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J Am Acad Audiol 3 : 315-323 (1992)
Comparison of Etymotic Insert and TDH
Supra-aural Earphones in Auditory Brainstem
Response Measurement
Luann E. Van Campen*
Carol A. Sammetht
James W. Hall 1111
Barbara F. Peek§
Abstract
There are few systematic comparisons of Etymotic ER-3A insert earphones versus supraaural earphones in auditory brainstem response (ABR) measurement. We compared ER-3A
insert earphones and two types of supra-aural earphones (TDH-39P and TDH-49P) in a
group of normal hearing adults . Acoustic analyses revealed spectral and temporal differences among earphones. Behavioral and ABR thresholds to click stimuli were slightly
elevated with the ER-3A compared to the TDH earphones . The ER-3A earphones produced
a latency delay, relative to the TDH earphones, that varied from about 0.8 to 1 .0 msec, and
increased at lower stimulus intensity levels . In addition, ABR wave I amplitude was
significantly reduced with the ER-3A earphone . Based on these data, we recommend collection of normative data with the ER-3A earphones prior to their use in ABR measurement.
Key Words: Auditory brainstem response (ABR), brainstem auditory evoked response
(BAER), transducers, insert earphones, tubephones
n 1984, Etymotic Research (61 Elk Grove
Village, IL, 60007) introduced a new line
of insert earphones (called "tubephones"),
which are composed of a transducer and a 278
mm plastic tube that funnels the sound to an
EARTH plug for coupling to the ear (Killion,
1984). The model ER-3A insert earphone reportedly provides a number of advantages over
*tDivision of Hearing and Speech Sciences, Vanderbilt
University School of Medicine, Nashville, Tennessee
*Current Affiliation : Section of Audiology and Speech
Pathology, Department of Otorhinolaryngology, University of
Oklahoma School of Medicine, Oklahoma City, Oklahoma
tCurrent Affiliation : Departmentof Speech and Hearing
Science, Arizona State University, Tempe, Arizona
t Division of Hearing and Speech Sciences, and Department of Otolaryngology, Vanderbilt University School of
Medicine, Nashville, Tennessee
§Audiology and Speech Pathology Services, Department of Veterans Affairs Medical Center, Nashville, Tennessee
Reprint requests : Carol A . Sammeth, Department of
Speech and Hearing Science, Arizona State University,
Tempe, AZ 85283-0102
supra-aural-type earphones for behavioral audiometric testing, including increased interaural
attenuation (Killion et al, 1985 ; Hosford-Dunn
et al, 1986 ; Sklare and Denenberg, 1987), increased ambient noise attenuation (Clemis et
al, 1986 ; Clark and Roeser, 1988), elimination
of ear canal collapse (Killion, 1984), and increased patient comfort (Clark and Roeser,
1988). Only small correction factors need to be
applied in the low and high frequencies to
equate pure-tone behavioral thresholds obtained
with the ER-3A with those obtained with the
TDH-39 (Wilber et al, 1988 ; Borton et al, 1989) .
Other studies also have reported similar
behavioral thresholds between ER-3A and TDH49 earphones (Clemis et al, 1986), and TDH-50
earphones (Clark and Roeser, 1988 ; Larson et
al, 1988).
The advantages of insert earphones for
behavioral audiometry also have been ascribed
to auditory brainstem response (ABR) measurement (Killion, 1984 ; Clemis et al, 1986). The
manufacturer states that insert earphones produce an acoustic click with very little transducer ringing, and reduce electrical stimulus
artifact in ABR measurement (Killion, 1984).
315
Journal of the American Academy of Audiology/Volume 3, Number 5, September 1992
The manufacturer also states that ABR waveforms obtained with the ER-3A earphone are
similar to those obtained with a supra-aural
earphone, except for an approximately 1.0 msec
delay in absolute wave latencies introduced by
the length of tubing.
While the benefits ofinsert earphones have
been verified empirically for behavioral audiometry, few studies have systematically examined Etymotic insert earphones in ABR measurement. For a neonatal intensive care nursery
population, Gorga et al (1988) reported that the
distribution of ABR thresholds was similar for
ER-3A insert earphones versus Beyer DT48
circumaural earphones. Absolute wave latencies
also were similar, once the acoustic delay produced by the insert earphones was subtracted .
