AD- 7 85 386
HEARING PROTECTION OF EARMUFFS WORN
OVER EYEGLASSES
Charles W. Nixon, et ai
Aerospace Medical Research Laboratory
W r igh t - Pa t te r s on Air Force Base, Ohio
June 1974
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HEARING PROTECTION OF EARMUFFS WORN OVER
EYEGLASSES
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IS. KEY W090S fCwilinvt on roooroo oldo It nocomoory ond Idonllty by block number)
Hearing Protection Sound Protection
Ear Protectors Personal Protection
Earmuffs Attenuation Devices
Earmuff Compatibility
20. ABSTRACT (Contlnvo on roooroo otdo It nocoooocy ond Idonllty by 6/oeJr numb or)
The hearing protection ordinarily provided by earmuffs is reduced when worn by
persons who also wear eyeglasses because sound enters the device throught air
leaks around the eyeglass temple - earmuff cushion interface. This study ex¬
amined the acoustic fit of different earmuff protectors and various types of eye¬
glass frames found in a population, measured the loss of attenuation due to
programmed air leaks and measured differences in earmuff protection for subjec
subjects while wearing and not wearing eyeglasses. Results demonstrated that
DD l JAN 7) 1473 EDITION OP I NOV $5 IS OBSOLETE^
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iCCuni f . ■•. ..‘.Oiiqw or TSI5 PAGEfniiw Cm
earmuffs worn over eyeglasses lose from 1 dB to 10 dB of attenuation at individ-1
ual frequencies. The amount of loss is related to type of earmuff, type of eye- j
glasses as well as frequency of the sound. Two remedial approaches were
identified as (1) authorizing for use by eyeglass wearers only earmuffs that
demonstrate by test satisfactory sound protection with eyeglasses and (2)
the use of an insert or pad at the eyeglass temple - earmuff cushion interface
to minimize and eliminate the acoustic leak. j
10/
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FOR THE COMMANDER
sff* — — i «- (£- Vvw
HENNlfiwE.
VON GIERKE
Director
Biodynamics and Bionics Division
Aerospace Medical Research Laboratory
I
t
AIR FORCE/S«7tO/J S«pt»mo«r 1»?4 - 100
SUMMARY
Laboratory evidence, both of a physical and psychophysical nature,
substantiates informal subjective reports from operational situations
that attenuation of earmuffs is reduced when worn over eyeglasses.
This reduction ranges from about 1 da to 10 dB at individual frequencies
and is shown to be associated with air leaks created when the eyeplass
frame keeps the earcup seal away from the side of the head. The amount
and the patterns of the losses vary from earmuff to earmuff and with type
of eyeglass temple. The greater the distance of the earcups away from
the head, the greater is the air leak and subsequent attenuation loss.
Within limits , the size of the air leak corresponds to the amount of
attenuation loss with larger leaks showing larger losses . Attenuation
losses were greater at the low and high frequencies than at the middle
frequencies .
Well-trained subjects demonstrated via psychophysical methods that
the standard AF issue zylonite eyeglass frame does contribute to losses
of attenuation when worn with standard AF circumaural ear protectors.
The amount of the loss varies with the particular earmuff worn and to
some extent with the type of eyeplass bow, as well as with variations
in the configuration of the wearer's head. In addition, the nominal
amount of loss for an earmuff-eyeglass combination at each of the individual
test frequencies can be specified, on the average. The incidence of eye¬
glass users in the population is relatively high and of sufficient proportion
that the loss of sound attenuation is considered an operational problem.
The critical issue is whether eyeglass wearers experience more noise
induced hearing loss (with earmuffs; than non-wearers as a result of
the reduced protection. An investigation of hearing levels of AF
1
personnel who wear the earmuff-eyeglass combination vs those who do not
wear eyeglasses but work in the same noise environs should br conducted.
