Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Audiology and hearing health professionals in developed and developing countries wikipedia , lookup
Sensorineural hearing loss wikipedia , lookup
Soundscape ecology wikipedia , lookup
Noise-induced hearing loss wikipedia , lookup
Evolution of mammalian auditory ossicles wikipedia , lookup
Sound from ultrasound wikipedia , lookup
Sound localization wikipedia , lookup
Dosimetry measurements using a probe tube microphone in the ear canal Lawrence I. Shotland National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Building 10, Room 5C306, Bethesda, Maryland 20892 ~Received 4 January 1995; revised 25 August 1995; accepted 8 September 1995! Federal and international standards recommend use of microphone placement either on or in the vicinity of the shoulder for dosimetry to minimize deviations from the undisturbed sound field. Probe microphone measurements from the ear canal were compared to shoulder and chest measures in order to investigate the validity of current dosimetry methodologies. Six subjects were monitored in an industrial setting. As expected, ear-canal levels exceeded other measures for all subjects. Shoulder and chest measures showed very low intersubject variability whereas ear-canal measures resulted in large intersubject variability. The ear-canal methodology has the potential to identify individuals whose external ear gain exceed the mean, putting them at increased risk of noise-induced permanent threshold shift ~NIPTS!. It is proposed that overall external ear pressure gain be used as an index to adjust exposure levels when predicting NIPTS using ISO 1999. A normative database of external ear pressure gain was constructed from 30 ears for this purpose. PACS numbers: 43.50.Yw, 43.64.Wn, 43.66.Ed INTRODUCTION Effects of the external ear transfer function gain from the sound field to the eardrum are well documented ~Kuhn, 1979; Shaw, 1965, 1974; Wiener and Ross, 1946!. Transformations from either a diffuse or free field to the eardrum show a maximum pressure increase of 15–25 dB at a resonance frequency of 2.6 –3.0 kHz in the normal population. Furthermore, the frequency response of the ear canal is relatively independent of azimuth ~Shaw, 1965; Wiener and Ross, 1946!. Frequency and magnitude of the transfer function largely determine the amount of energy transferred through the middle ear enroute to the cochlea. Because the resonant peak sound-pressure level ~SPL! at the eardrum of some individuals may be as much as 10 dB greater than others exposed to an identical noise source, the risk of noiseinduced permanent threshold shift ~NIPTS! may be increased substantially ~Brüel, 1977; Price, 1979; Shaw, 1976; Tonndorf, 1976!. It has been suggested that pressure levels measured at the eardrum be used as the basis for setting maximum noise exposure levels ~Kuhn, 1979!. Although this concept is not widely disputed, it has not been practical to make direct measures at the eardrum outside of the laboratory due to instrumentation issues ~Brammer and Piercy, 1977; Hart, 1991; Kuhn, 1979!. Dosimetry is typically performed with the microphone mounted in various positions on or about the torso. ISO 1999 ~ISO, 1990! recommended placement 0.10 m from the entrance of the ear canal of the ear closer to the sound source, e.g., shoulder placement. The Mine Safety and Health Administration ~MSHA!, ~1994! requires shoulder placement of the microphone for personal dosimetry. MSHA based this requirement on the work of Seiler ~1982!, on the assumption that measurement differences at various microphone placements are the result of ‘‘errors’’ caused by body baffle and absorption effects. The approach does not take into consid979 J. Acoust. Soc. Am. 99 (2), February 1996 eration resonance of the external ear and the resulting transfer function gain ~Brammer and Piercy, 1977; Hart, 1991; Kuhn, 1979! due to anatomical or physiological influences of the individual ~Shaw, 1974!. Although this phenomenon is well known, the variability between individuals and its influence on NIPTS has received limited attention. The Occupational Safety and Health Administration ~OSHA! ~1983! also recommends shoulder placement, but showed flexibility in its recommendations based on the realization