There was a 50 percent decrease in the number
of neonates who failed ABR screening when the
insert earphones were used, presumably due to
elimination of ear canal collapse . For normal
adult subjects, Hood and Morehouse (1985) and
Yang and Henrickson (1988) compared ER-3A
earphones with TDH-39 supra-aural earphones,
and Beauchaine et al (1987) compared the ER3A with Beyer DT48 earphones . Both Hood and
Morehouse (1985) and Beauchaine et al (1987)
found small, nonsignificant differences between
earphones for behavioral thresholds to click
stimuli. For ABR measurements, the insert
earphones reportedly had comparable testretest reliability, an absolute wave latency delay varying from 0.8 to 1 .0 msec, comparable
interwave latencies, comparable latency-intensity functions, and similar overall waveform
morphology . Beauchaine et al (1987) reported
that ABR thresholds were slightly higher with
the ER-3A than with the Beyer earphones.
There are, however, some discrepancies in
the reported data . First, Hood and Morehouse
(1985) reported that wave I amplitude was
slightly reduced with the insert earphones.
This resulted in a significantly greater mean
V/I amplitude ratio for the insert versus the
TDH-39 earphone . Yang and Henrickson (1988)
reported no statistically significant difference
in V/1 amplitude ratios between the earphones.
Second, Hood and Morehouse reported that
early peaks of the ABR (I, 11, 111) were more
identifiable with the insert earphone than with
the TDH-39 earphone, even at low stimulus
intensities and fast repetition rates. In contrast, Beauchaine et al (1987) reported that
early peaks were less identifiable with the ER3A than with the Beyer earphones as stimulus
intensity was reduced. Third, after subtracting
a 0.9-msec correction factor from the ER-3A
values, Yang and Henrickson (1988) reported
that the absolute latencies of waves 1, III, and V
at all intensities, and the I-V interwave latency
at 60 dB nHL were significantly shorter with
the ER-3A earphone than with the TDH-39
earphone . These findings differ from those of
Hood and Morehouse (1985) and Beauchaine et
al (1987) .
Some of the discrepancies in the literature
should be resolved before the Etymotic ER-3A
earphones are routinely applied to clinical ABR
measurement. The purpose ofthe present study,
therefore, was to compare ABR latency, amplitude, and threshold data obtained with Etymotic
ER-3A insert earphones with those obtained
with two earphones commonly used in clinical
practice (TDH-39P and TDH-49P supra-aural
earphones) in a group of normal hearing adults .
Amplitude spectra and temporal waveforms of
click stimuli transduced by each earphone also
were evaluated.
METHOD
Subjects
Subjects were ten adult volunteers (five
males and five females) . Ages ranged from 22 to
35 years, with a mean age of 26 .4 years . All
subjects had normal hearing, as defined by
bilateral auditory sensitivity of 20 dB HL or
better for octave frequencies from 250 to 8000
Hz inclusively (ANSI, 1969), and no history of
otologic and neurologic pathology. ABRs were
recorded only for right ear stimulation.
Stimuli and Instrumentation
Stimuli were 100- gsec rectangular electrical pulses produced by the model 1007A stimulus generator of a Nicolet CA-1000 signal
averager, presented at a rate of 21 .1 per second .
Stimuli were led to one of three sets of earphones : Etymotic ER-3A insert earphones, or
Telephonics TDH-39P or TDH-49P supra-aural
earphones mounted in MX-41/AR cushions . The
EARTH plug of the ER-3A was inserted so that
its outer edge was flush with the opening of the
ear canal. Both ears of a subject were occluded
(with the ER-3A or TDH earphones) during
testing. Acoustic clicks were of rarefaction polarity (as verified by using the technique recommended by Gorga et al, 1985).
Acoustic measurements ofthe stimuli were
made by coupling the right earphone of each
transducer to the ear of a Knowles Electronic
316
RAIN 1 11 :11~f 1 1
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Mf3 ,1
Comparison of Earphones /Van Campen et al
Mannikin for Acoustic Research (KEMAR). The
output of the coupler was led through a Bruel
and Kj aer 2608 measuring amplifier to a Hewlett
Packard model 3561A spectrum analyzer .
For ABR measurements, silver disc electrodes were attached with paste following
cleansing of the skin with alcohol and abrasion
with a mild pumice solution. Recording electrodes were located at the high forehead (Fz;
noninverting) and the right earlobe (A2; inverting), with a ground electrode at the nasion
(Fpz) . Electrode impedance did not exceed 3000
ohms and was within 1000 ohms between electrodes . The raw EEG was amplified by 100,000,
and bandpass filtered from 30 to 3000 Hz .