AF Forms 1490, Hearing Conservation Data and 1491, Reference Audiogram
already contain information regarding the use of earmuffs as well as
eyeglasses (including safety glasses). Therefore, these data should be
available for investigation from the USAF Hearing Conservation Data
Registry, Brooks AFD as.
In view of the eyeglasses-earmuf f hearing protection problem discussed
herein, some remedial action seems desirable. Of the various alternatives
dicussed, two appear to be most workable: (1) evaluation of earmuff
performance with eyeglasses would be routinely accomplished when earmuffs
are initially evaluated for potential AF use. Those items showing little
attenuation loss with eyeglasses would be identified for use by eyeglass
wearers while those showing significant loss would not be approved for
eyeglass wearers. (2) Another approach would be to provide removable
inserts or pads to be used at the temples of all earmuff-eyeglass
wearers to effectively minimize or eliminate the leakage-protection
problem. A short-term applied research effort could identify suitable
materials and configurations, their relative efficiency in terms of
increased protection and provide guidance for implementation by earmuff-
eyeglass wearers.
PREFACE
This study was accomplished by the Bioacoustics Branch, Biodynamics
and Bionics Division of the Aerospace Medical Research Laboratory, Wright
Patterson Air Force Base, Ohio. The research was conducted by Charles
W. Nixon, Ph.D. and SSgt William C. Knoblach under Project 7231, "Bio¬
mechanics of Aerospace Operations," Task 723103, "Biological Acoustics
in Aerospace Environments," and Work Unit 16 , "Auditory Responses to
Acoustic Energy Experienced in Air Force Activities." Acknowledgement
is made to Mr. Jack Kelly, formerly of the University of Dayton Research
Institute, and Capt David Krantz of the Bioacoustics Branch for their
support .
3
TABLE OF CONTENTS
PAGE
Summary . . 1
Preface . 3
Introduction . 4
Purpose ... . ......... . b
Approach . ........... 5
Lyeglass-Earmuff Interaction . 6
Programmed Air Leaks . 10
Psychoacoustic Tests . 16
Discussion . - . . 23
References ......... . 26
4
LIST OF TABLES
Page Wo.
Mean Distance of Eyeglass Temples From Side of Head 8
When Worn With Four Different Earmuff Protectors
Mean Difference Scores Between Earmuff Attenuation When 21
Worn With vs_ Without Eyeglasses
Differences in Attenuation of Earmuffs Worn With and 22
Without Eyeglasses (In Decibels)
LIST OF ILLUSTRATIONS
Page No.
Physical System Used to Measure Effect of Various 12
Programmed Air Leaks on Earcup Attenuation
Loss of Earcup Attenuation Due to Programmed Air Leaks 14
In Four Different Levels of Ambient Noise
Variation in Amount of Attenuation Loss Due to Different
Sizes of Air Leaks As A Function of Earcup and Test ID
Frequency
Differences in Attenuation l'J
Average Attenuation of Several Earmuf f-Eyeglass Combinations
With and Without A Foam Insert (Pad) At The Temple 2b
INTRODUCTION
United States A.ir Force noise sources comprise some of the most
intense acoustic environments in existence. These environs require major
ongoing programs of noise control and hearing conservation to insure that
Air Force personnel are not unnecessarily exposed to noise levels exceeding
the limits specified in Air Force Regulation 161-35, Hazardous Noise
Exposure* 27 July 1973 (7). When noise control measures for maintaining
exposures to within limiting values are not feasible, personal hearing
protective devices are required. A variety of earmuff type and insert
earplug type protectors are provided to individuals routinely exposed to
intense noise. Currently, AF standard earmuffs are distributed in the field
as personal equipment items and AF standard earplugs are dispensed as medical
service items. Both types of sound protectors are in widespread use
throughout the world.
Observations over the years and informal subjective reports from
personnel working in noise environments have indicated that some of the
types of earmuffs in use do not appear to provide adequate sound protection
for persons wearing eyeglasses. Presumably air leaks occur at the points
where the earmuffs fit over the eyeglass temples’'' and these leaks result
in reduced sound protection.