that new technology will, in all likelihood, facilitate new anticipated evaluation possibilities. Within the limits of instrumentation availability, investigators have addressed the problem of accurate measures by placing a microphone in the concha of the external ear. Brammer and Piercy ~1977! used a miniature microphone in the cavum of the concha to determine transfer functions from the sound field to the external ear. They then applied a shaped spectra in an attempt to reconstruct an ‘‘average’’ concha-to-center of the head transfer function for the purpose of estimating exposure. Hart ~1991! also used a microphone in the concha in a similar effort to develop transfer functions from the free field to the ear. Reviewed studies have the common shortcoming of actual transfer functions or pressure gains to the eardrum not being directly measured. The purpose of this study was twofold. First, to make direct pressure measures adjacent to the eardrum in a real world industrial setting in order to assess the effect of the outer ear on estimations of sound field exposure. Using this technique, it was possible to investigate the validity of traditional measurement routines. The second goal was to establish normative data for overall sound field-to-eardrum pressure gain functions which could then be applied as an exposure index to traditional dosimetry measures. 979 I. EXPERIMENT 1—METHODS A. Subjects After informed consent was obtained, six adult subjects were tested. All subjects were employed at the National Institutes of Health ~age range 29– 46 years!. Five subjects were paid volunteers from the Maintenance Engineering Branch; the author served as an unpaid volunteer. Five subjects were male; one female. All subjects were in good health and denied positive recent or active otologic history. Normal middle ear function was verified by the presence of tympanometric peaks 650 daPa using standard immittance measurement instrumentation and procedures. B. Instrumentation A Larson–Davis model 700 dosimeter was interfaced to a Lectret SA model 3000 microphone configured with a silicon probe and connected to a preamplifier ~Nicolet! and custom built power supply. The microphone was held in place on a headband ~Nicolet FlexSys! which allowed for anterior–posterior, superior–inferior, and medial–lateral positioning of the probe tube in the ear canal. Frequency response of the microphone was characterized by resonant peaks at 0.5 and 4 kHz, the second peak being approximately 20 dB down. Further roll-off occurred above 4 kHz. Frequency contributions above 4 kHz were negligible. Ear-canal resonance and length were measured using a Nicolet FlexSys real ear system in the laboratory. Length is defined as a straight line between the entrance to a point roughly halfway between the vertical axes formed by the top of the eardrum and the umbo ~Chan and Geisler, 1990!. For the entrance position, the junction of the posterior canal wall and floor of the concha was arbitrarily chosen because of relative ease in visualization and consistency between subjects ~Shotland et al., 1986!. Using an optical method ~Chan and Geisler, 1990! showed repeatability for mediolateral probe placement, i.e., 62 mm. Shotland et al. ~1986! demonstrated repeatability 61 dB for the first resonant peak of the transfer function. Placement using this system has been validated by Chan and Geisler ~1990!. Pre- and post-exposure psychophysical thresholds were measured from 0.25– 8 kHz using a sweep frequency stimulus presented through a Grason–Stadler model 10 audiometer. All testing was done in a double-walled sound suite ~Industrial Acoustics Company!. C. Procedures The study was completed at two refrigeration plants located on the campus of the National Institutes of Health. A major rationale for choosing these two sites was the extremely stable nature of the noise source, which typically varied by no greater than 1–2 dB between adjacent 2-s sampling points. Microphone sites were used in the ear canal, on the right shoulder, and on the left breast pocket. One ear was randomly selected and the probe positioned approximately 5-mm lateral to the eardrum, based on the measured earcanal length. The microphone probe pointed in an upward direction for shoulder placement