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26
FM Systems and Communication
Access for Children
Dawna Lewis and Leisha Eiten
Introduction
With the expansion of newborn hearing screening programs, infants with hearing loss are being identified at
very early ages. Once hearing loss has been identified,
an essential goal for pediatric audiologists is to provide
children with communication access across a wide variety of listening environments. Although there are many
factors that may impact communication access, audibility of the signal of interest is of primary importance.
In addition to degree and configuration of hearing loss
and amplification issues, audibility will be affected by
distance between the sound source and listener, background noise, and reverberation.
Communication Access
As the distance between listeners and talkers varies
throughout the day, the audibility of speech also will
vary. This affects both the level of the primary signal
reaching the listener’s ears and the level of that signal
relative to background noise. For example, the overall level of a classroom teacher’s voice one meter from
the listener is approximately 71 dB SPL and at four
meters is approximately 61 dB SPL (Pearsons, Bennett,
& Fidell, 1977). The level of average conversational
speech at one meter is approximately 60 dB SPL (Cox
& Moore, 1988). Instances of “conversational speech at
one meter” may not be as common for children as for
adults. Any parent of a young child will be quick to
report that the distance between a talker and a young
listener can vary considerably in a very short period of
time. In addition, there will be many times when the
talker and listener will not be face-to-face. For infants
and young children, there will be times when the level
of speech reaching their ears will be higher than that of
speech at one meter. The level of speech at the near ear
of an infant held in the cradle position is approximately
68 dB SPL and at the near ear of a young child held on
the hip is approximately 76 dB SPL (Stelmachowicz,
Mace, Kopun, & Carney, 1993). At other times the level
of speech reaching an infant or young child’s ears may
be lower, such as when the talker is farther away or the
orientation of talkers is not face-to-face. For example,
in the United States infants up to 20 pounds or one
year of age are placed in rear-facing infant seats in the
back seat of cars. Even when they move to front-facing
car seats, it is recommended that they remain in the
back seat until they are 12 years of age (American Red
Cross, 2007). In this environment, the combined effects
of distance, orientation, and car noise may significantly impact the audibility of speech.
For school-age children, the ability to hear and
understand verbal information is critical to learning in
the classroom. However, in many schools poor classroom acoustics may impact this ability. Background
noise can be defined as unwanted sounds that interfere
with the primary signal. Noise can come from sources
outside the school building (e.g., traffic, playground
noise), inside the building but outside the classroom
(e.g., sounds from the hallway, a nearby music class),
or in the classroom (e.g., heating and air conditioning
systems, the students themselves). Reverberation is the
repeated reflection of sound and typically is reported
in terms of the time it takes for a sound to decrease
60 dB from its original intensity once the sound stops
(reverberation time). Long reverberation times may
negatively impact perception of the primary speech
signal.
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In 2002, the American National Standards Institute
(ANSI) published guidelines for classroom acoustics.
These guidelines recommended maximum background
noise levels of 35 dB(A) and maximum reverberations
times of 0.6 to 0.7 seconds (ANSI, 2002) for typical classrooms. However, many classrooms fail to meet these
criteria (Crandell, 1991; Crandell & Smaldino, 1994;
Finitzo-Heiber, 1981). For example, Knecht, Nelson,
Whitelaw, and Feth (2002) evaluated background noise
levels and reverberation times in 32 elementary classrooms in three different school districts. Noise levels
ranged from 34 to 66 dB(A), with only four classrooms
having noise levels below the ANSI recommended levels.
Reverberation times ranged from 0.2 to 1.27 seconds, with
13 classrooms exceeding the ANSI recommendations.
The deleterious effects of noise, distance and
reverberation on the speech perception of listeners
with hearing loss are well known (e.g., Johnson, Stein,
Broadway, & Markwalter, 1997; Ross & Giolas, 1971).
Finitzo-Heiber and Tillman (1978) examined word recognition for 12 children with hearing loss and 12 children with normal hearing under various conditions of
noise and reverberation. In all conditions, children
with hearing loss performed more poorly than their
peers with normal hearing. In adverse listening conditions, children with minimal hearing loss perform
more poorly than their peers with normal hearing on
tasks of speech perception in noise, especially when
speech is directed to the poor ear and noise to the good
ear for those with unilateral hearing loss (Bess, Klee, &
Culbertson, 1986; Crandell, 1993; Kenworthy, Klee, &
Tharpe, 1990; Ruscetta, Arjmand, & Pratt, 2005).