A number of different earmuff protectors are found in the USAF
inventory (5). Earmuff protectors are procured by the AF in large quantities
through a central nurchasinp, procedure. Procurement is accomplished on a
competitive basis which involves the selection of one specific device for
purchase from a gioup of items which are qualified as technically acceptable
by test (6) and are included on a Qualified Products List (QPL) . Each
’•The terms eyeglass "temple" and eyeglass "bow" are used interchangeably.
0
central procurement selects one earmuff from the QPL for acquisition and
placement in the world-wide inventory. Subsequent procurements may and
do select different items from the same list. As a consequence of this
method, which employs a list of qualified products, several different
earmuffs are now in use in AF operational situations. The evaluation of
earmuffs for consideration of their inclusion on the QPL and possible use
by the AF does not include tests of their effectiveness when worn over
eyeglasses .
PURPOSE
The purpose of this investigation was to evaluate the hearing protection
performance of earmuff protectors, some of which are presently on the AF
Qualified Products List , when they are worn by persons who also wear eye¬
glasses. This effort considered if problems existed, the nature of the
problems and recommendations for remedial action where appropriate.
APPROACH
Decrements in the amount of protection provided by earmuffs when they
are worn over eyeglasses may be a function of the inability of the earcup
cushion to fit closely around the eyeglass frame and form a good acoustic
seal against the head. The degree to which this acoustic seal is or is not
accomplished and the extent of the resulting air leak determines the
reduction in protection from that obtained when eyeglasses are not worn.
Since earmuffs differ in shape, size, material flexibility, and the like,
from manufacturer to manufacturer, some may be better than others when
worn over eyeglasses. This effort was carried out in three phases, each
of which was directly concerned with determining the compatibility of
7
earmuffs with the use of eyeglasses, and it attempted to quantify the
amount of difference between attenuation of earmuffs when worn with and
without eyeglasses.
The first phase of the study considered the relationship of a
muff-type protector to the various types of eyeglass frames found in a
typical population, primarily the fit or seal of the muffs to the head.
The second portion was concerned with physical measures of loss of
attenuation for earmuffs due to programmed leaks created by using various
sizes of hollow tubing inserted under the earcup cushion. The third phase
involved measurements of the actual differences in attenuation provided
with the QPL earmuffs for the same subjects while wearing and not wearing
eyeglasses .
EYEGLASS-EARMUFF INTERACTION
Typically an earmuff protector in use encircles the pinna and the
earcup cushion rests against that area of the head immediately surrounding
the ear to provide an acoustic seal against the outside noise. Maximum
sound protection demands that a good acoustic seal be accomplished and
maintained. Ideally, an earmuff cushion should fit equally the individual
who wears eyeglasses as well as it does those who do not wear them.
However, observation and experience suggest that eyeglasses do interfere
with the proper fit and seal of the earmuff cushion.
Eyeglass Temple Disp lacement of Earmuff
Earmuffs rest against the bows of the user’s eyeglasses just in
front of the pinna. Some types of bows appear to "bend" inward under
the weight or tension of the muffs and to rest against the sides of
8
the head, while others hold the earmuff seal away from the head creating
an obvious air leak that is visible to an observer. The actual displace¬
ment or distance of the eyeglass bows from the head of each subject was
measured with various earmuffs in place on the head and compared to the
same measurements when no earmuff was worn.
All subjects who participated in the measurement survey normally
wore eyeglasses and measurements were taken with their own personal
eyeglasses which had been professionally fitted to them by their own
physicians. Consequently, the data are representative of the types of
frames and the kinds of fits that might be expected in typical populations.
All measurements were taken by an individual with training and experience
in the fitting of eyeglasses.
The eyeglass temple displacement with and without earmuffs was
measured on more than 100 volunteers, both left and right ears, and the
various types of eyeglass bows observed were tabulated. Approximately
80% of the bows were of various sizes and thicknesses of plastic, about
10% were metal and about 10% of thin wire. The mean displacement values
measured on these individuals are shown in table 1.
