Distortion Product Otoacoustic Emissions:
Comparison of Sequential versus
Simultaneous Presentation of
Primary-Tone Pairs
Randall C. Beattie*
Abstract
This Grason-Stadler GSI-60 system for measuring distortion product otoacoustic emissions (DPOAEs) allows the examiner to present one set of
primary-tone pairs at a time (i.e., sequential presentation), or to present as
many as four sets of primary-tone pairs at a time (i.e., simultaneous presentation). The Sequential and Simultaneous protocols were used to compare
administration times, DPOAEs, and noise floors (NFs) on normal-hearing subjects at three frequencies (f2 = 1000, 2000, and 4000 Hz) and eight intensities
(L1 = 40–75 dB SPL in 5 dB steps; L2 = 30–65 dB SPL). The Simultaneous
protocol was completed in less than half the time (mean = 2 minutes, 21 seconds) required for the Sequential protocol (mean = 5 minutes, 13 seconds).
When stimulus intensity (L1) was <60 dB SPL, the Sequential and Simultaneous
protocols yielded similar DPOAEs and NFs. However, at the higher L1 intensities, the NFs for the Simultaneous protocol were larger than those for the
Sequential protocol. The higher Simultaneous NFs reflect the greater system
distortion/noise generated by the GSI-60 instrumentation. Reliability was
assessed using the standard error of measurement of the difference between
two scores. The data revealed no significant differences between protocols,
and suggest that differences between two DPOAEs are statistically significant
if they exceed ~7 dB (95% confidence interval).
Key Words: Distortion product otoacoustic emissions, Grason-Stadler GSI60, input-output functions, sequential presentation of primary-tone pairs,
simultaneous presentation of primary-tone pairs, test-retest reliability
Abbreviations: ANSI = American National Standards Institute; DPOAE =
distortion product otoacoustic emissions; f1 = lower value in Hertz of two
paired primary tones; f2 = higher value in Hertz of two paired primary tones;
FFT = fast Fourier transform; L1 = sound pressure level in decibels of the f1
frequency; L2 = sound pressure level in decibels of the f2 frequency; NF =
noise floor; SPL = sound pressure level in decibels; RMS = root mean
squared
Sumario
Este sistema Grason-Stadler (GSI-60) para la medición de emisiones otoacústicas por productos de distorsión (DPOAE) permite al examinador la presentación
de un juego de pares de tonos primarios al mismo tiempo (p.e., presentación
secuencial), o presentar hasta cuatro juegos de pares de tonos primarios a
la vez (p.e., presentación simultánea). Se utilizaron los protocolos Secuencial
y Simultáneo para comparar los tiempos de administración, las DPOAE y los
pisos de ruido (NF) en sujetos normo-oyentes, en tres frecuencias (f2 = 1000,
2000 y 4000 Hz) y ocho intensidades (L1 = 40-75 dB en pasos de 5 dB; L2 =
30-65 dB SPL). El protocolo Simultáneo se completó en menos de la mitad
del tiempo (media = 2 minutos, 21 segundos) requerido por el protocolo
Secuencial (media = 5 minutos, 13 segundos). Cuando la intensidad del estí-
* Department of Communicative Disorders, California State University, Long Beach
Reprint requests: Randall C. Beattie, Department of Communicative Disorders, California State University, Long Beach, 1250
Bellflower Blvd., Long Beach, CA 90840; Phone: 562-985-5281; E-mail: BeattieR@csulb.edu
471
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
mulo (L1) fue £60 dB SPL, los protocolos Secuencial y Simultáneo rindieron
DPOAE y NF similares. Sin embargo, en las intensidades L1 más altas, los
NF para el protocolo Simultáneo fueron mayores que aquellas para el protocolo Secuencial. Los NF Simultáneos más altos reflejan la mayor distorsión/ruido
del sistema, generado por la instrumentación del GSI-60. La confiabilidad fue
evaluada utilizando el error estándar de medición de la diferencia entre los
dos puntajes. Los datos revelaron que no había diferencias significativas entre
los protocolos, y sugieren que las diferencias entre dos DPOAE son estadísticamente significativas si exceden ~7 dB (intervalo de confianza del 95%).
Palabras Clave: Emisiones otoacústicas por productos de distorsión; GrasonStadler GSI-60; funciones input/output; presentación secuencial de pares de
tonos primarios; presentación simultánea de pares de tonos primarios; confiabilidad test/retest.
Abreviaturas: ANSI = Instituto Nacional Americano de Normal; DPOAE =
emisiones otoacústicas por productos de distorsión; F1 = valor inferior en
Hertz de dos tonos primarios pareados; f2 = valor superior en Hertz de dos
tonos primarios pareados; FFT = transformación rápida de Fourier; L1 = nivel
de presión sonora en decibeles de la frecuencia f1; L2 = nivel de presión
sonora en decibeles de la frecuencia f2; NF = piso de ruido; SPL = nivel de
presión sonora en decibeles; RMS = media de la raíz cuadrada.
D
istortion product otoacoustic
emissions (DPOAEs) result from the
simultaneous presentation of two
tones of frequencies f1 and f2, where f2 is
higher in frequency than f1 (Kemp, 1979).
DPOAEs are thought to be generated by the
outer hair cells and may correspond to the
region where the f1 and f2 forward traveling
waves maximally overlap (Brown and Kemp,
1984; Kemp, 1986; Brown, 1987; LonsburyMartin and Martin, 1990; Probst et al, 1991;
Sun et al, 1996). In human ears the
difference-intermodulation distortion product
at the frequency 2f1-f2 (cubic difference tone)
is the most prominent DPOAE. Cochlear
emissions have been employed to screen
newborns, evaluate difficult-to-test patients,
identify middle-ear pathology, and to monitor
the status of Meniere’s disease, noise-induced
hearing loss, and ototoxicity (Kemp et al,
1986; Lafreniere et al, 1991; Lonsbury-Martin
et al, 1993; Probst et al, 1993; Hunter et al,
1994; Kimberly et al, 1997; Mulheran and
Degg, 1997; Yardley et al, 1998; Hall, 2000).
DPOAEs are typically obtained by
presenting pairs of primary tones sequentially
at different frequencies. However, to reduce
test time, one manufacturer (Grason-Stadler,
Madison, WI) produces a DPOAE system in
which more than one pair of primary tones
472
can be presented simultaneously. This system
(GSI-60) allows the examiner to present one
set of primary-tone pairs at a time (i.e.,
sequential presentation), or to present as
many as four sets of primary-tone pairs at a
time (i.e., simultaneous presentation). The
GSI-60 instruction (Grason-Stadler, 1996)
manual states that the Simultaneous protocol
is advantageous because DPOAE data can be
obtained in less time than the Sequential
protocol. A shortened test may be
advantageous because it allows (1) completion
of testing before a difficult-to-test patient
becomes uncooperative, (2) testing more
patients within a given time period, and/or
(3) a reduction in the cost of testing so that
the procedure is affordable to more patients.
