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J Am Acad Audiol 2 : 156-163 (1991)
Effect of Reference Microphone Location and
Loudspeaker Azimuth on Probe Tube
Microphone Measurements
Michelle A. Ickes*
David B. Hawkins t
William A. Cooper t
Abstract
The effects of loudspeaker azimuth and reference microphone location on probe tube
microphone measures were assessed . The real ear unaided response (REUR), real ear
aided response (REAR), and real ear insertion response (REIR) were obtained on a KEMAR .
Aided measures were obtained with both a behind-the-ear and an in-the-ear hearing aid . All
three measurements were affected by changes in the loudspeaker azimuth and reference
microphone location . Responses obtained with a 90 degree loudspeaker azimuth or with the
reference microphone located at-the-ear revealed greater disparity than those obtained
under other conditions . Most of the differences occurred at frequencies above 2000 Hz, with
measurements utilizing the behind-the-ear hearing aid showing greater dispersion . These
results suggest that the location of the loudspeaker and the reference microphone are
important variables when utilizing probe tube microphone measurements .
Key Words : Probe tube microphone measurements, hearing aids, reference microphone
location, loudspeaker azimuth
espite the current popularity of real
ear probe tube microphone measureD ments (Cranmer, 1990), there is no
standard test protocol . Relatively little effort
has been made to delineate the sources of variability in the measurements . Manufacturer's
recommendations often dictate the specific techniques employed by clinicians for hearing aid
selection and/or validation .
Two procedural issues that may differ among
equipment specifications and clinicians, and
which could affect test results and variability,
are the loudspeaker azimuth and the reference
microphone location . Killion and Revit (1987)
" Dorn Veterans Administration Medical Center, Columbia, South Carolina
lina
t University of South Carolina, Columbia, South Caro-
Reprint Requests : Michelle A. Ickes, Audiology and
Speech Pathology (126), Dorn Veterans Administration Medical Center, Columbia, SC 29201
156
investigated the effects of four different loudspeaker locations on measurements of real ear
insertion gain (REIG) . The locations studied
were 070° (directly in front), 0°/45° (in the horizontal plane, 45 degrees offfrom center), 45°/45°
(45 degrees up and 45 degrees off center), and
90°/0' (directly overhead). REIG was quite similar for 070° and 0°/45° . However, the 45745°
azimuth showed increased REIG at 4000 and
5000 Hz, and the 90°/0° azimuth showed substantially higher gain above 500 Hz . The best
test-retest reliability was obtained with the 45°/
45° and 0745' loudspeaker locations . It is not
clear at this point how different loudspeaker
locations affect the three main probe tube measurements : real ear unaided response (REUR),
real ear aided response (REAR), and REIG .
Many probe tube measurement systems
utilize a reference microphone to maintain a
constant sound pressure level or to correct for
minor deviations in the sound field. Based upon
currently available instruments, the three most
popular locations for this reference microphone
Probe Tube Measurements/Ickes et al
are over-the-ear, on-the-cheek, and at-the-ear
(lateral to the external ear) . If the transfer
functions from these three positions to the
eardrum and/or hearing aid microphone location were different, it is possible that different
values for probe tube measurements would be
obtained. Variations in sound pressure distribution at different locations on the head have
been examined by Madaffari (1974) and Kuhn
and Burnett (1976) . Madaffari measured
changes in acoustic pressure as a function of
frequency due to head diffraction and baffle at
25 locations on and about the pinna with the
loudspeaker location held constant at a 0 degree
azimuth. Significant differences in SPL were
evident when comparing one location to another, with the greatest variations occurring in
the higher frequencies. Kuhn and Burnett found
minor changes in SPL alongside the head for
frequencies of 2000 Hz and below. Above 4000
Hz, however, larger changes in SPL were found
with small variations in position about the
pinna.
Revit (1987) indicated that there may be as
much as 3 inches between a behind-the-ear
(BTE) hearing aid microphone and the reference microphone of a real ear measurement
system . This difference in location can create
significant differences in the SPL being received by each microphone . The effect will be
seen for frequencies which have half wavelengths, that are less than the distance between
the two microphones. Differences should therefore be expected at frequencies near and above
2000 Hz . Minor head movements may change
the SPL arriving at each microphone location
and cause inconsistencies between various real
ear measurements . Due to these problems, Revit
suggested that the reference microphone should
be as close as possible to the hearing aid microphone.