Sweep time was 12 msec (including a prestimulus baseline of 1 .2 msec) . There were 512 data
points per sweep.
ABRs for stimulus intensities greater than
40 dB nHL were averaged for 1000 sweeps,
whereas those for stimulus intensities of 40 dB
nHL or less were averaged for 2000 sweeps . All
responses were replicated . Data were stored on
floppy disc by a Nicolet DC-2000 disk controller
for later analysis .
Procedures
Subjects were seated comfortably in a reclining chair in a sound-treated booth throughout testing. The order in which the three sets of
earphones were examined was counterbalanced
across subjects .
Prior to the initiation of data collection for
this study, dB nHL was established for the right
earphone of each set by obtaining behavioral
thresholds to 21 .1 per second,100-psec clicks on
19 normal hearing subjects (three ofwhom were
subsequently included in this study) . A modified ascending procedure was used with 2-dB
increments . Subjects were instructed to hold
down a response button for as long as they heard
the stimuli, and to let it up when the stimuli
were no longer audible. The mean dial setting
on the stimulus generator for behavioral threshold was 4 .8 dB with the TDH-39 (SD = 4 .8 dB),
6.2 dB with the TDH-49 (SD = 4.7 dB), and 6.9
dB with the ER-3A (SD = 5 .5 dB). Since the
differences in dB between the mean values were
relatively small (2 .1 dB or less), and we were
using a 5-dB step size for ABR measurement, a
5-dB dial setting was selected as 0 dB nHL for
m
v
0
Figurel Temporalwave£orms and spectra obtained
with each earphone set on
KE1b1AR and averaged over
25 presentations of 100gsec clicks at 75 dB nHL.
Amplitude scales for the
temporal waveforms were
arbitrary. Notice the greater low frequency energy in
the spectrum for the ER3A, and reduced acoustic
ringing in the temporal
waveform relative to the
TDH earphones.
317
Journal of the American Academy of Audiology/Volume 3, Number 5, September 1992
sented at 80 dB nHL, and the intensity was
decreased in 20-dB steps until no replicable
response could be obtained . The level was then
increased in 5-dB steps until a response again
was observed .
ABRs were analyzed by two of the authors
(LVC and CS). Thresholds, and latency and
amplitude data, were accepted only if both
judges were in agreement. Repeated-measures
analyses of variance were accomplished on the
data using program 4V from the Biomedical
Data Package* statistical software, with Tukey
a posteriori comparisons performed as needed.
For all statistics reported in this study, significance was set at p < 0 .05 .
RESULTS
Acoustic Measurements
Figure 1 shows temporal waveforms and
spectra for click stimuli transduced by each
earphone and measured on KEMAR. Both TDH
earphones had greater ringing than the ER-3A
earphone for stimulus intensity levels down to
15 dB nHL. The peak equivalent (baseline-topeak) SPL (dB peSPL) for the initial pulse of the
click stimuli was measured using the peak
equivalent of a pure-tone acoustic output displayed on a storage oscilloscope . For a 0-dB nHL
click, peSPL was 30 dB for the TDH-39 earphone, 27 dB for the TDH-49, and 21 .5 dB for the
ER-3A.
Threshold Data
f
1 .2 ms/div
Stimulus
Figure 2 ABR threshold searches from one subject
obtained with each earphone set. Stimuli were 100-psec
clicks referenced to dB nHL. Notice the reduced stimulus
artifact and relative latency delay with the ER-3A insert
earphones.
all three earphone sets used in the study.
Behavioral thresholds to the click stimuli also
were obtained from the 10 subjects used in this
study for comparison with these previously
established normative values .
ABR thresholds were obtained for each set
of earphones . The stimuli were initially pre318
Representative waveforms from ABR
threshold searches for one subject under each
earphone set are shown in Figure 2. As expected, the ER-3A earphones produced a latency shift and reduced stimulus artifact, compared with the supra-aural earphones.
