Download Dosimetry measurements using a probe tube

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.
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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
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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!.
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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.
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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
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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
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Lawrence I. Shotland: Probe microphone dosimetry measurements
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