Only limited data are available comparing the
Sequential and Simultaneous protocols. Kim
et al (1997) tested normal-hearing and
hearing-impaired subjects at a single
condition (L1 = 65 and L2 = 50 dB SPL) and
found that the two protocols yielded similar
mean DPOAE and NF amplitudes. These
investigators measured recording times for
six frequencies and found that the Sequential
and Simultaneous protocols required typically
40–80 seconds and 11–25 seconds,
respectively. That is, the Simultaneous
protocol was completed in approximately
Sequential versus Simultaneous/Beattie
one-third to one-half the time as the
Sequential protocol. Kim et al (1997) also
evaluated test-retest reliability on three
normal-hearing subjects and found that the
standard deviation of the test-retest
differences was less than 4 dB for both the
Sequential and Simultaneous protocols.
Some investigators suggest that it may
be useful to obtain DPOAEs at more than one
intensity (Stover and Norton, 1993; Hall,
2000). For example, monitoring input-output
functions may be useful to identify changes
in hearing following the administration of
ototoxic drugs or exposure to loud noises.
Input-output functions can be examined for
(1) DPOAE amplitudes over a range of
intensities, (2) DPOAE threshold, (3) slope of
the function, and/or (4) shape of the function
(Nelson and Kimberly, 1992; Lasky et al,
1994; Nelson and Zhou, 1996; Stover et al,
1996). Obtaining input-output functions also
may provide a measure of loudness growth
that may be useful when selecting
amplification characteristics such as
compression ratio (Dorn et al, 2001). When
input-output functions are generated, as
compared to testing at a single intensity, the
potential time savings should be even greater
for the Simultaneous protocol as compared to
the Sequential protocol.
Reliability is an essential aspect of any
clinical procedure because it provides a
measure of the degree of confidence that can
be placed in an individual DPOAE or between
DPOAEs. These decisions require that
clinicians know how much of a difference in
the DPOAE is necessary before they can be
reasonably certain that the DPOAE change
is due to an alteration in the auditory system
and is not simply due to measurement error.
These questions require the establishment of
reliability values that estimate the degree of
confidence that can be placed in an individual
DPOAE or in differences between DPOAEs.
In view of the limited research comparing
the Sequential and Simultaneous protocols,
the present study was designed to gather
additional data comparing these protocols
when testing over a wide range of intensities.
Normal-hearing subjects were tested and
retested at eight intensities (L1 = 40–75 dB
SPL in 5 dB steps; L2 = 30–65 dB SPL) and
at three frequencies (f2 = 1000, 2000, and
4000 Hz). The comparability of the two
protocols was assessed by measuring (1) test
administration times for obtaining DPOAEs
at eight intensities for each of the three
frequencies, (2) DPOAE amplitudes and noise
floor (NF) amplitudes in the vicinity of the
DPOAEs, and (3) test-retest reliability.
METHOD
Subjects
One ear was randomly selected from 62
normal-hearing women ranging in age from
19 to 26 years (mean = 23). The subjects
reported no ear infections, and a negative
history of long-term noise exposure, ototoxic
drugs, and hereditary hearing loss. They
passed a pure-tone screening test for the
octave frequencies from 500 Hz to 4000 Hz at
15 dB HL (ANSI, 1996), had normal otoscopic
findings, and had normal tympanograms
using a 226 Hz probe tone (static admittance
ranged from 0.3 to 1.4 mmhos and the
pressure peak was 0 ± 50 daPa). Additionally,
no spontaneous OAEs were present within
100 Hz of the cubic difference tone (2f1-f2) for
the three pairs of primary tones in which f2
= 1000, 2000, and 4000 Hz.
Instrumentation
Testing was conducted with a
commercially available system for measuring
spontaneous OAEs and DPOAEs (GrasonStadler GSI-60; Application Software Version
4.7). The probe system employed a
measurement microphone and two separate
speakers for presenting each of the primaries.
The primaries were delivered via tubing that
was coupled to the ear canal with a probe tip
and rubber eartips supplied by the
manufacturer. The microphone directed the
output to an associated analysis
board/computer that averaged ear canal
responses. A sampling rate of 16 kHz was
used. The data were continuously recorded
into a series of 32 msec segments (frames)
that were subjected to spectral analyses using
a Fast Fourier Transform yielding 512 bins
of data; each bin of data was 31.25 Hz wide.
Test Stimuli
Three pairs of frequencies in which f2 was
set at 1000, 2000, and 4000 Hz were
presented at an f2:f1 ratio of about 1.20
(Harris et al, 1989; Gaskill and Brown, 1990;
473
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
Lonsbury-Martin et al, 1993; Nielsen et
al,1993; Rasmussen et al, 1993; Brown et al,
1994; Whitehead et al, 1995). This
investigation presents data as a function of
the f2 frequency. The associated distortion
product frequencies (2f1-f2) were 686, 1312,
and 2686 Hz. The L1 primary tone varied in
5 dB decrements from 75 to 40 dB SPL. The
L2 primary tone was 10 dB less than L1
because several studies suggest that this
relative level yields DPOAEs that approach
maximal values (Gaskill and Brown, 1990;
Hauser and Probst, 1991; Whitehead et al,
1995; Beattie and Jones, 1998). The primary
tones were presented in 5 dB decrements
from L1 = 75 dB SPL and L2 = 65 dB SPL to
L1 = 40 dB SPL and L2 = 30 dB SPL.
Data Acquisition Criteria
Testing at a given frequency was
terminated when a signal-to-noise ratio of 10
dB was attained. However, a minimum of
200 frames and a maximum of 1,000 frames
were averaged. Moreover, a frame was
rejected if it exceeded the 30 dB SPL rejection
criterion or if L1 or L2 differed by more than
2 dB from the target values. These test
acceptance criteria and test rejection criteria
were selected because they are consistent
with the values that are employed in clinical
settings with this instrumentation (GrasonStadler, 1996; Kim et al, 1997; Painter, 1997,
2000). Testing was completed in a minimum
of 6.4 seconds if all criteria were met, or in a
maximum of 32 seconds even if all the test
acceptance criteria were not met.
Noise Floor Measurements
The root mean squared (RMS) noise
calculation averaged two bins of data (62.5
Hz) on either side of the DPOAE bin. For
each sweep, the average noise value from
these four bins was compared to the rejection
criterion (30 dB SPL) for either acceptance or
rejection of the sweep. If the noise level in the
four bins was less than 30 dB SPL, the sweep
was accepted and the complex FFT values
were averaged in an accumulator. The noise
floor levels were computed from two bins on
either side of the 2f1-f2 frequency for the
Sequential protocol, where f2 = 1000, 2000,
or 4000 Hz. For the Simultaneous protocol,
however, the GSI system uses the noise floor
corresponding to the first set of primary-tone
474
pairs per group when applying the data
acquisition criteria for frame rejection, test
rejection, and test acceptance. Because the
frequency pairs were always presented from
low to high frequency for the Simultaneous
protocol, the noise floor was measured around
2f1-f2 where f1 = 843 Hz and f2 = 1000 Hz.