Behavioral and ABR threshold data in dB
nHL for each earphone type are illustrated in
Figure 3 . Analysis of variance (ANOVA) revealed a significant difference between
behavioral thresholds as a function of the earphone set. Tukey a posteriori comparisons revealed that the mean behavioral threshold for
the ER-3A earphone was significantly higher
than for the TDH-39 earphone . The mean differences between ER-3A and TDH-49 earphones,
and TDH-39 and TDH-49 earphones, were not
*Biomedical Data Package (BMDP) Statistical Software . (1985) . P .O . Box 24A26, Los Angeles, CA 90024
Comparison of Earphones /Van Campen et al
Behavorial
Threshold
ABR
Threshold
Sensation
Level
Figure 3 Bar graphs of mean behavioral and ABR
thresholds, and the individually calculated sensation level
(difference between the ABR and behavioral threshold)
obtained with each earphone set. Plus and minus one
standard deviation is indicated for each mean .
statistically significant . All subjects had ER-3A
thresholds that were higher than TDH-39
thresholds .
For ABR thresholds, ANOVA and Tukey
comparisons also revealed a significant difference as a function of the earphone set (see Fig.
3) . The mean ABR threshold for the ER-3A
earphone was significantly higher than for the
TDH-39 earphone . The mean threshold for the
ER-3A earphone did not differ significantly
from the TDH-49, nor did the mean thresholds
differ significantly between the TDHearphones.
Five of the ten subjects had higher ABR thresholds for the ER-3A earphone than for the TDH39 earphone .
Finally, the sensation level (i .e ., the difference between ABR threshold and behavioral
threshold) was calculated for each individual
subject under each earphone set (see Fig. 3) . An
ANOVA revealed no statistically significant
differences between any of the mean sensation
levels .
and standard deviations of these difference
scores are shown in Table 1 . The mean latency
delay for the ER-3A earphone relative to the
TDH earphones increased as stimulus intensity decreased. A two-way ANOVA (earphone
set, stimulus intensity level) and Tukey comparisons revealed that the TDH-39/ER-3A mean
latency difference at 40 dB nHL was significantly greater than that at both 60 and 80 dB
nHL. The mean latency difference at 60 dB nHL
did not differ significantly from that at 80 dB
nHL. The TDH-49/ER-3A mean latency difference was significantly greater at 40 dB than
at 80 dB nHL, but there were no significant differences between 40 and 60, or between 60 and
80 dB nHL.
The theoretical acoustic delay produced by
a 278-mm tubing length is 0.8 msec . Table 2
illustrates the means and standard deviations
of the latencies of identifiable peaks at each
stimulus level for each earphone set, following
subtraction of this correction factor from the
ER-3A data . Subtracting 0.8 msec brings the
ER-3A mean data in line with the TDH data at
higher stimulus levels, but undercorrects at
lower stimulus levels .
Another ANOVA was performed on the
wave V latency data at 80, 60, and 40 dB nHL
after subtraction of 0.8 msec from the ER-3A
absolute latency values . The ER-3A earphone
still produced significantly longer wave V
latencies at 60 and 40 dB nHL than either TDH
earphone, although there was no significant
12
11
10
9
s
7
6
5
ABR Latencies
Figure 4 displays the mean wave V latencyintensity function for each earphone set. To
further examine the relative latency delay produced by the ER-3A earphone, wave V latency
obtained on each subject with each TDH earphone set was subtracted from that obtained
with the ER-3A earphone set for stimulus intensity levels of 80, 60, and 40 dB nHL. Means
4
0
10
20
30
40
50
60
70
80
90
Intensity (dB nHL)
Figure 4 The mean wave V latency-intensity function
for each earphone set. The shaded area represents the
range of normative values used in our clinic for the TDH39 earphone . Values for the ER-3A earphone in this graph
are not corrected for the acoustic delay produced by the
length of tubing.
Journal of the American Academy of Audiology/Volume 3, Number 5, September 1992
Table 1
ABR Amplitudes
Wave V Latency Difference Scores
between Earphones
Earphones
(ER-3A)-(TDH-39)
Wave V latency
Stimulus Intensity
X
(msec)
(ER-3A)-(TDH -49)
Wave V Laten cy
SD
N
X
(msec)
SD
N
80 dB nHL
60
0 .80
0 .92
0 .10
0 .22
10
10
0 .83
0 .92
0.10
0 .12
10
10
20
1 .26
0 .19
8
1 .14
0 .13
8
40
1 .15* 0 .24
10
1 .00*
0.17
10
These difference scores represent the calculated difference for each individual subject between the ER-3A and
TDH-39 latencies, and between the ER-3A and TDH-49
latencies, at each intensity level . Stimuli were 100-sec
clicks .