and in a horizontal attitude 980 J. Acoust. Soc. Am., Vol. 99, No. 2, February 1996 away from the torso when positioned on the chest. Each subject was fitted with ear protection in both ears, except when the ear canal position was used, in which case that ear was not protected. The dosimeter was programmed to record according to OSHA ~1983! regulations, i.e., slow A-weighted integration time with 5 dB exchange rate. Using this paradigm, A-weighted levels were compared between the two torso microphone placements and the ear canal. The use of A weighting can introduce frequency biasing; however, because the noise spectrum was relatively flat, this was essentially canceled out, accounting for less than 0.5 dB error when external measurement levels were subtracted from those measured in the ear canals. In addition, unweighted peaks were recorded. Monitoring was done for 20 min at each microphone placement with data sampling every 2 s. The instrument was set for auto shutoff. For one subject ~LS! data collection consisted of 5 min at each microphone setting. For all subjects, measurements were first made in the ear canal, followed by shoulder and chest measurements. Because randomization was not used, the possibility existed that an order effect might occur. It was rejected after the following considerations. First, sound field measurements without the subjects present showed no large fluctuations or change in SPL over time. Second, because the subjects were not required to respond to a stimulus, no relevant learning effect occurred. Last, because subjects were not given prior briefing, their behavior was not influenced by the expected outcome. Thus any measurement differences are thought to be mediated exclusively by microphone placement. The subjects were given minimal instructions prior to monitoring. They were requested to follow their normal routine, which consisted of inspection or light repairs of the refrigeration units in the building. Their routes and azimuths relative to the sound source varied in a quasirandom fashion. Thus the microphone varied between being in the direct field and the sound shadow. No further attempts were made to control for exposure, other than to visually monitor the subjects. All of the subjects complied with the instructions. For the author only ~LS! exposures were systematically maintained at all microphone positions by use of a predetermined route, ensuring equal representation of measures made at the three microphone positions. Because the route was strictly adhered to, the quasirandom exposure conditions were identical between microphone positions, eliminating the need for an additional 15 min per condition. To eliminate experimenter bias, the subject was blinded to the results until after the measurements were completed on all subjects. A colleague ~MK! acted in the full capacity of the experimenter while the author was used as a subject. D. Data analysis Following each experimental session, the data were downloaded to a PC and examined. Sound-pressure levels were calculated by the software program for each measurement condition and analyzed for each of the three microphone placements using a one-way repeated analysis of variance ~ANOVA!. A projected dose was computed for each measure using Table G-16a ~OSHA!. Lawrence I. Shotland: Probe microphone dosimetry measurements 980 FIG. 1. One-third octave-band analysis for two sites used for the study. Measurements were made during normal operations using a Larson–Davis 800 B sound level meter set to ‘‘slow’’ and mounted on a tripod in the sound field. II. RESULTS AND DISCUSSION One-third octave-band analyses for the two refrigeration plants are shown in Fig. 1. Both sites yielded similar wideband spectra with energy present through 10 kHz. Overall A-weighted level, in dB typically varied between 93 and 99 dB over the course of the study, depending upon operating conditions and location of the recording microphone relative to the refrigeration units. Within a given recording session, sound field levels were very stable and essentially free of impulses. Further, equivalent exposure levels varied somewhat between subjects because outside air temperature dictated the number of refrigeration units actually used. One to five refrigeration