A comparison of manufacturer-specific
prescriptive procedures for infants
By Richard Seewald, Jillian Mills, Marlene Bagatto, Susan Scollie, and Sheila Moodie
Early hearing detection and communication development
a function of frequency. Prescriptive algorithms provide
(EHDCD) programs enable us to identify hearing loss in
the foundation on which the hearing instrument perforearly infancy, making intervention by 6 months of age a
mance characteristics are selected for the infant or child to
reasonable goal. For infants and children fitted with hearbe fitted. Recent survey data indicate that most audioloing instruments, it is critical that the devices be adjusted
gists fitting hearing aids to infants and young children use
appropriately so that the pre-lingual listener has full and
a published hearing aid prescriptive procedure.6,9 At preconsistent access to sound, without discomfort for loud
sent, prescriptive procedures can be divided into two classounds.
sifications: (1) evidence-based generic prescriptive algorithms
Several published pediatric amplification guidelines
and (2) manufacturer-specific proprietary algorithms.
are available to assist clinicians with the hearing aid fitEvidence-based generic prescriptive algorithms for chilting process for infants and children.1-5 While the guidedren include the DSL m[i/o]10 and the NAL-NL1,11 both
lines were developed in different jurisdictions, similar
of which are intended for use with any hearing aid.
recommendations are made for all stages of the hearing
Proprietary prescriptive methods incorporate prescriptive
instrument fitting process. For example, the use of a sysalgorithms developed by manufacturers for use with their
tematic, evidence-based approach to hearing aid fitting
specific hearing instruments. The lack of published inforthat accounts for an infant’s external ear acoustics at the
mation regarding the development of pediatric manufacassessment, selection, and
turer-specific proprietary
verification stages and that
algorithms makes it diffi“…an infant can end up with
includes the measurement
cult for the audiologist to
of hearing aid output limmake an informed choice
substantially different hearing
iting is a requirement of
about which procedure
every current pediatric
will be best for their young
aid performance depending
amplification protocol.
patient.12 Some manufacWhile it is encouraging
turers have decided to
on which prescriptive
to see consensus in pediforego the development of
atric hearing aid fitting
a proprietary pediatric
algorithm is chosen..."
guidelines, there continue
algorithm and use either
to be substantial differthe NAL or DSL prescripences among clinical practice behaviors. Surveys have inditive algorithms for fitting children and have implemented
cated that the majority of pediatric audiologists use sound
these formulas in their fitting software. These generic prefield-aided threshold measures to verify the electroacoustic
scriptive algorithms may have been adapted by the manperformance of hearing instruments for young infants.6
ufacturer to account for the unique characteristics (e.g.,
Probe-microphone measures were used by roughly 20%
compression kneepoint) of their hearing aids.
of the audiologists fitting hearing aids to infants.6 It has
Several investigators have compared proprietary prebeen well-documented that behavioral measures (i.e., aided
scriptive algorithms for use with adults. Keidser and colthreshold measures) do not provide accurate estimates of
leagues found a 10-dB variation in the amount of gain
the aided audibility of soft, average, and loud speech or
prescribed by NAL-NL1, DSL[i/o], and four proprietary
the real-ear saturation response of the hearing aid and that
algorithms for the same audiogram.12 A study by Hawkins
infants 6 months of age and younger are not developmenand Cook compared simulated 2-cc versus actual 2-cc gain
tally capable of performing this task.7,8 Failure to approas well as simulated insertion versus actual insertion gain
priately verify the electroacoustic performance of the hearing
measures from the fitting software for 28 hearing aids of
aid in terms of predicted speech audibility and maximum
various styles from four major manufacturers.13 Results
hearing instrument output can result in obstructing the
indicated a clear trend for the simulated values to overeslanguage benefits an infant would have otherwise received
timate what was actually provided by the hearing aids for
from being identified at an early age and optimally fitted.
both the 2-cc coupler and insertion gain comparisons.13
It is also important to consider the prescriptive targets
Differences of as much as 10 to 20 dB were noted for the
used to determine the recommended levels for amplified
various hearing aids and prescriptive algorithms in the
speech and target limits for hearing instrument output as
study.