Procedures
The GSI DPOAE system allows the
examiner under the “Custom DP Stimulus”
menu to select a Sequential or Simultaneous
presentation of test stimuli. In the Sequential
mode, a single pair of frequencies are
presented, one at a time, in as many as 16
combinations of frequency and intensity.
Growth functions were collected for each f2
frequency in the following order: 1000, 2000,
and 4000 Hz. In the Simultaneous mode, the
GSI system allows a maximum of four pairs
of frequencies to be presented simultaneously,
followed by up to three additional groups of
four pairs (i.e., a total of 16 conditions).
Initially, three pairs of frequencies were
presented simultaneously at L1 = 75 dB SPL
and L2 = 65 dB SPL. The three pairs of
frequencies were then presented in 5 dB
decrements down to L1 = 40 dB SPL and L2
= 30 dB SPL.
The subjects were placed in a reclining
chair situated in a sound-treated room
(Industrial Acoustics Corporation, Series
400). They were instructed to remain quiet
and very still during testing (Hall, 2000,
197). All subjects were tested with both the
Sequential and Simultaneous protocols. The
order of testing for the two protocols was
randomized for each subject. The subjects
were tested three times in each condition
(T1, T2, and T3), with a 10–20 minute break
that involved removal and reinsertion of the
probe tip (Hall, 2000) between T1 and T2
and between T2 and T3.
Data Analysis and Statistical Tests
When comparing the Sequential and
Simultaneous protocols, the median of the
three measurements on each subject was
used when calculating administration times,
DPOAEs, and NFs. To evaluate the statistical
significance of administration times between
the Sequential and Simultaneous protocols,
paired t-tests were performed using the
Statistical Package for the Social Sciences
Sequential versus Simultaneous/Beattie
(SPSS for Windows 11.01S). Using this
statistical package, a two-way analysis-ofvariance (ANOVA) for repeated measures
was performed on each frequency to assess
the statistical significance of eight levels of
the factor “intensity” and two levels of the
factor “protocol.” Tukey’s post hoc test was
used to identify significant differences
between means (Linton and Gallo, 1975;
Bruning and Kintz, 1987).
Several authors state that the standard
error of measurement (SEM) is the most
useful statistic for assessing test-test
reliability (Guilford and Fruchter, 1973;
Anastasi, 1976; Lemke and Wiersma, 1976;
Ghiselli et al, 1981; Demorest, 1984;
Demorest and Walden, 1984; American
Psychological Association, 1985; Brown,
1989). This statistic may be used to
estimate the true DPOAE from a single
measurement and may be particularly
useful for screening applications to assess
whether an individual score deviates
significantly from the norm or from a
predetermined cut-off score. The SEM can
be computed from the following formula:
SEM = SD √1 - r
where the SD is the combined standard
deviation of test and retest DPOAEs and r
is the Pearson product-moment correlation
coefficient. The SEM allows construction
of a confidence interval that contains the
t r ue D PO AE. Th e pr obabi lit y is
approximately 68 percent that the true
DPOAE will fall within one standard error
of the observed DPOAE; there is a 95
percent probability that the true DPOAE
will be within two standard errors of the
obtained DPOAE; and there is a 99 percent
probability that the true DPOAE will lie
within 2.58 standard errors of the obtained
DPOAE.
To assess w h eth er tw o DPOA E
measurements are statistically significant,
the standard error of measurement of the
difference between two values (SEM∆) can
be employed (Anastasi, 1976; Demorest and
Walden, 1984; Mehrens and Lehmann,
1984; Brown, 1989). The two measurements
may represent DPOAEs before and after
noise exposure or drug administration,
DPOAEs from the right ear versus the left
ear, or DPOAEs from two individuals. The
SEM∆ can be computed from the following
formula:
SEM∆ = SEM √2
where the SEM is the standard error of
m ea surem ent of t he t est a nd ret est
DPOAEs, and the square root of two is
1.414. Demorest and Walden state that the
SEM and the SEM∆ are “the only way to
ensure that measurement error is explicitly
taken into account in score interpretation”
(1984, 234).
RESULTS
Administration Time
When each of the three f2 stimuli (1000,
2000, and 4000 Hz) were presented
sequentially at eight levels (L1 = 40–75 dB
SPL), the mean administration time for all
subjects for data collection was 313 seconds
(5 minutes, 13 seconds). Test times ranged
from 194 to 640 seconds, with a standard
deviation of 110 seconds. When the three f2
stimuli were presented simultaneously at
each of the eight L1 levels, the mean
administration time was 141 seconds (2
minutes, 21 seconds), the standard deviation
was 63 seconds, and test times ranged from
64 to 253 seconds. The 172-second mean
difference between the two protocols was
statistically significant using the t-test (p <
.01, t = 15.3). It is evident that the
Simultaneous protocol was completed in less
than half the time required for the Sequential
protocol.