units were operational at any given time. During shoulder monitoring of one subject ~CW!, one refrigeration unit lost power. The net effect was negligible, as a repeat measurement yielded an identical A-weighted level, in dB. Table I contains the summary statistics for the six subjects. No permanent threshold shifts were noted and any TTS resolved. Mean A-weighted pressure level from the six ear canals was 100.9 dB, versus 93.2 dB at the shoulder and 93.3 dB at the chest. The mean differences were 7.7 dB between the ear canal and shoulder and 7.6 dB between the ear canal and chest. Figure 2 illustrates the means with 99% confidence limits for A-weighted level, in dB for the six subjects. As this figure clearly illustrates, the ear canal position resulted in the majority of variability across subjects. The main effect of microphone placement showed statistical significance @F~2,10!544.4, p,0.001#. Post hoc analysis ~least FIG. 2. Mean pressure levels normalized to an 8-h workday and 99% confidence limits at the ear canal, shoulder, and chest. Shoulder and chest values are true L EX,8h measurements which approximate sound field levels. Because ear canal levels incorporate the external ear pressure gain into the measurements, they are not direct predictors of exposure levels, per say. squares means for microphone placement! showed significant differences between ear canal and shoulder ~t58.25, p,0.001! and ear canal and chest ~t58.07, p,0.001!. Shoulder versus chest was nonsignificant ~t520.177, p50.863!. MSHA requires ~and OSHA recommends! shoulder microphone placement to eliminate effects of body baffle, diffraction, and presumably resonance, to approximate the undisturbed sound field. The purpose of this experiment was to perform dosimetry measurements in the ear canal to investigate influences of the resonant characteristics upon measured pressure levels. In this way, it was possible to validate the premise of MSHA’s measurement rationale. Not surprisingly, A-weighted levels measured in the ear canal exceeded those at the shoulder and chest positions. Earlier laboratory studies, although designed somewhat differently in that spectra were preserved, nonetheless showed similar effects of earcanal resonance upon sound field levels ~Kuhn, 1979; Shaw, 1965, 1974; Wiener and Ross, 1946!. The virtually identical shoulder and chest exposure levels obtained in the present study gave an accurate representation of sound field levels and are supported by earlier reports for both continuous noise and puretones ~Basch, 1972; Burks, 1993; Kuhn, 1979; TABLE I. Summary statistics for L EX ~dB! and eardrum levels ~SPL!. 981 Subject Eardrum Shoulder Chest Ear canal resonance ~kHz! 1 2 3 4 5 6 106 100.5 96 100 99.5 103.5 94 95.5 90.5 93.5 91 94.5 94.5 95 90 94 93.5 93 1.8 2.5 2.8 2 2.5 2.2 24 20 24 24 27 21 Mean A-weighted levels ~dB! Standard deviation 100.9 93.2 93.3 2.3 23.3 2 1.8 0.37 2.5 3.5 J. Acoust. Soc. Am., Vol. 99, No. 2, February 1996 Ear canal length ~mm! Lawrence I. Shotland: Probe microphone dosimetry measurements 981 Nichols et al., 1947; Muldoon, 1973! as well as for impulse noise ~Shotland et al., 1994!. For the purpose of predicting sound field exposure levels, MSHA’s measurement protocol appears to be both valid and reliable under a variety of recording conditions. In contrast, the greater than expected variability found in the ear-canal measurements has important implications for risk assessment and hearing conservation. While it is of no concern for those having pressure gains equal to or less than the mean, those having eardrum levels exceeding the mean are at increased risk. Further, this risk is not identified using traditional measurement techniques because only the mean gain is incorporated into MSHA and OSHA regulations. Thus the potential utility of probe microphone measures is in identification of these individuals. These data provide validation to earlier recommendations that transfer function gain be included as an additional variable for NIPTS risk assessment ~Brüel, 1977; Kuhn, 1979; Price, 1979; Shaw, 1976; Tonndorf, 1976!. For illustrative purposes, the data in the present study were compared to predictions of NIPTS published by ISO.1 Differences between exposure levels at the shoulder and