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A more recent study examined behindthe-ear (BTE) hearing aids from six major
manufacturers programmed using their
“default” fitting procedure.14 For some
this was a generic fitting formula and others applied their own proprietary prescriptive method. The OSPL90 was measured
at the default program settings with all
special features deactivated. Results indicated maximum output values ranging
from 90 to 109 dB SPL (re: 2-cc coupler)
at 2000 Hz across the six hearing aids for
a flat 50-dB-HL hearing loss.14
RATIONALE
Despite the unexpected degree of variation
among prescriptive methods, for everyday listening the average adult listener
may be able to adjust the volume control
of the hearing instruments to compensate
for the 10- or 20-dB variation provided
from one algorithm to the next. However,
in the case of a 6-month-old infant who
can neither manipulate the gain of the
hearing aid nor verbalize concerns, these
recent findings would seem to be clinically important. Infants and young children must live and learn with the hearing
instrument characteristics provided (or
not provided) to them. They cannot
accommodate a 10-dB variation in prescribed overall gain by adjusting a volume
control wheel as an adult would. They
also cannot “fill in the blanks” when words
or speech sounds are inaudible the way
adults do because they have not yet developed sufficient knowledge of spoken language.
The results reported above make it
apparent that for the same audiometric
hearing loss there may be differences in
the prescribed gain from various prescriptive procedures. In addition, it may be
that there is some variation in the gain
prescribed from manufacturer-specific
implementations of the same generic prescriptive procedure. As a result, an infant
can end up with substantially different
electroacoustic characteristics in hearing
aid performance depending on which prescriptive algorithm is chosen by the audiologist within the hearing aid fitting
software. It is important to systematically
quantify the differences that may exist
among manufacturer-specific implementations of prescriptive algorithms that
potentially affect audibility and comfort
of amplified speech.
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hearing aids’ responses in the ear canal.15
Nine audiograms, ranging from mild to
profound, were used to program each
hearing aid (see Table 1).
Simulated real-ear measurements
(S-REM) were performed for soft (55 dB
SPL), average (65 dB SPL), and loud
(75 dB SPL) speech and maximum power
output (MPO), using the Audioscan®
Verifit VF-1 real-ear measurement system
(Software Version 2.0.18). Each BTE
instrument was coupled to the standard
HA-2 coupler available on the VF-1 and
placed inside the test chamber. The test
chamber was closed prior to making all
electroacoustic measurements. The equipment (i.e., cables, programming boots)
supplied by the manufacturers specific to
each BTE instrument was connected to
a HI-PRO box for programming each
hearing instrument.
For each BTE instrument programming, the 1/3-octave band measurements
obtained from the VF-1 were collapsed
into a single Speech Intelligibility Index
(SII) value, based on the 1/3-octave band
SII calculation procedure.16 The SII value
ranges from 0 to 1, and was converted to
a value between 0% and 100% for the
purposes of this study. A Microsoft Excel
spreadsheet was used for all SII calculations.17 Separate SII values were calculated for each BTE instrument across the
nine audiograms and for each of the three
input levels. The nine audiograms were
converted to equivalent adult hearing
levels18 before SII calculations were
performed. It has been documented that
young children are not able to use
RESEARCH QUESTIONS
This study was designed to address three
questions:
(1) Will the prescribed output-limiting
levels differ for the same 6-month-old
infant depending on the manufacturerspecific prescriptive algorithm chosen?
(2) Will the prescribed gain by frequency
characteristics differ for the same 6-monthold infant, as a function of input level,
depending on the manufacturer-specific
algorithm chosen?
(3) Will predicted speech audibility differ across manufacturer-specific prescriptive algorithms evaluated?
METHOD
BTE instruments recommended for use
with the pediatric population were chosen from five manufacturers. The hearing
aids were programmed using the most
current version of each of the manufacturer’s proprietary fitting software, installed
as modules in the NOAH® client module software system. Where proprietary
prescriptive algorithms for pediatric fittings were not available, three of the BTE
instruments were programmed using the
manufacturer’s default prescriptive method.
In two cases this was the manufacturer’s
implementation of the NAL-NL1 procedure and in another case this was the manufacturer’s implementation of the DSL
method. Advanced features (i.e., noise
reduction, directional microphone) were
deactivated in all devices. Average realear-to-coupler difference (RECD) values
for a 6-month-old infant were applied to
2-cc coupler measures to predict the
Table 1.
Nine audiograms (A-I) in dB HL, used in this study.