X = mean ; SD = standard deviation ; N = sample size .
*Indicates significance at p < .05 .
difference at 80 dB nHL. There were no significant differences in absolute latencies between
the TDH-39 and TDH-49 earphones at any
stimulus level. All 10 subjects had longer
latencies for the ER-3A earphone than for the
TDH earphones at both 60 and 40 dB nHL after
subtraction of 0 .8 msec .
Finally, an ANOVA revealed that the I-V
interpeak latency was statistically equivalent
across all three earphones at 80 dB nHL. At
lower stimulus levels, wave I data were too
sparse for analysis .
Peak-to-trough amplitudes of waves I and
V and the V/I amplitude ratio at 80 dB nHL for
each earphone set are shown in Table 3. ANOVA
and Tukey comparisons revealed no significant
difference in mean wave V peak-to-trough amplitude across earphone sets at 80, 60, and 40
dB nHL, but at 80 dB nHL the ER-3A earphone
produced a significantly smaller wave I amplitude than either of the TDH earphones. As a
result of the smaller wave I (but comparable
wave V) amplitude, the ER-3A earphone also
yielded a greater V/I amplitude ratio at 80 dB
nHL than did the TDH earphones . This difference, however, failed to reach statistical significance. Seven of the ten subjects had larger
amplitude ratios for the ER-3A compared with
the TDH-39 . Eight of the subjects had larger
amplitude ratios for the ER-3A compared with
the TDH-49 .
DISCUSSION
W
e found significant differences forABRs
recorded with an ER-3A insert earphone versus TDH-39 and TDH-49 supra-aural
earphones. First, behavioral and ABR thresholds were significantly higher for the ER-3A
earphone versus the TDH-39 earphone . Second,
the ER-3A earphone had significantly longer
Table 2 Absolute Peak Latencies
Following Subtraction of a 0.8-msec Correction Factor from ER-3A Values
Earphones
Wave I
80 dB nHL
60
40
Wave III
80 dB nHL
60
40
Wave V
80 dB nHL
60
40
20
ER-3A
TDH-49
TDH-39
1 .61
1 .93
2 .96
0 .13
0 .05
0 .37
10
6
2
1 .62
2.27
2 .96
0 .12
0 .60
0 .34
10
4
6
1 .62
2 .52
3 .22
0 .09
0 .37
0 .37
9
4
3
3.78
4.11
0 .17
0.14
10
8
3 .76
0 .14
10
3 .88
0.33
10
4 .97
0 .33
7
5 .13
0 .34
5
5 .76
6 .13
6 .77
7 .81
0 .21
0 .26
0 .38
0 .53
10
10
10
9
5 .74
6 .13
6 .92
7 .89
4 .92
0 .37
6
4.15
0.14
0 .20
0 .30
0 .40
0 .54
7
10
10
10
9
4.34
5 .76
6 .25*
7 .12*
8 .07
7
0.26
0 .20
0 .33
0 .52
0 .29
10
10
10
8
The 0 .8 msec value represents the theoretical acoustic delay introduced by the length of tubing . Stimuli were 100-psec
clicks .
X = mean ; SD = standard deviation ; N = sample size .
*Indicates significance at p < .05 .
320
.
.
matillt 1
11 I I I s , d04 i
Comparison of Earphones /Van Campen et al
Table 3
ave
I
V
V/1
X
0 .34
0 .51
1 .88
TDH-39
Amplitude (lt V)
Peak-to-Trough Amplitudes
Earphones
TDH-49
ER-3A
Amplitude (p V)
Amplitude (w V)
SD
N
X
SD
N
0 .15
0 .13
1 .07
10
10
10
0 .35
0 .45
1 .63
0 .14
0 .14
1 .03
10
10
10
X
0 .29*
0 .52
2 .09
SD
N
0 .10
0 .14
0 .84
9
10
9
Shown are absolute amplitudes for waves I and V, and the V/I amplitude ratio, obtained with each earphone set . Stimuli
were 100-psec clicks presented at 80 dB nHL .
X = mean ; SD = standard deviation ; N = sample size .