Feigin, Nelson Barlow, and Stelmachowicz
(1990) examined the differences in input to the
microphone of a BTE hearing aid when the
reference microphone was located on the cheek
versus above the ear with the loudspeaker
maintained at a 0 degree azimuth. Sizeable
differences were found between the two locations, especially in the frequency region of 1200
to 2000 Hz . While these differences would affect
the REAR, it is unclear whether the REUR or
real ear insertion response (REIR) would be
affected by different reference microphone locations. Further, it remains to be seen whether
different loudspeaker azimuths would interact
with reference microphone location .
The purpose of this study was to examine
the effects of three loudspeaker azimuths (0
degrees, 45 degrees, 90 degrees) and three common reference microphone locations (over-theear, cheek, at-the-ear) on the REUR, REAR,
and REIR . Aided measures were obtained for a
BTE and an in-the-ear (ITE) hearing aid.
METHOD
M
easurements were made on a Knowles
Electronics Manikin for Acoustic Research (KEMAR) placed in an IAC double-walled
sound treated room .
AFrye 6500 probe tube microphone system
was utilized for all measurements . A60 dB SPL
composite weighted noise served as the stimulus. The loudspeaker was located 12 inches
from the KEMAR and positioned at horizontal
azimuths of 0 degrees, 45 degrees, and 90 degrees; the elevation angle was always 0 degrees.
All measurements were made using the manikin's right ear fitted with the large KEMAR
pinna.
The reference microphone was placed at
three locations around the KEMAR's pinna to
simulate common probe tube microphone systems . One location was over the ear, in which
the microphone was placed above the pinna on
a vertical line directly above the tragus . A
second location was on the cheek. A horizontal
line was drawn 15 mm anterior to the tragus,
with the reference microphone located 15 mm
directly below this point on the cheek. A third
location was at the ear or lateral to the ear. The
center of the top surface of the microphone was
located 25 mm lateral to the ear canal opening.
Due to concern regarding the possibility of
altering the probe tube location in the ear canal,
all unaided measures were obtained first. Based
on the length of the canal portion of the Zwislocki
ear simulator and the dimensions of the pinna,
for both unaided and aided measurements the
probe tube was inserted to a constant depth
past the tragus at a point calculated to be within
several millimeters of the eardrum location .
For the aided measurements with the BTE
hearing aid, the probe tube was inserted through
a custom drilled vent parallel to the bore of the
DB-111 earmold simulator. When aided measurements were made with the ITE hearing aid,
the probe tube was inserted through a parallel
vent with putty placed around the outer edge to
seal off the vent .
157
Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991
The reference microphone was initially
placed in the over-the-ear position, and measurements were made for the three loudspeaker
locations. The reference microphone was next
moved to the cheek and finally to the at-the-ear
location . The same procedure was then followed
for obtaining the REAR with the BTE hearing
aid and then with the ITE hearing aid. Each
measurement of the REUR and REAR was
repeated three times without alteration of the
location of the probe tube, loudspeaker, or reference microphone. The hearing aids were not
removed in between the three repetitions of
each measurement. The REIR was determined
by subtracting the REUR from the REAR .
The BTE instrument was a mild gain hearing aid with a front-facing microphone . The
hearing aid was coupled to the ear simulator via
a DB-111 earmold simulator. Earmold impression material was used to fill the concha to
simulate a fully occluding shell earmold. The
ITE instrument was a mild gain custom hearing aid manufactured to fit the ear of KEMAR
and the Zwislocki ear simulator. A specially
designed plastic ring allowed proper fit of the
ITE hearing aid to the manikin's ear. The volume control wheels on both the BTE and ITE
were set such that with the 60 dB SPL broadband input, the hearing aids were functioning
in a linear operating range.