Frequency (Hz)
Audiogram
250
500
750
1000
1500
2000
3000
4000
6000
A
30
30
35
35
40
40
45
50
50
B
30
30
35
40
45
50
60
70
70
C
30
30
35
45
50
60
80
90
90
D
50
50
50
50
50
50
50
50
50
E
50
50
50
55
55
60
65
70
70
F
50
50
55
55
65
70
80
90
90
G
70
70
70
70
70
70
70
70
70
H
70
70
70
75
75
80
85
90
90
I
90
90
90
90
90
90
90
90
90
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NOVEMBER 2008 • VOL. 61 • NO. 11
contextual information in the same way
as adults.19 Thus, an equal band importance function was used in calculating the
SII values.
150
140
Simulated real-ear saturation response
(S-RESR) measurements differed significantly across the five manufacturerspecific prescriptive algorithms (F[4,32] =
47.66, p<.001). Post hoc analyses indicated that this effect was present per
frequency (Tukey’s HSD, p<0.05). The
results of the S-RESR measurements for
the five manufacturer-specific algorithms
at 3000 Hz for the nine audiograms are
presented in Figure 1.
Substantial variation is evident in the
simulated RESR measurements across
manufacturers for the nine audiograms
used in this study. A review of the data
presented by individual audiograms further revealed the significant variation in
the output being produced by some of
the manufacturer-specific prescriptive algorithms. For example, for audiogram A,
values range from 100 dB SPL (Manufacturer 4) to 130 dB SPL (Manufacturer
1) in the higher frequencies. In the lower
frequencies, values range from 90 dB SPL
(Manufacturer 3) to 105 dB SPL (Manufacturer 4) (see Figure 2). Moreover, the
shapes of the output limiting responses
are different for each manufacturer. Some
responses are relatively flat across frequencies (Manufacturer 4), while others are
not (Manufacturer 1) (see Figure 2).
Determining the basis for manufacturer-specific prescriptive algorithm output limiting targets is difficult, as the
derivation of these algorithms is often neither clearly described nor accessible. However, from the present study, it can be seen
that there is a large range of variation
among the S-RESR for the manufacturerspecific prescriptive algorithms evaluated
in this study. Clearly the output limits of
some manufacturer-specific prescriptive
algorithms may be inappropriate for pediatric hearing aid fittings.
Prescribed gain by frequency for
different input levels
Simulated real-ear aided response (SREAR) measurements for soft, average,
and loud speech inputs differed
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120
Manufacturer 1
110
Manufacturer 2
100
Manufacturer 3
Manufacturer 4
90
Manufacturer 5
80
70
60
Audiogram
An example of S-RESR measurements for five manufacturer-specific algorithms
at 3000 Hz for the nine audiograms used in this study.
Figure 1.
150
140
130
S-RESR (dB SPL)
Output limiting
S-RESR (dB SPL)
130
RESULTS AND DISCUSSION
Manufacturer 1
120
Manufacturer 2
110
Manufacturer 3
100
Manufacturer 4
Manufacturer 5
90
80
70
60
100
1000
10000
Frequency (Hz)
Figure 2. S-RESR measurements obtained for the five manufacturer-specific algorithms
using Audiogram A (gently sloping, mild to moderate hearing loss).
significantly across manufacturers: soft
inputs (F[4,32]=18.11, p<0.001); average inputs (F[4,32]=24.67, p<0.001);
loud inputs (F[4,32]=31.89, p<0.001).
Post hoc analyses indicated that these
differences were significant at most test
frequencies (Tukey’s HSD, p<0.05). An
example is shown for Audiogram E
(Figure 3, A-C). It can be seen that, in
this example, S-REAR values differ by
as much as 20 dB at some frequencies
for soft, average, and loud inputs.
Predicted speech audibility
Substantial differences among SII values
were noted from the various manufacturer-specific prescriptive algorithms’
responses for soft speech input levels. Relatively less difference in these values was
Pe d i a t r i c A m p l i f i c a t i o n : A s p e c i a l i s s u e
noted for average level inputs and even
smaller differences for loud input levels
(Figure 4 A-C).