*Indicates significance at p < .05 .
absolute latencies than either supra-aural earphone at low stimulus intensities, following
subtraction of a 0.8-msec correction factor . Third,
significantly smaller wave I amplitudes were
recorded with the ER-3A earphone than with
either supra-aural earphone . Acoustic evaluation of the ER-3A earphone also revealed that it
had a lower peak pressure output (peSPL), less
acoustic ringing, and more low-frequency energy for transduced clicks than the supra-aural
earphones.
Threshold Differences
Significantly higher behavioral and ABR
thresholds for the ER-3A have not been reported previously . Hood and Morehouse (1985)
reported no statistically significant difference
between the ER-3A and TDH-39 earphones for
behavioral thresholds to click stimuli, but did
not evaluate ABR thresholds . In the Beauchaine
et al (1987) study, behavioral thresholds for the
ER-3A and Beyer DT-48 earphones reportedly
differed by only 2 dB . The mean ABR threshold
for the ER-3A earphone was 5 dB higher than
for the Beyer earphone, but no statistical analysis was accomplished .
Our finding of significantly higher behavioral and ABR thresholds for the ER-3A
could be due to the fact that the ER-3A earphone
had the lowest peSPL value of the three earphones and that we were limited by the
attenuator to using a 5-dB stepsize . Although
the difference in behavioral thresholds that we
found between the earphones indicates that
separate normative data for dB nHL should be
collected with the ER-3A earphone, the more
important aspect of our data is that ABR thresholds were obtained at equivalent sensation levels across the earphones. This latter finding
implies that all three earphones are equally
effective for the purpose of estimating behavioral
thresholds from ABR thresholds .
Latency Differences
Our second significant fording was that the
ER-3A latency delay, relative to the TDH-39 or
TDH-49 latencies, increased with decreasing
intensity. This resulted in a slightly steeper
latency-intensity function for the ER-3A. No
previous authors have reported this finding.
Although the absolute value of the acoustic
latency delay produced by the ER-3A may depend on several factors, such as the type of
cushion used with the supra-aural earphone, it
would be expected to be constant across stimulus intensity levels . We considered the possibility that the mean 3-dB difference found in
stimulation level between the ER-3A and TDH39 earphones (due to behavioral threshold differences for the subjects in this study) might
have meant that we were measuring on a lower,
steeper portion of the latency-intensity function for the ER-3A earphone than for the TDH39 . This hypothesis is not supported, however,
by examination of the data of Stockard et al
(1979), who examined the dB/msec shift in wave
V latency with TDH-39 earphones. The relative latency delays that we demonstrated with
the ER-3A are greater at lower stimulus intensities than would be accounted for by the
small difference in stimulation level. We also
examined individual data of those subjects
who had behavioral thresholds that were the
most and least disparate from the mean and
found no correspondence with the slope of the
ABR latency-intensity function . Based on these
observations, we conclude that the small difference in mean stimulation level did not largely
influence the data .
Several other factors also may be proposed
to explain the slightly longer latencies found at
lower stimulus intensity levels with the ER-3A.
First, it is possible that this finding is the result
of the lower peSPL measured with the ER-3A
Journal of the American Academy of Audiology/Volume 3, Number 5, September 1992
earphone . At high stimulus levels, the relatively small output difference between earphones may not produce significant peak latency changes, while at lower intensities, this
same intensity difference might result in a
larger peak latency shift.
Beauchaine et al (1987) reported that the
latency delay of the ER-3A relative to the Beyer
DT48 did not vary across stimulus intensity,
but these researchers also reported equivalent
peSPL between the earphones. Yang and
Henrickson (1988) also did not report an intensity dependence, but the 0.9-msec correction
factor that they used appears to have overcorrected for the ER-3A latency delay at all
stimulus intensity levels .
A second factor that may account for the
differences is that the ER-3A earphone had
greater low-frequency energy than the TDH
earphones, as measured on KEMAR (see Fig. 1) .
Although both the manufacturer and Hood and
Morehouse (1985) have reported similar spectra between the transducers, their measurements were made with 2-cc and 6-cc closed
couplers, which provide an acoustic seal for
both the insert and supra-aural earphones.
When supra-aural earphones are placed on
KEMAR, however, there is substantial roll-off
oflow-frequency energy due to acoustic leakage.