It was assumed, prior to initiation of the measurement survey, that
placement of the earmuff over the eyeglass temples would reduce the
distance of the bows from the sides of the head. Contrary to this assump¬
tion, it was observed that three of the four earmuffs measured with eye¬
glasses showed bow displacements from the side of the head that were
greater with the earmuff than when no earmuff was worn (table 1). It
appeared, on inspection, that the muff may have exerted pressure on that
poition of the eyeglass bow behind the pinna in such a way that the
9
TABLE I
MEAN DISTANCE OF EYEGLASS TEMPLES FROM SIDE OT HEAD
WHEN WORN WITH FOUR DIFFERENT EARMUFF PROTECTORS
EYEGLASSES
EYEGLASSES
EYEGLASSES
EYEGLASSES
EYEGLASSES
WITHOUT
WITHOUT
WITHOUT
WITHOUT
WITHOUT
EARMUFF
EARMUFF A
EARMUFF B
EARMUFF D
EARMUFF E
RIGHT SIDE
6.33*
6.11
6.61
6.49
6.78
LEFT SIDE
6.30
5.67
6.75
6.50
6.68
^DISTANCE IN MILLIMETERS
10
forward part of the bow "bulged out" at the temporal area of the head
and the effectiveness of the seal around the bow in front of the pinna
was reduced.
The exception to this finding was demonstrated by Earmuff A, which
was the only device for which the measured displacement of the eyeglass
bows was less with than without the earmuff. The earcup opening for this
unit is quite large and the cushion is relatively narrow. This configur¬
ation appeared to allow the cushion to seal against the head behind that
portion of the bow which extends behind the ear of the subject instead
of resting against the end of the frame. The other earmuff s examined
have smaller openings and wider cushions which press against the end of
the frame. On this basis, device A would be expected to show the least
amount of attenuation decrement of the muffs examined when worn with
eyeglasses.
Earcup Cushion Material
Perhaps the most common, and possibly most important source of air
leak when earmuffs are worn with eyeglasses, is the degree to which the
material of the earcup cushion fails to conform to or around the eyeglass
bow. The more compressible and flexible materials are better able to
mold or form themselves around the temples providing a more effective
seal than with the less conforming cushions. This characteristic and
its relationship to attenuation loss is clearly demonstrated in a report
by Webster and Rubin (4) which examined earmuff protection for individuals
wearing eyeglasses as a function of three types of cushion material on
the earmuffs.
11
Si ze of Eyeglass Temple
Another factor which contributes to loss of attenuation due to air
leaks, which is not independent of cushion material, is the physical
thickness or size of the eyeglass bow. Generally, the greater the thickness
of the bow, the greater is the possibility of loss of attenuation due to
air leaks. Effects of military issue type frames are reflected in the
psychoacoustic measurements which appear later. The effects of thin wire
bows would ordinarily be expected to be negligible in front of the pinna,
all other variables excluded.
The amount of air leak and attenuation loss appears to be a function
of various combinations of at least the three factors mentioned above,
the displacement of the temples from the sides of the head, the ability
of the earcup cushion material to conform around the temples, and the
thickness of the temples or bows. In addition, the shape of the head
of the wearer, the amount of headband tension, the degrees of freedom
of the headband suspension, and the like, may all contribute singly or
in combination to a reduction in acoustic seal and attenuation of an
earmuff worn over eyeglasses. The earmuff itself would appear to be
the most controllable factor of those identified.