It is evident that the observed differences in frequency response characteristics of the manufacturer-specific
algorithms resulted in differences in SII
values. Currently, there is no way to
predict real-life outcome differences from
SII value differences for a 6-month-old
infant. However, from a habilitative
audibility perspective, increased audibility should result in more languagelearning opportunities. Research has
shown that children require and prefer
audibility of the entire speech range and
at higher sensation levels than adults in
order to understand speech.20,21 Therefore it is prudent to consider that more
NOVEMBER 2008 • VOL. 61 • NO. 11
100
1000
Frequency (Hz)
10000
Manufacturer 1
104
103
102
101
100
90
80
70
60
50
40
100
b)
S-REAR (dB SPL)
a)
S-REAR (dB SPL)
S-REAR (dB SPL)
140
130
120
110
100
90
80
70
60
50
40
1000
Frequency (Hz)
10000
Manufacturer 3
Manufacturer 2
140
130
120
110
100
90
80
70
60
50
40
c)
100
Manufacturer 4
1000
Frequency (Hz)
10000
Manufacturer 5
S-REAR measurements obtained for (a) soft (55 dB SPL), (b) average (65 dB SPL), and (c) loud (75 dB SPL) input levels
across the five manufacturer-specific algorithms using Audiogram E (gently sloping moderate to moderately severe).
Figure 3.
100
100
100
b)
c)
80
80
60
60
60
40
SII (%)
80
SII (%)
SII (%)
a)
40
20
20
20
0
40
0
A
B
C
D
E
F
Audiogram
G
H
A
I
Manufacturer 1
B
Manufacturer 2
C
D
E
F
Audiogram
Manufacturer 3
G
H
I
Manufacturer 4
0
A
B
C
D
E
F
Audiogram
G
H
I
Manufacturer 5
Predicted SII values for (a) soft (55 dB SPL), (b) average (65 dB SPL), and (c) loud (75 dB SPL) input levels for the five
manufacturer-specific algorithms for the nine audiograms used in this study.
Figure 4.
audibility is desirable for the pediatric
population.
Clinical implications
Despite the published recommendations
for pediatric amplification, real-ear or
simulated real-ear verification of hearing
instrument performance is not routine
clinical practice for many pediatric audiologists.6,9 With the recent proliferation
in EHDCD programs, more infants are
being fitted with hearing aids by 6 months
of age.
The current study has demonstrated
the substantial variation that is generated
among manufacturer-specific prescriptive
algorithms for the same 6-month-old
infant. Some manufacturer-specific algorithms resulted in unexpectedly high output limiting values, generating concern
about the potential for discomfort in a
6-month-old fitted with these algorithms.
Many of the manufacturer-specific
prescriptive algorithm output-limiting
values were substantially higher than
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output-limiting values recommended by
published evidence-based generic procedures for output limiting in pediatric hearing aid fittings.10,11 Additionally, the
variation in manufacturer-specific prescriptive algorithm hearing aid response
characteristics to speech inputs resulted
in substantial differences in predicted audibility as measured by the SII.
It is important in any pediatric hearing aid fitting to ensure that speech inputs
are audible in order to facilitate speech and
language learning. The results of this study
suggest that the manufacturer-specific prescriptive algorithms examined provide very
different simulated real-ear responses even
for the same infant. This research highlights the importance of verifying and comparing the output limiting and responses
to speech inputs provided by various manufacturer-specific implementations of
prescriptive algorithms.
Regardless of the approach taken for
electroacoustic prescription, the responsible clinician should want to know the
Pe d i a t r i c A m p l i f i c a t i o n : A s p e c i a l i s s u e
levels of amplified sound that hearing aids
deliver to the child’s ear. Therefore, pediatric audiologists must apply comprehensive and evidence-based electroacoustic
verification strategies that are compatible
with the population they are working with.
Acknowledgment
This work was supported by the Masonic Help-2Hear Foundation of Ontario.
Richard C. Seewald, PhD, is Distinguished University Professor at the Child Amplification Laboratory, National
Centre for Audiology, University of Western Ontario. Jillian
Mills, MSc, is an Audiologist at ListenUP! Canada.
Marlene Bagatto, AuD, is a Research Audiologist;
Susan Scollie, PhD, is Assistant Professor; and Sheila
Moodie, MClSc, is a Research Audiologist, all at the Child
Amplification Laboratory, National Centre for Audiology. Readers may contact Dr. Seewald at seewald@nca.uwo.ca or at
National Center of Audiology, Elborn College Building, Western Road, University of Western Ontario, London, Ontario,
Canada N6G 1H1.