DPOAE and NF Amplitudes
Table 1 presents mean DPOAEs and NFs
in dB SPL and standard deviations for the
Sequential and Simultaneous protocols for
three frequencies at L1 = 40–75 dB SPL. The
mean data also are shown in Figure 1 for 1000
Hz (top panel), 2000 Hz (middle panel), and
4000 Hz (bottom panel).
The 1000 Hz DPOAE data show similar
mean SPLs (<0.5 dB) for both the Sequential
and Simultaneous protocols at L1 = 40–60 dB
SPL. However, somewhat higher DPOAEs
were observed for the Simultaneous protocol
at L1 = 65–75 dB SPL. The ANOVA revealed
a statistically significant interaction (p < .01)
475
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
Table 1 Mean Distortion Product Otoacoustic Emissions (DPOAEs) and
Noise Floors (NFs) in dB SPL and Standard Deviations for the Sequential and Simultaneous
Protocols for f2 = 1000, 2000, and 4000 Hz at L1 = 40–75 dB SPL
f2
Frequency
Protocol
Sequential
Mean DPOAE
SD
Mean NF
SD
L1 in dB SPL
55
60
40
45
50
-3.0
0.5
2.6
4.9
6.8
4.6
5.9
-12.3 -12.1
-12.8
65
70
75
6.7
6.4
6.1
5.6
6.4
6.5
7.4
6.4
5.7
-10.8
-11.9
-11.7
-10.2
-11.6
8.0
4.4
6.1
6.2
5.8
6.3
6.9
6.9
-3.4
0.1
2.8
4.7
6.8
8.1
9.1
10.6
6.4
5.9
5.8
6.3
6.2
5.8
5.1
5.7
-12.3 -10.1
-9.8
-9.2
-7.0
-3.1
2.8
1000 Hz
Simultaneous Mean DPOAE
SD
Mean NF
SD
Sequential
Mean DPOAE
SD
Mean NF
SD
-12.8
6.5
5.8
7.1
5.2
4.6
3.7
3.0
3.9
-9.1
-7.5
-4.1
-0.3
3.1
5.5
7.1
8.9
8.0
8.0
7.1
6.7
6.0
7.2
7.2
6.6
-18.0 -19.0
-17.6
-17.6
-17.0
-16.6
-14.2
-17.6
6.4
7.6
4.9
5.3
5.4
5.6
6.5
5.8
-9.2
-6.7
-4.6
-0.5
2.2
5.4
6.9
6.6
10.1
8.0
7.1
7.3
7.5
6.1
5.5
6.0
-18.0 -17.1
-16.4
-14.5
-14.8
-10.0
-4.5
2000 Hz
Simultaneous Mean DPOAE
SD
Mean NF
SD
Sequential
Mean DPOAE
SD
Mean NF
SD
-18.5
7.7
6.8
7.2
6.3
8.7
4.6
5.8
4.5
-10.3
-7.4
-2.9
0.9
4.2
6.9
7.0
5.2
5.6
6.7
6.9
6.4
5.5
5.1
6.0
7.0
-22.0 -21.2
-21.0
-20.7
-20.4
-20.3
-20.5
-22.9
5.0
4.8
5.1
5.5
5.7
6.0
7.9
5.3
-10.3
-6.8
-2.9
0.3
4.1
6.4
6.9
6.4
7.1
6.4
6.7
6.2
5.8
5.2
5.3
6.9
-22.9 -22.9
-21.1
-20.2
-15.1
-6.8
-0.2
5.5
4.7
3.3
3.4
8.3
4000 Hz
Simultaneous Mean DPOAE
SD
Mean NF
SD
between the intensity and protocol factors.
Tukey’s post hoc test showed statistically
significant differences (p < .01) between the
Sequential and Simultaneous protocols at
L1 = 70 and 75 dB SPL. The Simultaneous
protocol yielded means that were 3 dB higher
than the Sequential protocol at L1 = 70 dB
SPL and 5 dB higher at L1 = 75 dB SPL.
Significant differences between protocols
476
-21.6
9.3
5.3
5.7
were not observed at the lower L1 intensities.
Standard deviations were similar across
intensities and were about 6 dB for both
protocols. The top panel in Figure 1 shows
that the NFs associated the Sequential and
Simultaneous protocols differed by less than
3 dB for L1 = 40–60 dB SPL; these differences
were not statistically significant (p > .01)
using the Tukey test. However, the
Sequential versus Simultaneous/Beattie
Figure 1 Mean DPOAEs in dB SPL are presented
for the Sequential protocol (closed circles, solid line)
and the Simultaneous protocol (stars, dotted line) for
f2 = 1000 Hz (top panel), 2000 Hz (middle panel), and
4000 Hz (bottom panel) at L1 = 40–75 dB SPL. The
associated mean NFs in dB SPL also are shown for
the Sequential protocol (triangles, dashed line) and
the Simultaneous protocol (closed squares, dotteddashed line).
Simultaneous NFs were higher than the
Sequential NFs at L1 = 65–75 dB SPL (p <
.01). The figure shows that the Simultaneous
minus Sequential differences increased from
4.7 dB at L1 = 65 dB SPL to 14.4 dB at L1 =
75 dB SPL.
The 2000 Hz data in Figure 1 shows that
the DPOAEs for the Sequential and
Simultaneous protocols differed by less than
1 dB for L1 = 40–70 dB SPL. At L1 = 75 dB
SPL, however, the Sequential protocol yielded
a DPOAE that was higher (8.9 dB SPL) than
the Simultaneous DPOAE (6.6 dB SPL). The
two-way ANOVA yielded a statistically
significant interaction between the protocol
and intensity factors, and the Tukey test
revealed that the Sequential DPOAE was
significantly higher (p < .01) than the
Simultaneous DPOAE at L1 = 75 dB SPL.
The DPOAEs for the two protocols did not
differ significantly (p > .01) at the other
intensities. For L1 = 40–65 dB SPL, the
middle panel in Figure1 shows that the
Sequential and Simultaneous NFs associated
with 2000 Hz differed by <3.1 dB. These
differences were not statistically significant
using the Tukey test. The noise floor was
significantly higher (p < .01) for the
Simultaneous protocol than with the
Sequential protocol for L1 = 70–75 dB SPL.
These differences were 6.6 dB at 70 dB SPL
and 9.7 dB at 75 dB SPL.
The bottom panel in Figure 1 shows that
the Sequential and Simultaneous DPOAEs
for 4000 Hz were similar (<1.2 dB) as L1
increased from 40 to 75 dB SPL. The effects
of protocol and the interaction between
protocol and intensity were not statistically
significant (p < .01) when tested with a twoway ANOVA for repeated measures. The 4000
Hz NF data in the bottom panel of Figure 1
illustrate similar values (<2 dB) for the
Sequential and Simultaneous protocols for L1
= 40–60 dB SPL. However, the Tukey test
indicated that the NFs associated with the
Simultaneous protocol were higher (p < .01)
than the Sequential protocol at L1 = 65, 70,
and 75 dB SPL. These differences increased
from approximately 5 dB at L1 = 65 dB SPL
to 20 dB at L1 = 75 dB SPL.
Reliability
Table 2 presents SEM values for the
Sequential and Simultaneous protocols at
three frequencies (f1 = 1000, 2000, and 4000
477
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
Table 2
L1 in
dB SPL
Standard Errors of Measurement for the Sequential (Seq)
and Simultaneous (Simul) Protocols
1000 Hz
Seq Simul Com
2000 Hz
Seq Simul Com
Seq
4000 Hz
Simul Com
Combined
1000–4000 Hz
Seq Simul Com
75
4.2
2.3
3.3
1.5
1.5
1.5
2.5
2.7
2.6
2.7
2.2
2.5
70
2.9
2.3
2.6
2.3
2.5
2.4
1.7
1.2
1.5
2.3
2.0
2.2
65
2.6
2.9
2.7
2.2
2.0
2.1
1.4
1.5
1.5
2.1
2.1
2.1
60
2.8
2.5
2.7
2.1
2.9
2.5
1.9
1.7
1.8
2.3
2.4
2.3
55
2.5
3.2
2.8
2.2
2.7
2.5
2.6
2.2
2.4
2.4
2.7
2.5
50
2.3
2.3
2.3
2.5
3.3
2.9
2.8
3.3
1.1
2.5
3.0
2.8
45
3.1
4.1
3.6
3.0
3.2
3.1
2.5
1.9
2.2
2.9
3.1
3.0
40
4.2
4.8
4.5
4.3
5.2
4.7
3.2
3.7
3.5
3.9
4.6
4.3
Combined
40–75
3.1
3.1
3.1
2.5
2.9
2.7
2.3
2.3
2.3
2.6
2.8
2.7
Combined
45–75
2.9
2.8
2.9
2.4
2.6
2.5
2.2
2.1
2.1
2.5
2.5
2.5
Note: The first column presents L1 intensities in dB SPL. The next nine columns present data for the three f2 frequencies.