eardrum were determined, and pressure gains determined. The mean pressure gain from the shoulder to the eardrum for the six subjects was 7.7 dB. Subject #1 showed a pressure gain of 12 dB, 4.3 dB above the mean L EX,8h. It is this at-risk subject that illustrates the benefits to be gained by probe microphone dosimetry. Examining Table E.3 of Annex E ~ISO, 1990!, median NIPTS of 26 dB at 4 kHz is predicted from exposure to an A-weighted level of 95 dB for 40 years. For the at-risk subject falling above the mean, add 5 dB for L EX,8h5100 dB. One would instead refer to Table E.4, i.e., 100 dB. The NIPTS at 4 kHz after 40 years exposure at 100 dB is predicted to be 41 dB for the at-risk subject. This is a 15-dB increase in NIPTS and is greater than the 10 dB increase from the 50th to the 10th percentile ~26 dB versus 36 dB NIPTS! predicted from the 95-dB exposure. Conversely, subject #3 had the smallest pressure increase of 5.5 dB, and would be expected to incur less NIPTS than predicted by Tables E.3– 4. In consideration of this concept, resonant frequency of the outer ear may possibly influence NIPTS, thus adding another dimension to the predictions. III. EXPERIMENT 2—METHODS A. Subjects B. Instrumentation Pink noise was digitally generated by a Wavetek model 132 noise generator, amplified/attenuated, equalized, filtered and output through a Klipsch KG4.2 loudspeaker. All measurements were made in a sound attenuating, double-walled suite ~Industrial Acoustics Company! having a reverberation time of 0.155 s. Sound field and ear-canal measurements were made using a B & K type 4182 probe microphone J. Acoust. Soc. Am., Vol. 99, No. 2, February 1996 interfaced to a Larson–Davis 800B sound level meter. Microphone frequency response was flat with roll-off <5 dB through 5 kHz, and <10 dB at 10 kHz. Point-to-point variations in the sound field surrounding the position of the subjects’ heads were 61 dB. The sound level meter was connected via an extension cable and read in the adjacent room of the sound suite. C. Procedures External ear transfer functions were constructed using the substitution method ~Wiener and Ross, 1946!. Acoustically shaped bandpass filtered pink noise ~0.5– 6.3 kHz! was output through the speaker and SPL measured and recorded at the location of the middle of the subject’s head without the subject present. This measurement is referred to as the ‘‘reference’’ and was repeated for each subject. All measurements were made with the sound level meter set to ‘‘slow’’ and A-weighting. As with the field measurements, the use of A-weighting accounted for less than 0.5 dB of error. Following each reference measure, the subject was then seated with his/her ear canal 1.5-m distance from the speaker diaphragm at 0° azimuth and the measurement repeated. Subtraction of the reference from the ear-canal measure yielded the external ear pressure gain. IV. RESULTS AND DISCUSSION Thirty subjects were used ~11 male, 19 female!. They were recruited from ongoing NIH clinical protocols. Subjects were required to meet the same inclusion criteria as for experiment 1. 982 FIG. 3. Frequency distribution of the sound field-to-eardrum gain function. The abscissa represents transfer function gain in decibels, and the ordinate contains the number of subjects who demonstrated the various gain values shown. Bandpass-filtered pink noise was output through a loudspeaker and measured using a B & K 4182 probe microphone to take advantage of its relatively flat frequency response. The electrical signal was connected via an extension cable to a sound level meter in the adjacent room of the sound suite. Two methodologies have been described for determining exposure levels at the eardrum. The previously described technique made direct comparisons between dosimetry measurement conditions. A second method is based upon the application of laboratory norms to individual measurements. The data generated in experiment 2 are reported herein. Shown in Fig. 3 is a frequency distribution of the data for 30 subjects. A normal distribution was obtained, with a mean gain of 9.8 dB, range of 6.5 dB and standard deviation ~s.d.! of 1.8 dB. When compared to Table E of ISO 1999, 50% of the exposed population would be presumed to have transfer functions exceeding