The effects of distance, noise, and reverberation
also may negatively impact some groups of children
with normal hearing (e.g., Bradley & Sato, 2004; Crandell & Smaldino, 1996; Jamieson, Kranjc, Yu, & Hodgetts, 2004; Nelson, Kohnert, Sabur, & Shaw, 2005).
Over 25 years ago, Zentall and Shaw (1980) compared
the performance of children who had been identified
as hyperactive with children who were not hyperactive on classroom tasks in the presence of background
noise with a high linguistic content. When the task was
familiar, the hyperactive children performed more
poorly in high noise levels than the other children.
When the task was unfamiliar, both groups performed
poorly in high noise levels. More recently, Bradlow,
Kraus, and Hayes (2003) showed that children with
normal hearing who have learning disabilities (LD)
also may experience difficulties understanding speech
in adverse environments. They compared sentence
perception in noise (−4 and −8 dB signal-to-noise ratio)
for children with and without LD. Children with LD
performed more poorly than their peers and were
more adversely affected by decreasing signal-to-noise
ratio. Children with normal hearing sensitivity who
may be negatively impacted in adverse listening environments include young children and children who
are English language learners, as well as those with
the following:
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Attention deficit disorders,
Auditory processing disorders,
Developmental delays,
Dyslexia,
Recurrent middle-ear dysfunction,
Speech and language disorders,
History of conductive hearing loss,
Hyperactivity.
Challenges experienced by children with hearing
loss in adverse listening environments may negatively
impact development/performance in a number of areas.
For example, difficulties perceiving speech in noise
may impede a listener’s ability to follow conversations
with multiple talkers. In addition, because most communication takes place in environments where noise
is present, children with hearing loss may experience
difficulties following conversations from a distance or
acquiring information via overhearing. These difficulties may, in turn, affect speech and language development as well as social-emotional functioning. Subtle
difficulties in language abilities might be expected to
have an impact on advanced language skills, such as
social reasoning (e.g., emotion understanding, theory
of mind), discourse participation, complex narratives
(justifying opinion, explanations), complex syntax (cohesive ties, complementation), and complex vocabulary.
Difficulties in advanced language skills may, in turn,
negatively affect psychosocial development. Numerous studies have suggested that children with hearing
loss experience difficulties in areas such as self-esteem,
stress, energy, peer relations, and social confidence (e.g.,
Bess, Dodd-Murphy, & Parker, 1998; Cappelli, Daniels,
Durieux-Smith, McGrath, & Neuss, 1995; Loeb & Sarigiani, 1986; Oyler, Oyler, & Matkin, 1988).
Educational performance also may be affected
since the acoustic conditions in typical classroom environments are less than ideal (Blair, Peterson, & Viehweg, 1985; Brookhouser, Worthington, & Kelly, 1991;
Culbertson & Gilbert, 1986; English & Church; 1999).
Oyler et al. (1988) reported results of teacher questionnaires completed for children with unilateral hearing
loss in a large school district. Twenty-four percent of
these students had repeated at least one grade (compared to a district average of 2%) and 41% were receiving
special services (compared to a district average of 8.6%).
FM Systems and Communication Access for Children
Listeners with hearing loss also may need to exert
more effort than their peers with normal hearing in the
same listening environment (Bess et al., 1998). Hicks and
Tharpe (2002) found that children (5–11 yrs.) with mildto-moderate or high-frequency hearing loss expended
more listening effort than children with normal hearing
in both easy and difficult classroom listening situations.
In 2006, the National Institute on Deafness and
Other Communication Disorders at the National Institutes of Health sponsored a workshop on outcomes in
research in children with mild-severe hearing loss.