The last three columns present data combined (Com) across the f2 frequencies. The bottom two rows present data that were
combined across protocols, frequencies, and intensities for all eight intensities (L1 = 40–75 dB SPL) and for the seven higher
intensities (L1 = 45–75 dB SPL).
Hz) and eight intensities (L1 = 40–75 dB
SPL). The table also presents the data
combined across protocols, frequencies, and
intensities. These SEM values represent the
mean of three measurements (T1 vs. T2, T1
vs. T3, and T2 vs. T3). Examination of the
SEM values in Table 2 reveals three major
observations. First, with one exception (1000
Hz at 75 dB SPL), the standard errors were
similar (<1 dB) for the Sequential and
Simultaneous protocols at all intensities and
frequencies. When the protocol data were
combined across all frequencies and
intensities, SEM values were 2.6 dB for the
Sequential protocol and 2.8 dB for the
Simultaneous protocol. Second, the SEM
values tended to decrease slightly as
frequency increased. That is, when the SEM
values were combined across protocols and
intensities, the standard errors were 3.1 dB
at 1000 Hz, 2.7 dB at 2000 Hz, and 2.3 dB at
4000 Hz. When the data were combined
across all conditions, the SEM was 2.7 dB.
Third, somewhat larger standard errors were
observed at L1 = 40 dB SPL than at the
higher intensities. That is, when combined
across protocols and frequencies, the standard
errors were 4.3 dB at L1 = 40 dB SPL and 2.5
dB for L1 = 45–75 dB SPL. Using the 2.5 dB
standard error value as representative of the
478
Sequential and Simultaneous protocols across
frequency and for L1 = 45–75 dB SPL, the
test-retest data suggest that there is a 95
percent probability that an individual’s true
DPOAE will fall within 5 dB (two standard
errors) of the obtained DPOAE.
The SEM ∆ was employed to assess
whether two DPOAE measurements are
statistically significant. Using the combined
standard error of measurement for the L1 =
45–75 dB SPL (2.5 dB), the SEM∆ is 3.5 dB
(2.5 dB X 1.414 = 3.5 dB). The SEM∆ can be
multiplied by 1.64 (90 percent confidence
interval), 1.96 (95 percent confidence
interval), or 2.58 (99 percent confidence
interval) to determine if two DPOAEs are
statistically significant at the selected
confidence level. Using the 95 percent
confidence interval (.05 level), the difference
between two DPOAEs is statistically
significant if it exceeds 6.9 dB (3.5 dB X 1.96
= 6.9 dB).
DISCUSSION
Administration Time
The Simultaneous protocol was
completed in less than half the time (two
Sequential versus Simultaneous/Beattie
minutes, 21 seconds) required for the
Sequential protocol (five minutes, 13 seconds).
These data are consistent with those reported
by Kim et al (1997), who stated that the
Simultaneous protocol was completed in onethird to one-half the time as compared to the
Sequential protocol. The times presented in
the current report represent the durations
required to test three frequencies at eight
intensities each (one trial only). Obtaining two
sets of data to assess reliability, which some
investigators advocate (Hall, 2000; Beattie et
al, 2003), would approximately double the test
times. The Simultaneous protocol may be
particularly useful when the examiner
requires data at numerous frequencies and/or
intensities. In addition to research
applications, this testing may include noise
or drug monitoring in which input-output
functions are generated. The Simultaneous
protocol also may be preferred when
evaluating difficult-to-test patients who are
unable or unwilling to remain still for the
required time period using the Sequential
protocol.
DPOAE and NF Amplitudes
The results showed that when stimulus
intensity (L1) was <60 dB SPL, the
differences between the DPOAEs and NFs
derived from the Sequential and
Simultaneous protocols were similar and
statistically nonsignificant at f2 = 1000, 2000,
and 4000 Hz. However, at the higher L1
intensities, the NFs for the Simultaneous
protocol were larger than those for the
Sequential protocol, and this difference
systematically increased as L1 increased
from 60 to 75 dB SPL. The largest differences
were observed for NFs associated with 4000
Hz, where the Simultaneous NFs were
approximately 5, 13, and 20 dB larger than
the Sequential NFs at the respective L1
intensities of 65, 70, and 75 dB SPL. At the
highest intensity (L1 = 75 dB SPL), the
Simultaneous DPOAEs also were slightly
greater (3–5 dB) than the Sequential
DPOAEs at 1000 and 2000 Hz.
For the Simultaneous NFs, signal
averaging continued at all frequencies (f2 =
1000, 2000, and 4000 Hz) until the preset
signal-to-noise ratio (+10 dB) of the first
frequency tested (i.e., f2 = 1000 Hz) was
achieved or until the 32-second time limit at
a particular intensity was reached. That is,
the GSI-60 Simultaneous protocol monitors
the noise floor status for the first set of
primary-tone pairs per group, which was f1
= 843 Hz and f2 = 1000 Hz. Because more
noise typically is measured around the
DPOAE associated with1000 Hz than for
2000 or 4000 Hz (see Figure 1), signal
averaging at 2000 and 4000 Hz for the
Simultaneous protocol may continue
somewhat longer and result in lower NFs
than for the Sequential protocol. To the
contrary, however, the Simultaneous protocol
yielded NFs that were higher than the NFs
for the Sequential protocol at L1 = 65–75 dB
SPL. A likely explanation is that the
Simultaneous protocol produced higher
system distortion and/or system noise because
of the interaction of the six primary tones
(three sets of two tones). That is, the
interaction of the six primary tones generated
system harmonic and/or intermodulation
distortion products that fell within the NF
bins.