RESULTS
Real Ear Unaided Response
Reference Microphone Location . Figure 1 shows the effect of reference microphone
location on the REUR at the three loudspeaker
azimuths .* Also shown for each azimuth are
data from Shaw (1974) [numerical values were
taken from Shaw and Vaillancourt (1985)] which
utilized the substitution method . The best agreement with the Shaw data is seen at the 0 degree
azimuth. The differences among the three reference microphone locations are also the least at
Although values for the REUR and REAR are reported and
plotted in dB SPL, they are actually transformed values .
When the reference microphone is active on the Frye 6500 as
it was in this study, the actual values are shown as "Gain,"
and represent the difference in dB between the SPL measured by the probe microphone and the reference microphone . Since the convention is to express the REUR and
REAR as dB SPL in the ear canal, the input (60 dB SPL) was
simply added to all of the gain values in orderto presentthese
data as dB SPL measured in the ear canal .
158
REAL EAR UNAIDED RESPONSE
0 AZIMUTH
1000
10000
1000
10000
v0
100
J 90
0-
N so
0 70
60
50
100
110
100
90 AZIMUTH
90
so
70
60
50
100
1000
FREQUENCY (in Hz)
10000
Figure 1. Real ear unaided response as a function of
reference microphone location for the three loudspeaker
azimuths . Also shown are the data from Shaw (1974)
which utilized a substitution method .
this location . The three reference microphone
locations give similar results except above 3000
Hz for the at-the-ear location, where an increase in SPL is observed .
The 90 degree azimuth results show the
largest deviation from Shaw's data, as well as
among the three reference microphone locations . It is clear that a reference microphone
based REUR is different than a substitution
method REUR, especially with the loudspeaker
at the 45 degree and 90 degree azimuths . This
should be expected, as the reference microphone effectively reduces some of the sound
pressure build up along the head due to the head
baffle, effects that will be present with the
substitution method .
Loudspeaker Azimuth. Figure 2 shows
the effect ofloudspeaker azimuth for each reference microphone location . Differences are observed primarily above 2500 Hz for each loudspeaker azimuth for all reference microphone
locations. Large peaks are present in the REUR
for both the 45 degree and 90 degree azimuth
location for the at-the-ear reference microphone
location . The 90 degree azimuth loudspeaker
orientation again gives the most deviant result
for each of the three reference microphone locations.
Probe Tube Measurements/Ickes et al
110
REAL EAR AIDED RESPONSE (BTE)
REAL EAR UNAIDED RESPONSE
100
0 AZIMUTH
OVER EAR
90
go
70
BO
50
too
110
100
90
a
N so
m
1000
10000
1000
- 0 AZ
--- 45 AZ
90 AZ
10000
- OVER EAR
-- CHEEK
CHEEK
45 AZIMUTH
AT EAR
90
e0
u 70
70
90
100
110
120 Tr
too
110 }
100
90
9o
so
90 AZIMUTH
70
so
100
10000
9o
70
501
1000
1000
FREQUENCY (in Hz)
10000
Figure 2. Real ear unaided response as a function of
loudspeaker azimuth for the three reference microphone
locations.
9o
100
1000
FREQUENCY (in Hz)
10000
Figure 3. Real ear aided response for a behind-the-ear
(BTE) hearing aid as a function of reference microphone
location for the three loudspeaker azimuths .
REAL EAR AIDED RESPONSE
Reference Microphone Location . Figure 3 shows the REAR as a function of reference
microphone location for each of the three loudspeaker azimuths with the BTE hearing aid.
The reference microphone location has the least
effect with the loudspeaker at a 0 degree azimuth . Good agreement is seen at the 45 degree
azimuth among the reference microphone locations across most of the frequency range tested .
The at-the-ear location does, however, show a
large peak above 5000 Hz .
As with the REUR, the largest discrepancies as a function of reference microphone location for the REAR are seen with the loudspeaker at the 90 degree azimuth. The REAR
obtained with the at-the-ear location becomes
divergent from the other two response curves
beginning at 2500 Hz and displays two or three
large peaks in the higher frequencies. The overthe-ear and cheek reference microphone locations produced similar REARS through 4000
Hz, with the cheek location resulting in less
output between 4000 and 6000 Hz .
Figure 4 shows that differences among
REARS with the three reference microphone
locations were not as large when an ITE hearing
aid was used . There was little effect ofreference
microphone location at the 0 degree azimuth.