ISO values. Of the 50%, 16% lie above 1 s.d., i.e., 1.8 dB, with 2.5% above 2 s.d., which Lawrence I. Shotland: Probe microphone dosimetry measurements 982 equates to 3.6 dB. ISO 1999 provides the algorithm for calculating predicted NIPTS for the frequencies 0.5– 6 kHz at any desired L EX,8h. From an examination of the Gaussian distribution described above, it is estimated that a substantial subgroup of unidentified workers are at increased risk of NIPTS based on external ear resonance. OSHA ~1981! estimated that over 5 million American workers are exposed to hazardous noise in the workplace, while those of a recent study ~Caple, 1989! placed the number of individuals exposed to A-weighted levels of 85 to 90 dB between 5.2 and 8.9 million. Assuming a mean L EX,8h of 87.5 dB, roughly 0.832–1.4 million individuals are in all likelihood subjected to daily noise levels of 89 dB or greater ~1 s.d.!, based on the data of Caple ~1989!. For those exposed to levels approaching 90 dB, it is impossible to know how many are actually exposed to levels exceeding 90 dB, or how many more than the 5 million estimated by OSHA are exposed to hazardous levels. What is accepted as known is that 8 million factory-based employees have occupationally related NIPTS ~NIOSH, 1988!, in spite of federal and state laws attempting to prevent such occurrences. From the data presented herein, it is suggested that the unknown quantity pressure level at the eardrum, is contributing to the decrease in effectiveness of hearing conservation programs. V. GENERAL DISCUSSION A-weighting is the acceptable methodology for assessing risk of NIPTS. However, it’s validation and indications for use have been limited exclusively to the external sound field. The utilization of A-weighting to derive single-number levels introduces additional applicability issues. Unlike subtraction of spectra, in which the input spectrum and any weighting are independent, the comparison of two singlenumber values can result in bias. The bias can result from either the input spectrum or from weighting. In this study, the assumption is made that the noise field spectrum either is flat or approximates pink noise. Under these conditions, minimal error will occur. When the spectrum deviates from a flat response, such as when energy peaks occur that are coincident with ear canal resonance, the assumption has been violated, in which case risk of hearing loss will either be overor underestimated. However, this would be an isolated event. In comparison, the bias introduced by A-weighting is small and predictable, dependent upon the noise level and bandwidth. For typical wideband industrial noise having A-weighted levels exceeding 80 dB, bias will typically be less than 0.5 dB. A somewhat more involved but potentially useful method to circumvent the above issues would be the use of individual transfer functions. These could be measured in the field using a probe microphone and portable spectrum analyzer and compared to an A-weighted ‘‘average’’ transfer function, designated ‘‘T-weighted transfer function.’’ Use of this measurement might help to fully establish exposure risk relative to the average person, independent of the noise field spectrum. Many contributors to susceptibility of TTS and NIPTS have been identified in humans ~Davis and Ahroon, 1982; Hood et al., 1976; Lindgren and Axelsson, 1988; Royster 983 J. Acoust. Soc. Am., Vol. 99, No. 2, February 1996 et al., 1980; Spillmann, 1994; Thomas et al., 1979; Tota and Bocci, 1967!. Some of the many factors examined are pigmentation, race, exercise, smoking, menstrual cycle, as well as probable but not yet identified genetic causes. The majority of individual susceptibility to NIPTS is most likely caused by an interaction of several contributors which influence biochemical and metabolic processes. Unfortunately, at present it is not possible to isolate or control for these factors. External ear pressure gain, however, is a relatively simplistic parameter which can be quantified. The magnitude of gain is based largely on anatomical considerations, which are easily identified. Indirect but closely related evidence for this argument was shown by Caiazzo and Tonndorf ~1977!. They experimentally doubled the length of the ear canal and showed