Recall that each bin is 31.25 Hz wide
and that the NF values represent the RMS
average of two bins (62.5 Hz wide) on either
side of the DPOAE frequency bin. If system
distortion products or system noise are
present in any of the four bins around the
DPOAE bin, the overall (RMS) NF value will
be elevated. To estimate system noise, the
probe tip was sealed in a 2cc coupler, and NFs
were measured for the Simultaneous and
Sequential protocols. The NF values in dB
SPL are presented in Table 3 for each test
frequency and intensity. The Sequential NF
values are similar to those reported by Dorn
et al (2001). For the L1 intensities below 60
dB SPL, the NFs for the two protocols were
similar and varied <4 dB. The table also
shows that the Simultaneous protocol yielded
higher NFs than the Sequential protocol at
the highest L1 intensities. At L1 = 75 dB
SPL, these NF differences were 27 dB at
1000 Hz, 23 dB at 2000 Hz, and 35 dB at 4000
Hz. Moreover, at L1 = 65–75 dB SPL for the
Simultaneous protocol, the system
distortion/noise measurements are similar
to the NF values obtained under test
conditions. For 2000 Hz at L1 = 75 dB SPL,
for example, note that the system noise was
-6 dB SPL (Table 3) and the mean NF was
-4.5 dB (Table 1). Because the Simultaneous
NFs were greater than the Sequential NFs
only at the higher intensities (L1 > 65 dB
SPL) where greater system distortion was
479
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
Table 3
f2
Frequency
System Noise/Distortion (2cc Coupler Measurements) for the Sequential and
Simultaneous Protocols at Three Frequencies and Eight L1 Intensities
Protocol
L1 in dB SPL
55
60
40
45
50
65
70
75
Sequential
-28
-24
-31
-22
-26
-22
-28
-22
Simultaneous
-26
-25
-27
-24
-13
-7
-2
5
Sequential
-29
-34
-28
-29
-31
-28
-25
-29
Simultaneous
-28
-30
-29
-30
-29
-24
-16
-6
Sequential
-28
-32
-33
-31
-34
-33
-35
-32
Simultaneous
-29
-30
-30
-29
-27
-16
-7
3
1000 Hz
2000 Hz
4000 Hz
Note: System noise floor values are in dB SPL and represent RMS values for two bins of data on either side of the cubic
difference tone.
measured, we conclude that the higher
Simultaneous NFs reflect the greater system
distortion/noise generated by the interaction
of the six primary tones.
Larger DPOAEs were observed at the
higher intensities for the Simultaneous
protocol than for the Sequential protocol.
These larger DPOAEs also may reflect the
system distortions discussed above because
the DPOAE values represent both the
DPOAE and noise (biologic, environmental,
instrument) within the 31.25 Hz wide bin of
interest (Beattie and Ireland, 2000). Thus,
instrument noise in the form of distortion
products within the DPOAE bin may elevate
the DPOAE. Another explanation for the
higher Simultaneous DPOAEs was suggested
by Kim et al (1997), who state that presenting
several pairs of tones simultaneously is likely
to result in complicated interactions among
the distortion products that may be enhanced
or suppressed.
Although many investigators suggest L1
screening levels of ~60 dB SPL, high-level
testing may be used when testing hearingimpaired subjects or when generating inputoutput functions (Gorga et al, 1996; Hall,
2000; Robinette and Glattke, 2002). The
present study suggests that caution should
be used when moderate to high level (L1 > 65
dB SPL) primary tones are presented using
the Simultaneous protocol. That is, using
the GSI-60 instrumentation, the NFs for the
Sequential and Simultaneous protocols are
not equivalent when data are collected at
moderate to high levels. These findings should
be replicated with other instrumentation
480
(speakers) that may exhibit different degrees
of system noise/distortion. Moreover, different
findings would be expected if the bin width
is varied or if different bins are used to
calculate the NF (e.g., those without high
levels of distortion).
Reliability
Test-retest reliability was assessed in
which the retest followed a 10–20 minute
break that included probe tip removal and
replacement. The short time interval
minimizes the likelihood of changes in
hearing, environmental or subject noise,
procedures, or equipment (Franklin et al,
1992; Prieve et al, 1993; Roede et al, 1993;
Marshall and Heller, 1996; Zhao and
Stephens, 1999). DPOAE amplitude and noise
level differences on the test and retest may
be due to changes in probe placement,
changes in middle-ear status (e.g., swallowing
or coughing may alter middle-ear pressure),
and/or variable subject noise. Zhao and
Stephens (1999) state that changing the
position of the probe tip may affect (1) the
level of background noise in the ear canal,
particularly at the low frequencies, (2)
acoustic leakage, and (3) the interaction of the
ear canal resonances and the acoustic stimuli.
Beattie et al (2003) reported no test-retest
reliability differences between the very shortterm measurements (retest following probe
removal and a 10–20 minute break) and
short-term measurements (5–10 days
between test and retest). These authors
concluded that probe removal and
Sequential versus Simultaneous/Beattie
replacement was the major factor that
contributed to the increased variability (0.5
dB to 1.0 dB) between the immediate testretest measurements and the very shortterm and short-term measurements. Roede
et al (1993) state that middle-ear changes in
fluid or air pressure will have more effect on
the low-frequency DPOAEs than on the
higher frequencies.
The standard errors were similar for the
Sequential and Simultaneous protocols. When
combined across all frequencies and
intensities, the SEM values were 2.6 dB for
the Sequential protocol and 2.8 dB for the
Simultaneous protocol. When the data were
combined across all conditions, the combined
SEM was 2.5 dB. These protocol findings are
consistent with those of Kim et al (1997), who
reported no differences in reliability between
the Sequential and Simultaneous procedures.
We also found that the standard errors tended
to decrease slightly as frequency increased
from 1000 Hz (2.9 dB) to 4000 Hz (2.1 dB).
Larger standard errors may be observed at the
lower frequencies as compared to the higher
frequencies because the former is associated
with more variable immittance and more
variable noise (Roede et al, 1993; Zhao and
Stephens, 1999). However, Beattie et al (2003)
found similar standard errors of about 2.5
dB for 1000–4000 Hz.
When combined across protocols and
frequencies, somewhat larger standard errors
were observed for L1 = 40 dB SPL (SEM = 4.3
dB) than for L1 = 45–75 dB SPL (SEM = 2.5
dB). The improved reliability at the higher
intensities may be the result of reduced
variability in the background noise because
of a greater signal-to-noise ratio. That is, at
the higher intensities, DPOAEs may have
been sufficiently high in comparison to the
background noise so that the noise had little
effect on the signal in the DPOAE bin. The
DPOAE bin contains both the relatively stable
DPOAE and relatively variable noise.
Kimberly et al (1997) suggest that if the noise
is approximately 12 dB below the DPOAE, the
noise will have little effect on the DPOAE. In
contrast to the foregoing explanation,
however, Table 2 shows that the
Simultaneous protocol at L1 = 75 dB SPL
(1000 and 4000 Hz) yielded SEMs of ~2.3 dB
even though the associated signal-to-noise
ratios were similar to the L1 = 40 dB SPL
conditions.