PROGRAMMED AIR LEAKS
The necessity of obtaining a good acoustic seal with circumaural
devices to insure maximum hearing protection is demonstrated by a series
of physical measures of attenuation of earmuffs for which simulated air
leaks were created. A flat plate system for measuring sound pressure
levels inside an earcup was assembled and calibrated in accordance with
12
figure 1. The condenser microphone in the flat plate system recorded
the amount of sound pressure present inside the test earcups . Attenuation
of four different test earcups was measured first without an air leak
and then again with simulated air leaks. The sizes of the air leaks were
determined by selecting plastic tubing with inside diameters ranging from
0.046 mm to 0.233 mm. The plastic tubing (3/4" lengths) was positioned
between the flat plate and the earcup cushion for the measurements. Soft,
clay was used to seal around the plastic tubes and assure that the only
air leak was through and not around the tube. Care was taken to assure
that the tube was not collapsed by the weight of the earcup or by the
clay used for sealing around the tubes. A constant static pressure of
1000 grams was applied to each earcup during the measurements.
Earcup performance with and without the four simulated air leaks
was measured for various test frequencies at four different intensity
levels of broad band noise exposure: 70 dB, 80 dB, 90 dB, and 100 dB SPL.
Observation of the data reveals that the amount of attenuation loss
due to air leaks is reasonably constant with ambient level for the range
of measurements recorded and that the attenuation is generally the same
at 100 dB as it is at 70 dB, particularly at the frequencies most affected
by leaks. This "constancy" characteristic permits us to discuss loss
due to air leaks in terms of amount of loss, test frequency, and particular
earmuff involved, without specifying the various intensity levels (within
the range investigated).
Attenuation losses due to programmed air leaks were examined for
tubing with inner diameters covering a wide range of sizes, however,
13
AMPLIFIER
r
ATTENUATOR
OSCILLATOR
L_
NOISE GENERATOR
LOUDSPEAKER
SOUND SOURCE
CONSTANT STATIC
PRESSURE
(1000 grains) „
EARMUFF UNDER
EST
CONDENSER MICROPHONE
AND PREAMPLIFIER
PROGRAMMED
^X^AIR LEAK
I
FLAT PLATE
COUPLER
AUTOMATIC
FILTER
AUTOMATIC
GRAPHIC RECORDER
PHYSICAL SYSTEM USED TO MEASURE EFFECT OF VARIOUS
PROGRAMMED AIR LEAKS ON EARCUP ATTENUATION
FIGURE 1
14
major effects were observed primarily in the range between the 0.046 mm
and 0.233 mm openings. Attenuation was not significantly affected by
leaks smaller than 0.046 mm, and it changed little for those leaks greater
than 0.233 which were examined. Consequently, all subsequent measurements
were taken with four sizes of air leaks within the 0.046 to 0.233 mm
range.
Loss of attenuation due to two of the simulated air leaks for an
earmuff in various levels of noise is summarized in figure 2. These data
clearly demonstrate that the amount of loss is about the same in the
range of ambient levels from 70 dB to 100 dB. It is also observed that
as the size of the air leak is increased from 0.133 to 0.233 the amount
of attenuation loss also increases, as expected. The amount of attenuation
loss is frequency dependent with the greatest losses occurring at the low
frequency end of the scale (125 Hz and 250 Hz). The frequency dependency
is also directly related to the individual earmuf f s , as shown in figure 3.
It can be seen that a specific air leak caused different losses at the
various frequencies as well as different losses among the various earmuffs.
The extent of this variability is such that general trends or rules of
thumb describing amounts of loss as a function of air leak sizes are not
readily formulated. The exception to this statement is that very small
air leaks do, in fact, cause substantial losses in the low frequency
attenuation performance of earmuff devices. Further, that different
muffs show differing amounts of attenuation loss for the same air leak.
Therefore , air leaks introduced when earmuffs are worn with eyeglasses
would be expected to have different effects on the attenuation depending
upon which earmuff is worn.
