound level for steady-state and impulse sound using instrumented cadaver ears. ISVR Tech. Rep. No. 95, 1977, pp. 1-37. 20. Rawlinson, R. D., Wheeler, P. D. & Custard, G., The acoustical attenuation of some combinations of earplugs and earmuffs. Ann. Occup. Hyg., 31(3) (1987) 299-309.