This condition better simulates the real-ear
response where the seal to the pinna is not
complete . It is reasonable to assume that differences among the earphones in spectra at the
eardrum may effect the ABR. Even though the
ABR is primarily a high-frequency response,
increased low-frequency energy may result in
an overlap ofthe traveling waves in the cochlear
partition. Some authors (Klein and Teas, 1978 ;
Burkard and Hecox, 1983) have suggested that
responses to low-frequency stimuli are generated primarily in the same basal cochlear regions that respond to high-frequency stimuli. In
addition, Klein and Teas (1978) reported that
low-frequency stimuli produce a steeper latency-intensity function than high-frequency
stimuli . This issue could be further explored by
filtering the output of the ER-3A earphone to
approximate the real-ear response of a TDH
earphone .
Finally, differences were found among the
earphones in the degree of acoustic ringing
evident in the temporal waveform of the
transduced clicks . Weber et al (1981) reported
that excessive earphone ringing produces prolonged peak latencies, decreased amplitudes,
and disorganized waveform morphology . Our
data conflict with Weber et al (1981), however,
since the earphone with the least acoustic ringing (i .e ., the ER-3A) produced the longest latencies (after subtraction ofthe theoretical acoustic delay) and the smallest wave I amplitudes .
Clinically, the latency differences found between the earphones at lower stimulus intensity levels may have little impact . Although differences in peak latency are critical for diagnostic ABR testing, most testing is performed at
higher stimulus levels, and the differences we
found were small . Nevertheless, further evaluation of the latency differences between the ER3A and supra-aural earphones may be merited
with the use of hearing-impaired subjects .
Amplitude Differences
Our third significant finding was that wave
I amplitude with the ER-3A earphone was significantly smaller than with the supra-aural
earphones. This resulted in a larger, although
not statistically different, V/1 amplitude ratio .
These data are in agreement with Hood and
Morehouse (1985), who reported a significantly
smaller wave I amplitude and significantly
larger V/I amplitude ratio with the ER-3A than
with the TDH-39 . In contrast, Yang and
Henrickson (1988) reported no significant difference in the V/I ratio at 60 or 80 dB nHL, but
these researchers did not evaluate absolute
amplitude values .
Hood and Morehouse (1985) also reported
that early waves were more identifiable with
the ER-3A than with the TDH-39 earphone . In
contrast, Beauchaine et al (1987) reported that
early peaks (waves I and III) were more difficult
to identify with the ER-3A than with the Beyer
DT48 earphone . Although no formal evaluation
was accomplished, we also made a qualitative
observation that, at least for some subjects,
early waves of the ABR were of poorer morphology for some subjects with the ER-3A than with
the TDH-39 .
Spectral and temporal differences among
the earphones may again be invoked as a possible explanation for the amplitude differences
we observed with the ER-3A . For example, it is
possible that the additional low-frequency energy present in the ER-3A spectrum may produce upward spread of masking along the
cochlear partition, reducing the neural synchrony needed for good waveform morphology .
It is clear that further study into the effects on
the ABR of spectral and temporal waveform
differences among earphones is needed .
322
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Comparison of Earphones /Van Campen et al
Reduced amplitude of wave I with the ER3A earphone has important practical implications. If it is more difficult to identify wave 1
with the ER-3A earphone, then accurate determination of absolute and interpeak latencies
may be compromised. One possible solution to
this problem is the use of canal or tympanic
membrane electrodes, which enhance wave I
amplitude (Stypulkowski and Staller,1987). In
addition, our findings, and those of Hood and
Morehouse (1985), suggest that V/1 amplitude
ratio, an important diagnostic indicator, may
vary between earphone types . As a result, separate normative values will need to be collected
with the ER-3A.
The ER-3A insert earphone provides benefits over traditional supra-aural earphones in
terms of increased ambient noise attenuation,
elimination of ear canal collapse, and greater
patient comfort. The results of this study indicate, however, that there are significant differences between measurements made with the
ER-3A and supra-aural earphones. Until further research is accomplished to clarify these
differences, we recommend that each facility
develop separate normative data for the ER-3A
insert earphones prior to their use in ABR
measurement . These data should minimally
include behavioral thresholds in dB nHL and
V/I amplitude ratios . Further, we recommend
the use of a 0.8 msec correction factor at high
stimulus intensities and the use of canal or
tympanic membrane electrodes with the ER-3A
for enhancement of wave I amplitude.
Acknowledgment . Portions of this study were presented at the annual meeting of the American SpeechLanguage-Hearing Association, Boston, Massachusetts,
November, 1988 .
The authors thank Don Riggs for technical support
and for development of the illustrations used in this
article .
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