Although comparisons with previous
studies must be made with reservations
because of differences in experimental design,
the present SEM values (~2.5 dB) are
identical to those reported by Beattie et al
(2003) using similar procedures. Slightly
smaller standard errors were reported by
Beattie and Bleech (2000) and Franklin et al
(1992). The immediate test-retest standard
errors reported by Beattie and Bleech (2000)
under similar conditions (L1 = L2 = 55 dB
SPL) were ~1.7 dB for 1000, 2000, and 4000
Hz. Franklin et al (1992) reported similar
reliabilities for measurements taken over
four consecutive days and on four successive
weeks. For the L1 = L2 = 65 dB SPL condition,
they observed standard errors of about 3.1 dB
at 1000 Hz, 1.8 dB at 2000 Hz, and 1.3 dB at
4000 Hz (Franklin et al, 1992, table 2, 423).
To assess the consistency of DPOAEs,
clinicians may obtain two or three sets of
measurements. Hall (2000) suggests that, to
assess reliability, two or three measurements
should be obtained in which the probe tip is
removed between measurements. He states:
“False-negative screening outcomes can be
minimized by insisting on two replicable
DPgrams for each ear (DP amplitudes within
+1 or 2 dB for each frequency)” (Hall, 2000,
429). Our SEM∆ data suggest, however, that
the differences between two DPOAEs must
exceed approximately 7 dB at 1000–4000 Hz
to be statistically significant at the 0.05 level
of confidence. This value agrees with the 6–7
dB values suggested by Beattie and Bleech
(2000) and Beattie et al (2003). Test-retest
differences that exceed these values, in the
absence of any pathologic condition (e.g.,
ototoxic drugs, noise trauma, or middle-ear
infections), are unusual and warrant repeat
testing until stable findings are obtained. If
repeat measurements verify differences that
exceed these values, however, it is likely that
the differences represent an actual change in
the auditory system and are not the result of
measurement error.
SUMMARY AND CONCLUSIONS
T
he Simultaneous protocol was completed
in less than half the time required for the
Sequential protocol. The Simultaneous
protocol may be preferred when evaluating
individuals who have difficulty remaining
still for any period of time, and may be
particularly useful when data are required at
numerous frequencies and/or intensities.
Using the GSI-60 instrumentation, the
NFs for the Sequential and Simultaneous
481
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
protocols are not equivalent when data are
collected at moderate to high levels (L1 ≥ 65
dB SPL). The Simultaneous protocol yielded
higher NFs than the Sequential protocol,
which is believed to be due to higher system
distortion that results from the interaction of
the six primary tones. When stimulus
intensity (L1) was ≤60 dB SPL, the Sequential
and Simultaneous protocols yielded similar
DPOAEs and NFs at all tested frequencies.
At the highest intensity (L1 = 75 dB SPL), the
Simultaneous DPOAEs also were slightly
greater (3–5 dB) than Sequential DPOAEs at
1000 Hz and 2000 Hz. Depending on
procedural variables such as the minimum
number of frames averaged and the specified
signal-to-noise ratio, elevated NFs may result
in increased DPOAE amplitudes (Beattie
and Ireland, 2000).
No differences in reliability were
observed between the Sequential and
Simultaneous protocols when the data were
combined across all frequencies and
intensities. The SEM values tended to
decrease as f2 frequency increased. Using
the 2.5 dB standard error value as
representative of the Sequential and
Simultaneous protocols across frequency and
for L1 = 45–75 dB SPL, the test-retest data
suggest that there is a 95 percent probability
that an individual’s true DPOAE will fall
within 5 dB (two standard errors) of the
obtained DPOAE. Reliability also was
assessed using the SEM∆ (3.5 dB). Using the
95 percent confidence interval, the difference
between two DPOAEs is statistically
significant if it exceeds ~7 dB. Test-retest
differences that exceed these values are
unusual and warrant repeat testing until
stable findings are obtained.
REFERENCES
American National Standards Institute. (1996).
American National Standard Specifications for
Audiometers. ANSI S3.6-1996. New York: American
National Standards Institute.
American Psychological Association. (1985). Standards
for Educational and Psychological Testing.
Washington, DC: American Psychological Association.
Anastasi A. (1976). Psychological Testing. 4th ed. New
York: MacMillan.
Beattie RC, Bleech J. (2000). Effects of sample size
on the reliability of noise floor and DPOAE. Brit J
Audiol 34:305–309.
Beattie RC, Ireland A. (2000). Effects of sample size
on the noise floor and distortion product otoacoustic
emissions. Scand Audiol 29:93–102.
Beattie RC, Jones RL. (1998). Effects of relative levels
of the primary tones on distortion product otoacoustic
emissions in normal-hearing subjects. Audiol
37:187–197.
Beattie RC, Kenworthy OT, Luna CA. (2003).
Immediate and short-term reliability of distortionproduct otoacoustic emissions. Int J Audiol
42:348–354.
Brown AM. (1987). Acoustic distortion from rodent
ears: a comparison of responses from rats, guinea
pigs, and gerbils. Hear Res 31:25–39.
Brown AM, Kemp DT. (1984). Suppressibility of 2F1F2 stimulated acoustic emissions in gerbil and man.
Hear Res 13:29–37.
Brown AM, Sheppard SL, Russell PT. (1994). Acoustic
distortion products (ADP) from the ears of term infants
and young adults using low stimulus levels. Brit J
Audiol 28:273–280.
Brown JR. (1989). The truth about scores children
achieve on tests. Lang Speech Hear Services Schools
20:366–371.
Bruning JL, Kintz BL. (1987). Computational
Handbook of Statistics. 3rd ed. Glenview, IL: Scott,
Foresman and Company.
Demorest ME. (1984). Psychometric issues in speech
recognition testing. ASHA Reports 14:19–22.
Demorest ME, Walden BE. (1984). Psychometric principles in the selection, interpretation, and evaluation
of communication self-assessment inventories. J
Speech Hear Disord 49:226–240.
Dorn PA, Konrad-Martin D, Neely ST, Keefe DH, Cyr
E, Gorga MP. (2001). Distortion product otoacoustic
emission input/output functions in normal-hearing
and hearing-impaired human ears. J Acoust Soc Am
110:3119–3131.
Franklin DJ, McCoy MJ, Martin GK, LonsburyMartin BL. (1992). Test/retest reliability of
distortion-product and transiently evoked otoacoustic
emissions. Ear Hear 13:417–429.
482
Sequential versus Simultaneous/Beattie
Gaskill SA, Brown AM. (1990). The behavior of the
acoustic distortion product, 2f1-f2, from the human
ear and its relation to auditory sensitivity. J Acoust
Soc Am 88:821–839.
Ghiselli EE, Campbell JP, Zedeck S. (1981).
Measurement Theory for the Behavioral Sciences. San
Francisco: W. H. Freeman.
Gorga MP, Stover L, Neely ST. (1996). The use of
cumulative distributions to determine critical values
and levels of confidence for clinical distortion product otoacoustic emission measurements. J Acoust Soc
Am 100:968–977.
Grason-Stadler. (1996). Grason-Stadler GSI-60
DPOAE—Distortion Product Otoacoustic Emissions
System User Manual. Milford, NH: GSI GrasonStadler.