REFERENCES
1. American Academy of Audiology: Pediatric Amplification Protocol. Reston, VA: American Academy of
Audiology, 2003. Retrieved July 25, 2008 from
NOVEMBER 2008 • VOL. 61 • NO. 11
www.audiology.org/NR/rdonlyres/53D26792-E32141AF-850FCC253310F9DB/0/pedamp.pdf.
2. College of Audiologists and Speech-Language Pathologists of Ontario. Preferred practice guideline for the
prescription of hearing aids to children. 2002. Retrieved
July 25, 2008, from www.caslpo.com/Portals/0/ppg/
preshearingaidschild.pdf.
3. Ministry of Children and Youth Services, Ontario,
Canada: Ontario Infant Hearing Program provision
of amplification protocol and guidelines. 2007.
Retrieved July 25, 2008, from www.mountsinai.on.ca/
care/infant-hearing program/documents/amplification
revision2007_006.pdf
4. The Pediatric Working Group of the Conference on
Amplification for Children with Auditory Deficits:
Amplification for infants and children with hearing
loss. AJA 1996;5(1):53-68.
5. Guidelines for the Fitting, Verification and Evaluation
of Digital Signal Processing Hearing Aids within a
Children’s Hearing Aid Service. Modernising Children’s
Hearing Aid Services, University of Manchester,
England; 2005; 1-3. Retrieved August 1, 2008, from
www.psych-sci.manchester.ac.uk/mchas/guidelines/
fittingguidelines.doc.
6. Bamford J, Beresford, D Mencher G, et al.: Provision
and fitting of new technology hearing aids: Implications from a survey of some “good practice services”
in UK and USA. In Seewald RC, Gravel JS, eds. A
Sound Foundation Through Early Amplification: Proceedings of an International Conference. Stäfa, Switzerland: Phonak AG: 2001:213-219.
7. Stelmachowicz P, Hoover B, Lewis D, Brennan M: Is
functional gain really functional? Hear J 2002;55(11):
38-42.
8. Hawkins D: Limitations and uses of the aided audiogram. Sem Hear 2004;25(1):51-62.
9. Tharpe AM: Service delivery for children with multiple
impairments: How are we doing? In Seewald RC, ed.,
A Sound Foundation Through Early Amplification: Proceedings of an International Conference, Stäfa, Switzerland: Phonak AG, 2000: 175-190.
10. Scollie S, Seewald R, Cornelisse L, et al.: The Desired
Sensation Level Multistage Input/Output Algorithm.
Trends Amplif 2005;9(4):159-197.
11. Dillon H, Byrne D, Brewer S: NAL Non-linear (NAL
NL-1) User Manual, version 1.01. National Acoustic
Laboratories: Chatswood NSW, Australia, 1998.
12. Keidser G, Brew C, Peck A: How proprietary fitting
algorithms compare to each other and to some generic
algorithms. Hear J 2003;56(3):28-38.
13. Hawkins D, Cook J: Hearing aid software predictive
gain values: How accurate are they? Hear J 2003;56(7):
26-34.
14. Mueller HG, Bentler R, Wu Y: Prescribing maximum
hearing aid output: Differences among manufacturers found. Hear J 2008;61(3):30-36.
15. Bagatto M, Scollie S, Seewald R, et al.: Real-ear-tocoupler difference predictions as a function of age for
two coupling procedures. JAAA 2002;13:407-415.
16. American National Standards Institute: American
National Standard Methods for Calculation of the Speech
Intelligibility Index (ANSI S3.5-1997). New York;
ANSI, 1997.
17. Scollie S: Children’s speech recognition scores: The
speech intelligibility index and proficiency factors for
age and hearing level. Ear Hear 2008;29(4):543-556.
18. Dillon H: Hearing Aids. New York: Thieme, 2001.
19. Nittrouer S, Boothroyd A: Context effects in phoneme
and word recognition by young children and older
adults. J Acoust Soc Amer 1990;87(6):2705-2715.
20. Nozza R, Rossman R, Bond L: Infant-adult differences
in unmasked thresholds for the discrimination of
consonant-vowel syllable pairs. Audiology 1991;30:
102-112.
21. Bagatto MP, Moodie ST, Scollie SD, et al.: Clinical protocols for hearing instrument fitting in the Desired Sensation Level Method. Trends Amplif 2005;9(4):199-226.
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