that the frequency of TTS was lowered by one octave. Further, the magnitude of pressure gain at the eardrum is an isolated event, so as to preclude interactions with, or influences by other factors. In addition, anatomic contributors are constant and unlikely to change significantly without being noticed, unlike more subtle but important functional changes such as circulation or metabolism. With the above concepts in mind, a logical first step in modifying a hearing conservation program is to identify individuals who may be at increased risk of NIPTS by virtue of their external ear pressure gain. Those with pressure gains exceeding the sample population mean by a predetermined factor, e.g., 2 s.d., would be enrolled in a modified hearing conservation program such as required by OSHA regulations, in much the same way as individuals who have suffered standard threshold shifts. Secondly, once this individual has been placed in the hearing conservation program, dosimetry should be programmed to accommodate their external ear pressure gain. For example, assume a sound fieldto-eardrum gain function of 15 dB, or 5 dB greater than the population mean. For this person, a shoulder reading of 90 dB ~100% noise dose!, is equivalent to 105 dB at the eardrum, an effective noise dose of 200%. The dosimeter would be adjusted so that the criterion level is now 85 dB, thus normalizing the exposure level at the eardrum to that of the mean gain function. VI. CONCLUSIONS The use of a probe microphone for dosimetry measurements in the ear canal was shown to be a practical alternative to traditional torso or area surveys. Because the pressure gain of the individual’s external ear is directly incorporated into the measure, the measurement technique has the capability to delineate actual pressure levels reaching the middle ear, with potential value in identifying those with unsuspected, increased susceptibility to NIPTS. With use of the normative values presented, individual risk may be more fully explored using an easily individualized program. Lawrence I. Shotland: Probe microphone dosimetry measurements 983 ACKNOWLEDGMENTS Completion of this study would not have been possible without the enthusiastic cooperation of the Division of Safety and the Maintenance Engineering Branch at the NIH. The author wishes to thank Dr. Daniel Johnson and three anonymous reviewers for their comments on an earlier draft. Dr. Mauricio Kurc is gratefully acknowledged for his assistance in data collection. Portions of this study were presented at the American Academy of Audiology Sixth Annual Convention, Richmond, April 1994. ISO 1999 International Standard ~ISO, 1990! for acoustics attempts to predict risk of NIPTS and handicap as a result of equivalent A-weighted sound exposure level L EX,8h. Annex E provides expected NIPTS for sound exposure levels L EX,8h from 85–100 dB for duration of 10– 40 years. These predictions are based on mean exposure levels measured either at the head position of the worker with the worker absent, or located 0.1 m lateral to the entrance to the ear canal. A parametric approach is taken, with predictions given for population percentiles. The standard does not attempt to predict individual threshold shifts. 1 Basch, M. W. ~1972!. ‘‘A wearable pocket noise dosimeter,’’ Noise Measurement 5, 4 –10. Brammer, A. J., and Piercy, J. E. ~1977!. ‘‘Monitoring sound pressures within the ear: Application to noise exposure,’’ J. Acoust. Soc. Am. 61, 731–738. Brüel, P. V. ~1977!. ‘‘Do we measure damaging noise correctly,’’ Noise Control Eng. J. 8, 52– 60. Burks, J. A. ~1993!. ‘‘Effect of microphone placement on personal dosimeter measurements: A review,’’ Meeting of the American Industrial Hygiene Association, New Orleans, LA. Caiazzo, A. J., and Tonndorf, J. ~1977!. ‘‘Ear canal resonance and temporary threshold shift,’’ Meeting of the American Academy of Ophthalmology and Otolaryngology, Dallas, TX. Caple, G. E. ~1989!. ‘‘Effective hearing programs require assessment testing, enforcement,’’ Occup. Health Safety 58, 48 –9, 52. Chan, J. C. K., and Geisler, C. D. ~1990!. ‘‘Estimation of eardrum acoustic pressure and of ear canal length from remote points in the canal,’’ J. Acoust. Soc. Am. 87, 1237–1247. Davis, M. J., and Ahroon, W. A. ~1982!. ‘‘Fluctuations