110
100
J 90
0-
N go
m
v 70
90
so
100
1600
10600
110
100
9o
90 AZIMUTH
9o
70
901
50
100
Figure 4. Real ear aided response for an in-the-ear
(ITE) hearing aid as a function of reference microphone
location for the three loudspeaker azimuths .
159
Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991
At 45 degrees the curves are quite close except
above 4000 Hz, where the at-the-ear location
showed increased output . As with the BTE, the
largest differences are seen with the 90 degree
azimuth loudspeaker location, with the at-theear location again showing the most deviant response.
Loudspeaker Azimuth. Figure 5 shows
the REAR BTE data plotted so that the effect of
loudspeaker azimuth can be examined for each
ofthe reference microphone locations. It is clear
again that the 90 degree azimuth reveals the
greatest divergence from the other two loudspeaker azimuths for each of the reference
microphone locations. For the over-the-ear reference microphone location, loudspeaker azimuth begins to have an effect above 1500 Hz .
With the reference microphone on the cheek,
approximately a 3 to 10 dB difference is seen
above 400 Hz between the 0 degree and 90
degree azimuth loudspeaker locations.
The largest loudspeaker azimuth effects
are seen when the reference microphone is
located at-the-ear . Differences among the
REARS for the three azimuths begin primarily
above 1000 Hz, reaching a maximum of 23 dB at
2800 Hz, the location of a large resonant peak .
The ITE REAR data plotted to show the
effect of loudspeaker azimuth are shown in
Figure 6. In contrast to the BTE data, the
REAL EAR AIDED RESPONSE (ITE)
110
100
so
70
90
50
1000
-
10000
0 AZ
--- 45 AZ
CHEEK
90 AZ
100
1000
10000
1000
FREQUENCY (in Hz)
10000
110
100
90
70
80
e0
50
100
Figure 5. Real ear aided response for a behind-the-ear
(BTE) hearing aid as a function of loudspeaker azimuth
for the three reference microphone locations .
160
OVER EAR
100
1000
10000
1000
10000
1000
'
" " 10000
120
110
J 100
a
N 90
m
so
70
8o
100
120
110
100
9o
70
90
80 1
100
FREQUENCY (in Hz)
Figure 6. Real ear aided response for an in-the-ear
(ITE) hearing aid as a function of loudspeaker azimuth for
the three reference microphone locations.
differences are relatively minor in all conditions, with the exception of the 90 degree azimuth cheek position where a 6 to 10 dB reduction is seen between 2500 and 5000 Hz .
Real Ear Insertion Response
OVER EAR
90
REAL EAR AIDED RESPONSE (BTE)
Reference Microphone Location. Figure 7 shows the effect of reference microphone
location on the REIR for the three loudspeaker
azimuths for a BTE hearing aid. The three
reference microphone locations yield virtually
identical REIRs through 3000 Hz for the 0 and
45 degree loudspeaker azimuths and through
2500 Hz for the 90 degree azimuth. In the higher
frequencies the agreement is best between the
over-the-ear and cheek locations.
Similar results were obtained with the ITE
hearing aid. Figure 8 shows the ITE REIRs as
a function of reference microphone location .
The results are quite similar to those shown
above with the BTE . Excellent agreement is
seen in the low and middle frequencies, with
substantial differences occurring primarily with
the at-the-ear location above 3000 to 4000 Hz .
Loudspeaker Azimuth. Figure 9 shows
how the REIR changes with loudspeaker azimuth for each of the reference microphone loca-
Probe Tube Measurements/Ickes et al
50T
40
30
REAL EAR INSERTION RESPONSE (BTE)
0 AZIMUTH
REAL EAR INSERTION RESPONSE (BTE)
+0
30
OVER EAR
-_
20
10
20
10
0
0
100
so
1000
.10000
-10
100
1000
10000
10000
100
1000
10000
50
.~ 40
m
v 30
c
~. 20
? 1o
a
CD o
-10
100
-,
1000
.
50
90 AZIMUTH
AT EAR
40
30
:ate
20
10
-f0
100
1000
FREQUENCY (in Hz)
0
-10
100
10000
Figure 7. Real ear insertion response for a behind-theear (BTE) hearing aid as a function of reference microphone location for the three loudspeaker locations.