15
ATTENUATION LOSS IN DECIBELS
LOSS Of EARCUP ATTENUATION DUE TO PROGRAMMED
AIR LEAKS IN FOUR DIFFERENT LEVELS OF AMBIENT NOISE
FIGURE 2
16
VARIATION IN AMOUNT OF ATTENUATION LOSS DUE TO DIFFERENT
SIZES OF AIR LEAKS AS A FUNCTION OF EARCUP AND TEST FREQUENCY
FIGURE 3
An earmuff-eyeglasses combination with a small air leak could act
as a helmholtz resonator at particular test frequencies , producing a
sound pressure level under the earcup that is higher than the level
outside the earmuff. The "earcup-hollow tube" arrangement used in the
programmed air leak measurements constitutes such a resonator. The
resonance effects could not be seen in these data because measurement
were taken only at specific test frequencies. A continuously changing
or sweeping test signal moving across the frequency range of interest
could have identified the resonant peaks. Although this study did not
consider resonance effects of eartnuff-eyeglass combinations, it is pointed
out that these effects are encountered in use and generally reduce the
effectiveness of the protector under those conditions.
Physical data from the air leak measurements are not sufficient and
variability is too great to permit formulation of a simple scheme for
predicting these effects. Consequently, measurements of the actual
attenuation provided by earmuffs worn by persons with and without eyeglasses
was the next logical consideration of this study.
PSYCHOACOUSTIC TESTS
Five circumaural earmuff protectors, some of which appear on the Air
Force QPE and are known to be in use in the operational situation, were
evaluated when worn with eyeglasses. The method of measuring attenuation
closely followed the American National Standards Institute (ANSI) Method
for the measurements of Real Ear Attenuation of Ear Protectors at
Threshold (1) which is described in detail in an earlier report (5).
18
With this method subjects actually wearing the sound protectors determine
the amount of protection provided in a specified sound field. This is, in
effect, a real life test even though it is conducted in the laboratory at
very low sound pressure levels .
Since the primary purpose of this investigation was to evaluate a
potential AF problem, all subjects were personally fitted with standard
AF issue eyeglasses with standard zy Ionite frames but with no lenses. A medical
technician with training and experience in this special medical area
individually fit each subject with the appropriate size frames using a
"spectacle-fitting kit" which provides a basic selection of sizes. The
technique, method and purpose of this exercise were coordinated with an
ophthalmologist. All subjects were judged to be provided proper frames for
their head shape and configuration. It is understood by the investigators
that the fitting of eyeglasses is somewhat influenced by the individual
lenses required; however, for the purposes of this evaluation the procedure
employed was considered appropriate and correct. Standard AF frames were
used in the evaluation in order that findings might be related to the
actual operational situation.
Subjects who participated in this phase of the study were male
university students with normal hearing at the audiometric test frequencies
of from 125 Hz to 8000 Hz. Each subject participated in all tests, that
is , he wore the same eyeglass frames with each of the five earmuffs
investigated. Subjects, using the psychophysical method of adjustment (3),
determined their thresholds of hearing under three separate conditions,
(1) open-ear (eyeglass frames with no muff), (2) wearing an earmuff (no
eyeglass frames), and (3) wearing eyeglass frames and an earmuff.
19
Differences in the threshold of hearing between the open ear condition
and the two earmuff conditions are described as the attenuation attributed
to the muff or to the muff and eyeglasses combination worn in that condition.
The differences in attenuation between the earmuff and the earmuff-plus-
eyeglass condition is described as the attenuation loss due to eyeglasses.
Differences in the attenuation of the selected earmuffs worn with and
without eyeglass frames are summarized in figure 4. The amount of area
between the curves and the zero lines represents the average amount of
attenuation loss or reduction experienced by that particular muff when
it was worn over AF eyeglass frames. Several observations may be made from
these data.
First, the attenuation reduction is frequency selective. All devices
reveal greater losses of attenuation at the low and high frequency regions
of the spectrum than at the mid- frequency range. Also, minimum and
maximum reduction values occur at different test frequencies for each of
the different devices tested. Clearly, both attenuation and loss of
attenuation due to air leak are directly related to the frequency of the
test signal.
Second, all earmuffs show losses in attenuation at all frequencies
when eyeglass frames ara worn. Further, the amount of loss varies
significantly from earmuff to earmuff, confirming that reduction in
attenuation is a function of the individual earmuff. None of the items
showed improved protection with eyeglasses at any test frequency.