in susceptibility to noise-induced TTS as influenced by the menstrual cycle,’’ J. Aud. Res. 22, 173–187. Hart, M. D. ~1991!. ‘‘Measurement of sound using a microphone at the ear,’’ InterNoise 91 2, 1065–1068. Hood, J. D., Poole, J. P., and Freedman, L. ~1976!. ‘‘The influence of eye colour upon temporary threshold shift,’’ Audiology 15, 449– 464. ISO ~1990!. ISO 1999.2, ‘‘Acoustics: Determination of occupational noise exposure and estimation of noise-induced hearing impairment’’ ~International Standards Organization, Geneva, Switzerland!. Kuhn, G. F. ~1979!. ‘‘The pressure transformation from a diffuse sound field to the external ear and to the body and head surface,’’ J. Acoust. Soc. Am. 65, 991–999. Lindgren, F., and Axelsson, A. ~1988!. ‘‘The influence of physical exercise 984 J. Acoust. Soc. Am., Vol. 99, No. 2, February 1996 on susceptibility to noise-induced temporary threshold shift,’’ Scand. Audiol. 17, 11–17. Mine Safety and Health Administration ~1994!. 30 CFR; 70.506, ‘‘Noise exposure measurement procedures; instrument setting; calibration,’’ US Dept. of Labor, Washington, DC. Muldoon, T. L. ~1973!. ‘‘Response variations of a microphone worn on the human body,’’ Report No. 7810, Bureau of Mines, Washington, DC. National Institute for Occupational Safety and Health ~1988!. ‘‘Proposed National Strategy for the Prevention of Noise-Induced Hearing Loss,’’ DHHS Publication No. 89–135, edited by D. E. Dunn, NIOSH Working Group, 51– 63. Nichols, R. H., Marquis, R. J., Wicklund, W. G., Filler, A. S., Hudgins, C. V., and Peterson, G. E. ~1947!. ‘‘The influence of body-baffle effects on the performance of hearing aids,’’ J. Acoust. Soc. Am. 19, 943–951. Occupational Safety and Health Administration ~1981!. ‘‘Occupational noise exposure; hearing conservation amendment,’’ Fed. Regist. 46~11!, Part III ~US Dept. of Labor, Washington, DC!. Occupational Safety and Health Administration ~1983!. ‘‘Occupational noise exposure; Hearing Conservation Amendment; Final Rule,’’ Fed. Regist. 48~46!, Part II ~US Dept. of Labor, Washington, DC!. Price, G. R. ~1979!. ‘‘Loss of auditory sensitivity following exposure to spectrally narrow impulses,’’ J. Acoust. Soc. Am. 66, 456 – 465. Royster, L. H., Royster, J. D., and Thomas, W. G. ~1980!. ‘‘Representative hearing levels by race and sex in North Carolina industry,’’ J. Acoust. Soc. Am. 68, 551–566. Seiler, J. P. ~1982!. ‘‘Microphone placement factors for one-half inch diameter microphones,’’ unpublished M.S. thesis, University of Pittsburgh, Pittsburgh, PA. Shaw, E. A. G. ~1976!. ‘‘Diffuse field sensitivity of external ear based on reciprocity principle,’’ J. Acoust. Soc. Am. Suppl. 1 60, S102. Shaw, E. A. G. ~1965!. ‘‘Earcanal pressure generated by a free sound field,’’ J. Acoust. Soc. Am. 39, 465– 470. Shaw, E. A. G. ~1974!. ‘‘Transformation of sound pressure level from the free field to the eardrum in the horizontal plane,’’ J. Acoust. Soc. Am. 56, 1848 –1861. Shotland, L. I., Robinette, M. S., and Brey, R. H. ~1994!. ‘‘Effects of microphone position and azimuthal sound-incidence angle on peak sound pressure level of acoustic impulses,’’ Noise Control Eng. J. 42, 149–157. Shotland, L., Jenison, R., and Hecox, K. ~1986!. ‘‘The effect of probe tube depth on real ear measures,’’ American Speech-Language-Hearing Association, Detroit, MI. Spillmann, T. ~1994!. ‘‘Genetic diseases of hearing,’’ Curr. Opin. Neurol. 7, 81– 87. Thomas, G. B., Williams, C. E., and Hoger, N. G. ~1979!. ‘‘Some nonauditory correlates of the hearing threshold levels of a noise-exposed aviation population,’’ J. Acoust. Soc. Am. Suppl. 1 65, S119. Tonndorf, J. ~1976!. ‘‘Relationship between the transmission characteristics of the conductive system and noise-induced hearing loss,’’ Effects of Noise on Hearing, edited by D. Henderson, R. P. Hamernik, D. S. Dosanjh, and J. H. Mills ~Raven, New York!, pp. 159–177. Tota, G., and Bocci, G. ~1967!. ‘‘The importance of the colour of the iris on the evaluation of resistance to auditory fatigue,’’ Rev. Oto-neurooftalmolog. 42, 183–192. Wiener, F. M., and Ross, D. A. ~1946!. ‘‘The pressure distribution in the auditory canal in a progressive sound field,’’ J. Acoust. Soc. Am. 18, 401– 408. Lawrence I. Shotland: Probe microphone dosimetry measurements 984