1000
FREQUENCY (in Hz)
Figure 9. Real ear insertion response for a behind-theear (BTE) hearing aid as a function of loudspeaker location for the three reference microphone locations.
REAL EAR INSERTION RESPONSE (ITE)
REAL EAR INSERTION RESPONSE (ITE)
0 AZIMUTH
OVER EAR
100
1000
-
Q
U
10000
50
40
30
90 AZ
CHEEK
0.
-10
100
1000
10000
50
40
90 AZIMUTH
30
20
20
10
10
0
-10
100
10000
0 AZ
--- 45 AZ
m
T
c
1000
10000
AT EAR
0
1000
FREQUENCY (in Hz)
10000
Figure S. Real ear insertion response for an in-the-ear
(ITE) hearing aid as a function of reference microphone
location for the three loudspeaker locations.
-10
100
Figure 10 . Real ear insertion response for an in-the-ear
(ITE) hearing aid as a function of loudspeaker location for
the three reference microphone locations.
Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991
tions for the BTE hearing aid. For all three
reference microphone locations, the 90 degree
azimuth loudspeaker results in more insertion
gain between 400 and 4000 Hz . The agreement
is reasonably good between the 0 and 45 degree
azimuth REIRs for all reference microphone
locations.
The ITE loudspeaker azimuth data are
shown in Figure 10 . The REIR is similar for the
three azimuths through 3000 Hz for the overthe-ear and cheek reference microphone locations and through 2000 Hz for the at-the-ear
location . In the higher frequencies less REIG is
observed with the 45 and 90 degree azimuths,
especially with the at-the-ear reference microphone location .
Test-Retest Reliability. Three repetitions
were made for each condition. Since the probe
tube, loudspeaker azimuth, and reference microphone were not altered between the repetitions and the hearing aid was not removed, it
was expected that the test-retest reliability
would be excellent. The standard deviations
were less than 1 .0 dB and averaged approximately 0.3 to 0.7 dB for each condition, indicating excellent test-retest reliability .
DISCUSSION
be results of this study demonstrate that
T the REUR, REAR, and REIR are affected
by location of the loudspeaker and the reference
microphone . These findings are not surprising
given the known variations in sound pressure
level around the head and pinna and loudspeaker azimuth effects. The data of Shaw
(1974) clearly demonstrated the effect of loudspeaker azimuth on the REUR using the substitution method . There is an increase in SPL at
the eardrum across the entire frequency range
when the signal arrives from a 45 or 90 degree
azimuth rather than 0 degrees . When a reference microphone is placed on the head and the
SPL is kept constant at that location, the azimuth effect is altered, but differences among
the loudspeaker locations are still present . The
REUR can be altered with changes in the reference microphone location as a result of differing
transfer functions from the microphone to the
measurement point in the ear canal. The azimuth differences for the REUR are confined to
frequencies above 2500 Hz for all three reference microphone locations (see Fig. 2) . The
difference between the substitution method
REUR and the reference microphone REUR
162
increases substantially at the 45 and 90 degree
azimuths . If the substitution method REUR is
viewed as the more valid representation because of the presence of all head and body
diffraction effects, then closest agreement to
these data will be obtained with a 0 degree
azimuth loudspeaker when a reference microphone is employed . Even with a 0 degree azimuth, however, there can still be substantial
differences (up to 8 dB at some frequencies)
between the substitution and reference microphone REURs .
The findings of this study support the conclusion of Feigin et al(1990), that the location of
the reference microphone can alter the SPL
entering the hearing aid microphone and thus
affect the REAR . The differences observed in
this study were larger for BTE than ITE hearing aids and were confined mostly to frequencies above 2500 Hz . If one excludes all the 90
degree azimuth data and the at-the-ear reference microphone data for 45 degrees, then the
differences among the remaining REAR data
are relatively small.
The results of this study further show that
the REIR can also be affected by reference
microphone location and loudspeaker azimuth.