Guilford JP, Fruchter B. (1973). Fundamental
Statistics in Psychology and Education. 5th ed. New
York: McGraw-Hill.
Hall JW. (2000). Handbook of Otoacoustic Emissions.
San Diego, CA: Singular Publishing Group.
Harris FP, Lonsbury-Martin BL, Stagner BB, Coats
AC, Martin GK. (1989). Acoustic distortion products
in humans: systematic changes in amplitude as a
function of f2/f1 ratio. J Acoust Soc Am 85:220–229.
Hauser R, Probst R. (1991). The influence of systematic primary-tone level variation L2-L1 on the
acoustic distortion product emission 2F1-F2 in normal
human ears. J Acoust Soc Am 89:280–286.
Hunter MF, Kimm L, Cafarlli DD, Kennedy CR,
Thornton AR. (1994). Feasibility of otoacoustic emission detection followed by ABR as universal neonatal
screening test for hearing impairment. Brit J Audiol
28:47–51.
Kemp DT. (1979). Evidence of mechanical nonlinearity and frequency selective wave amplification in
the cochlea. Arch Otolaryngol 224:37–45.
Kemp DT. (1986). Otoacoustic emissions, traveling
waves and cochlear mechanics. Hear Res 22:95–104.
Kemp DT, Bray P, Alexander L, Brown AM. (1986).
Acoustic emission cochleography—practical aspects.
Scand Audiol Suppl 15:71–96.
Kim DO, Sun SM, Jung MD, Leonard G. (1997). A
new method of measuring distortion product otoacoustic emissions using multiple tone pairs: study of
human adults. Ear Hear 18:277–285.
Lemke E, Wiersma W. (1976). Principles of
Psychological Measurement. Chicago: Rand McNally.
Linton M, Gallo PS. (1975). The Practical Statistician:
Simplified Handbook of Statistics. Belmont, CA:
Wadsworth Publishing Company.
Lonsbury-Martin BL, Martin GK. (1990). The clinical utility of distortion-product otoacoustic emissions.
Ear Hear 11:144–154.
Lonsbury-Martin B, McCoy M, Whitehead M, Martin
G. (1993). Clinical testing of distortion-product otoacoustic emissions. Ear Hear 14:11–22.
Marshall L, Heller LM. (1996). Reliability of transient-evoked otoacoustic emissions. Ear Hear
17:237–254.
Mehrens WA, Lehmann IJ. (1984). Measurement and
Evaluation in Education and Psychology. New York:
Holt, Rinehart and Winston.
Mulheran M, Degg C. (1997). Comparison of distortion product OAE generation between a patient group
requiring frequent gentamicin therapy and control
subjects. Brit J Audiol 31:5–9.
Nelson DA, Kimberly BP. (1992). Distortion-product
emissions and auditory sensitivity in human ears
with normal hearing and cochlear hearing loss. J
Speech Hear Res 35:1142–1159.
Nelson DA, Zhou JZ. (1996). Slopes of distortion-product otoacoustic emission growth curves corrected for
noise floor levels. J Acoust Soc Am 99:468–474.
Nielsen LH, Popelka GR, Rasmussen AN,
Osterhammel PA. (1993). Clinical significance of
probe-tone frequency ratio on distortion product otoacoustic emissions. Scand Audiol 22:159–164.
Painter J. (1997). Basic instrumentation issues in
acquiring distortion product otoacoustic emissions.
In: Robinette MS, Glattke TJ, eds. Otoacoustic
Emissions. New York: Thieme, 333–346.
Painter J. (2000). Grason Stadler Incorporated (GSI).
In: Hall JW, ed. Handbook of Otoacoustic Emissions.
San Diego, CA: Singular Publishing Group, 291–303.
Prieve BA, Gorga MP, Schmidt A, Neely S, Peters J,
Schultes L, Jesteadt W. (1993). Analysis of transientevoked emissions in normal-hearing and
hearing-impaired ears. J Acoust Soc Am 93:3308–3319.
Probst R, Harris FP, Hauser R. (1993). Clinical monitoring using otoacoustic emissions. Brit J Audiol
27:85–90.
Kimberly BP, Brown DK, Allen JB. (1997). Distortion
product emissions and sensorineural hearing loss.
In: Robinette MS, Glattke TJ, eds. Otoacoustic
Emissions. New York: Thieme, 181–204.
Probst R, Lonsbury-Martin B, Martin G. (1991). A
review of otoacoustic emissions. J Acoust Soc Am
89:2053–2055.
Lafreniere D, Jung MD, Smurzynski J, Leonard G,
Kim DO, Sasek J. (1991). Distortion-product and clickevoked otoacoustic emissions in healthy newborns.
Arch Otolaryngol 117:1382–1389.
Rasmussen AN, Popelka GR, Osterhammel PA,
Nielsen LH. (1993). Clinical significance of relative
probe levels on distortion product otoacoustic emissions. Scand Audiol 22:223–229.
Lasky RE, Snodgrass E, Hecox K. (1994). Distortionproduct otoacoustic emission input/output functions
as a function of frequency in human adults. J Am
Acad Audiol 5:183–194.
Robinette MS, Glattke TJ. (2002). Otoacoustic
Emissions: Clinical Applications. 2nd ed. New York:
Thieme.
483
Journal of the American Academy of Audiology/Volume 14, Number 9, 2003
Roede J, Harris FP, Probst R, Xu L. (1993).
Repeatability of distortion product otoacoustic emissions in normally hearing humans. Audiol
32:273–281.
Stover L, Gorga MP, Neely ST, Montoya D. (1996).
Toward optimizing the clinical utility of distortion
product otoacoustic emission measurements. J Acoust
Soc Am 100:956–967.
Stover L, Norton SJ. (1993). The effects of aging on
otoacoustic emissions. J Acoust Soc Am 94:2670–2681.
Sun X , Kim DO , Jung MD, Randolph KJ. (1996).
Distortion product otoacoustic emission test of sensorineural hearing loss in humans: comparison of
unequal- and equal-level stimuli. Ann Otol Rhinol
Laryngol 105:982–990.
Whitehead ML, McCoy MJ, Lonsbury-Martin BL,
Martin GK. (1995). Dependence of distortionproduct otoacoustic emissions on primary levels in
normal and impaired ears. I. Effects of decreasing
L2 below L1. J Acoust Soc Am 97:2346–2358.
Yardley MP, Davies CM, Stevens JC. (1998). Use of
transient evoked otoacoustic emissions to detect and
monitor cochlear damage caused by platinumcontaining drugs. Brit J Audiol 32:306–316.
Zhao F, Stephens D. (1999). Test-retest variability of
distortion-product otoacoustic emissions in human
ears with normal hearing. Scand Audiol 28:171–178.
484