20
REDUCTION IN ATTENUATION (dB) WHEN EYEGLASSES ARE WORN
EARMUFF A
125 250 500 1000 2000 3000 4000 6000 8000
FREQUENCY (Hz)
FIGURE 4
DIFFERENCES IN ATTENUATION
21
Third, the earmuffs can be categorized or ranked in terms of their
susceptibility to loss of attenuation when worn with eyeglasses, or
conversely stated, in terms of their efficiency when used with eyeglasses.
Figure 4 ranks the muffs by inspection from the best at the top to the
poorest item at the bottom. When the difference values are actually
ranked and summed, the order of the numerical values for the last two
items are reversed with the item E showing the greatest loss of
attenuation, in terms of percentage change in reduction of attenuation,
earmuff A shows 8.3% loss, earmuff B 16.2%, earmuff C 21.1%, earmuff D
18 . rJ% , and earmuff E 21.6%. Clearly, item A is best re percentage change
when worn with eyeglasses; i.e., it shows the least attenuation loss, and
item C is the worst, although items C and D are very close to item E.
Forty-five t-tests, on 30 measures each of the differences between
attenuation of earmuffs worn with and without eyeglasses, are summarized
in table 2. Differences which were not statistically significant are
underlined and indicate that essentially the same attenuation is provided
with the eyeglasses as without them. This statistically significant
difference amounted to about 2.5 db. It is clear that earmuff A is least
affected by eyeglasses and that earmuff E is most affected. At the test
frequency of 2000 Hz, no significant differences between attenuation were
found for any of the devices.
Data on differences in earmuff attenuation with and without eyeglasses
as reported by Webster and Rubin (4) and by Fletcher and Loeb (2) are
summarized in table 3. Items V, W, and X show rather large differences.
Item II is the earmuff with foam- latex cushions which was essentially
unaffected by the eyeglasses.
22
TABLE 2
MEAN DIFFERENCE SCORES BETWEEN EARMUFF
ATTENUATION WHEN WORN WITH vs WITHOUT EYEGLASSES
EARMUFF
EARMUFF
EARMUFF
EARMUFF
EARMUFF
A
B
C
D
E
TEST
FREQUENCY
125
3. 351'*
2.71
5.26
7.05
4.48
250
3.23
4.63
5. 36
4.88
4.56
500
3.17
3. 12
5.19
4.76
5.34
1000
1.13
1.37
3.97
3.56
5.33
2000
0.17
0.02
0.99
0.45
1.72
3000
1.25
4.08
1.06
1.89
3. 38
4000
0.15
1.79
6.49
3.09
2.74
6000
0.30
3.57
4.32
8.71
3.32
8000
4.97
2.71
2.45
6.76
5.44
•'‘Entries are t-test s ores. Mean differences in excess of 2.46 are statistically significant,
underlined scores indicate that essentially the same attenuation was provided with the eyeglasses
as without them.
TABLE 3
DIFFERENCES IN ATTENUATION OF LARMUFFS WORN WITH
AND WITHOUT EYEGLASSES (IN DECIBELS)
TEST
FREQUENCY
V>‘
W*‘
EARMUFF
x**
Y*>V
z**
125
6 . 1 »■»*•'<
3.1
9
6
0
250
7.0
7.5
9
7
0
500
6.1
6.7
5
4
-1
1000
0.9
4.6
5
2
0
2000
8.7
5.1
0
0
-1
3000
7.2
-1.0
-
-
-
4000
11.8
7.0
5
5
0
6000
8.2
11.4
12
5
0
8000
4.7
9.3
-
-
-
•’‘FLETCHER AND LOEB
••‘•'■WEBSTER AND RUBIN
•■'•'‘^POSITIVE ENTRIES INDICATE AMOUNT OF ATTENUATION LOSS DUE TO THE EYEGLASSES
2k
DISCUSSION
It is the opinion of the investigators that the state-of-the-art
of earmuff design is sufficiently advanced that the loss of earmuff
attenuation when worn over eyeglasses is a technically solvable problem.