Since the REIR is the difference curve between
the REUR and REAR, it might be expected that
the REIR would be unaffected by changes in
these two variables. That is, ifchanges occurred
at different azimuths or were caused by reference microphone locations, these changes would
be the same for the REUR and REAR, and
therefore the REIR would not be affected . This
does not seem to be the case . An explanation
could be that as the loudspeaker azimuth or
reference microphone location is altered,
changes occur in the differences between the
transfer functions from the reference microphone and the hearing aid microphone to the
eardrum location . These differences would lead
to changes in the REIR . These changes all occur
in the higher frequencies and were larger for the
BTE than ITE hearing aid. If the REIR is being
measured with an ITE hearing aid at a 0 or 45
degree azimuth, only minor differences can be
expected to occur below 4000 Hz for the three
reference microphone locations. Other differences might be expected if various styles of ITE
hearing aids (e .g ., half shell or in-the-canal)
were employed, because of changes in the location of the hearing aid microphone .
The results ofthis study have some implications for the procedure of correcting prescription values for deviations of an individual's
Probe Tube Measurements/Ickes et al
REUR from the "average" REUR (Mueller,
1989). These data show that the "average" REUR
depends on the loudspeaker azimuth and the
location of the reference microphone . Therefore, the REUR chosen to represent the average
person should be obtained under the same conditions as would be employed for the clinical
measurements . A reasonable procedure would
be to place a KEMAR in the test environment
and obtain a reference REUR using the loudspeaker location and reference microphone position that will be used with the client . The
client's REUR would then be corrected by differences from the KEMAR REUR measured under
the same test conditions . If a KEMAR is not
available, REURs obtained from a group of
subjects could be averaged and serve as the
reference.
CONCLUSIONS
T
his study has demonstrated that the loudspeaker azimuth and reference microphone location affect REUR, REAR, and REIR .
Most of the effects are limited to frequencies
above 2000 to 3000 Hz and tend to be more
pronounced with BTE hearing aids . A loudspeaker azimuth of 90 degrees is not recommended, as the results can be quite deviant and
show large peaks and valleys. These deviations
appear most prominently with an at-the-ear
reference microphone location . Only minor differences, again confined to the higher frequencies, occur in the REIR if a 0 or 45 degree
azimuth is used with any of the three reference
microphone locations. Since it appears that
many factors affect the variability of probe tube
microphone measurements, especially in the
high frequencies, it may not be realistic for the
clinician to view results above 3000 Hz with the
same confidence as those obtained at lower
frequencies.
Acknowledgment . The authors would like to acknowledge the assistance of Lucille Beck and Ed Burnett in
providing in-the-ear hearing aids manufactured for a
KEMAR and adapters allowing for appropriate coupling
of the hearing aids to the ear simulator.
Portions of this paper were presented at the American
Speech-Language-Hearing Association Convention in
Seattle, Washington in November, 1990.
REFERENCES
Cranmer K. (1990) . Hearing instrument dispensing1990 . Hear Instrum 41(6):4-12.
Feigin J, Nelson Barlow N, Stelmachowicz P. (1990) . The
effect of reference microphone placement on sound pressure levels at an ear level hearing aid microphone . Ear
Hear 11(5):321-326 .
Killion M, Revit L. (1987) . Insertion gain repeatability
versus loudspeaker location : you want me to put my
loudspeaker WHERE? Ear Hear 8 (Suppl):68S-738 .
Kuhn G, Burnett E. (1976) . Acoustic pressure field alongside a manikin's head with a view towards in situ hearingaid tests. JAcoust Soc Am 62 :4161123 .
Madaffari P. (1974) . Pressure Response About the Ear.
Paper presented at the 88th Meeting of the Acoustical
Society of America, St. Louis, Missouri .
Mueller H. (1989) . Individualizing the ordering of custom
hearing instruments . Hear Instrum 40 :18-22 .
Revit L. (1987) . New Loudspeaker Locations for Improued
Reliability in Clinical Measures of the Insertion Gain of
HearingAids. Master's Thesis, Northwestern University .
Shaw E . (1974) . Transformation of sound pressure level
from the free field to the eardrum in the horizontal plane.
JAcoust Soc Am 56 :1848-1861 .
Shaw E, Vaillancourt M. (1985). Transformation ofsoundpressure level from the free field to the eardrum presented in numerical form. JAcoust SocAm 78 :1120-1123 .
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