The performance of earmuff A over that of earmuff E demonstrates that
better comoat ability is already achievable. Webster (4) found that an
earmuff with foam-laxtex cushions had little effect on attenuation
while vinyl covered cushions resulted in the usual noticeable low-
frequency loss when eyeglasses are worn. He suggested that a piece of
foam-latex or similar material be placed under/over the eyeglass temples
to form a more effective seal than would be obtained otherwise.
The problem of earmuff-eyeglars compatability may be approached in
a number of different ways. Une alternative is to provide special
earmuffs (clearly marked >n the units) for persons who work in noise and
who also wear eyeglasses. This would require a separate performance
specification and evaluation for these earmuffs. Another alternative
is to apply a correction factor to current earmuff performance specifica¬
tions to account for the attenuation losses due to air leaks. Unlike
the first approach, this correction factor(s) would be determined for
each earmuff at the time of its evaluation for potential AF use and
would be reflected in the performance data. No personnel, other than the
evaluators, would be directly involved. This method would adequately
protect tne eyeglass wearer and would overprotect the non-wearer of eyeglasses.
Eyeglass wearers could be provided with foam-latex (or similar
material with configuration to ue determined) inserts or applicators to
jt used at the eyeglass temple;; to i* '.ove the acoustic 3eal with all
earmuffs. A commercially available pad designed for this purpose was
evaluated recently in our laboratory. Its effectiveness in minimizing
the attenuation loss is seen as increased average protection at all test
frequencies in figure i>. The use of some material at the temple may
well be the most practical approach since it would be applicable to all
items already in use operationally, to those in the inventory and to
those procured in the future. The relative cost would be expected to
be small. An investigation into types of materials and of appropriate
configurations would precede final selection.
Thin wire eyeglass temples which rest close to the head have
essentially no effect on the attenuation of earmuffs worn over them.
Some personnel in the field have removed or Btripped the plastic off
the temples of AF standard eyeglass frames leaving only the thin metal
strip to minimize and eliminate air leaks. A brief examination of this
approach in our laboratory confirmed that temple-stripping does improve
attenuation. Earmuffs which seal poorly over unstripped temples show the
greatest improvement and as might be expected, earmuffs which initially
esel well show little improvement when worn over stripped temples .
Finally, the current procedure for the selection of earmuffs for
use by AF personnel in noise does not contain provisions for eyeglass
wearers who usually receive less protection than is indicated. In general,
if the attenuation values of earmuffs already on the QPL were corrected
(reduced) for protection lost due to air leaks around eyeglass bows, the
resulting values would not be expected to satisfy the performance
requirements in the earmuff specification, MIL-P-3826BB. The implication
for eyeglass-earmuff wearers in noise is clear.
26
REFERENCES
1. American National Standards Institute, "Method For The Measurement
of Real Ear Attenuation at Threshold." Z24.22, 1957.
2. Fletcher, J. and M. Loeb, "Evaluation of Willson and American
Optical Under The Helmet-Type Ear Protective Devices." USAMRL
Letter Report £ 3, Psychology Division, Ft. Knox, Kentucky, 1964.
3. Hirsh, I. J. , Measurement of Hearing, McGraw-Hill Book Company, 1952.
4. Webster, J. C. and E. R. Rubin, "Noise Attenuation of Ear-Protective
Devices," Sound, Vol. 1, No. 5, September-October, 1962.
6. Nixon, C. W,, D. T. Blackstock and R. G. Hansen, "Performance of
Several Ear Protectors," WADC Technical Report 58-280, May 1959.
6. Military Specification, Mil-P-38268B , Protector, Ear, Sound PRU-1A/P,
3 August 1964.
7. Air Force Regulation, AFR 161-35, Hazardous Noise Exposure, 27 July
1973.
28
r'U.S. Government Printing Office: 1974 - 657-013/5!