Among the outcomes from that workshop was a series
of papers on the current state of knowledge regarding
this population of children. Topics included psychosocial development (Moeller, 2007), language and literacy (Moeller, Tomblin, Yoshinaga-Itano, Connor, &
Jerger, 2007), perceptual processing (Jerger, 2007), speech
recognition and production (Eisenberg, 2007), as well
as research considerations (Tomblin & Hebbeler, 2007)
and implications for future research (Eisenberg et al.,
2007). These papers provide an overview of many of
the challenges experienced by children with hearing
loss as well as directions for future research.
Communication Solutions
When hearing loss is first identified, hearing instruments typically are the most common technology option
for achieving auditory access. Cochlear implants also
are an option available for children with severe to profound hearing loss. Although these devices provide
communication access in many situations, access will
continue to be negatively impacted by noise, distance
and reverberation (Anderson & Goldstein, 2004; Anderson, Goldstein, Colodzin, & Iglehart, 2005; Schafer &
Thibodeau, 2006). Leavitt and Flexer (1991) used the
Rapid Speech Transmission Index (RASTI) to examine
the integrity of a speechlike signal at multiple seating
positions in a classroom in relationship to a sound
source at the front of the room. Perfect reproduction of
the signal was only obtained 6 inches from the sound
source. The integrity of the signal reaching typical
seating positions in the classroom was far from ideal.
Although direct correlations between RASTI scores and
speech perception were not examined, these results
reveal that the signal reaching a hearing instrument
or cochlear implant microphone will be significantly
degraded at all but the closest listening distances. One
way to ensure a high quality speech input to the hearing instrument or cochlear implant is to maintain a
close distance between the talker and listener; a solu-
tion that is neither practical nor desirable. A more feasible solution is to place a microphone on the talker,
transmitting the signal directly to a receiver worn by
the listener.
Remote microphone technologies can provide
reproduction of close, un-degraded input signals with
no reverberation effects and reduced impact of noise.
A variety of remote-microphone hearing assistance technology (HAT) choices are available for classroom and
home use. Large area systems are often used for educational and public venue applications. They may utilize
induction loops/mats, sound-field FM (frequency modulation) or IR (infrared) transmission. Personal systems
can be adapted to both in-school and out-of-school use.
Personal remote-microphone systems include portable
desktop FM or IR systems and individual FM systems.
Expansion of the use of wireless connections has
resulted in many questions about the use of Bluetooth
technology as a remote-microphone HAT. Wikipedia
defines Bluetooth as:
A wireless protocol utilizing short-range communications technology facilitating data transmission over
short distances from fixed and/or mobile devices,
creating wireless personal area networks. The intent
behind the development of Bluetooth was the creation of a single digital wireless protocol, capable of
connecting multiple devices and overcoming problems arising from synchronization of these devices.
(Wikipedia, 2008)
A number of factors impact the application of
Bluetooth technology as a remote microphone HAT
option (Fabry, 2008). These include:
■ High power consumption,
■ Limited transmission range,
■ Latency (delay) in signal transmission.
Though not a direct remote-microphone HAT, Bluetooth technology is being used as a means of connecting the listener to other electronic accessories such as
cell phones, MP3 players, and computers.
FM Solutions to Enhance
Communication Access
Frequency-modulated (FM) systems are an effective
solution to the problems of noise, distance and reverberation because they use a microphone near a talker’s
mouth to maintain a close listening distance between
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the talker and the listener. FM systems use a low-power
radio transmitter to send FM radio signals from this
remote microphone to a miniature receiver worn by
the listener. The FM transmitter/microphone is worn
by the primary talker who is often a parent or teacher.
The use of a remote microphone provides an advantage because the transmitted signal preserves a consistent input level even if the distance between the talker
and the listener changes. Because the speech signal is
picked up inches from the talker’s mouth, the transmitted signal maintains a positive signal-to-noise ratio
advantage above the level of background noise without
reverberation effects.
FM Developments
The changes in FM technology in the last 20 years have
been significant. Prior to the mid 1990’s most FM systems transmitted in the 72- to 76-mHz frequency range.
Transmission in this range was prone to interference
and required strong antennae. Many FM systems used
one dedicated transmission channel that could not be
changed. FM receivers were body-worn units that originally required the hearing instruments to be removed
entirely, and later could be connected to a child’s personal hearing instrument(s) via direct audio input cords.
Environmental (local) microphones on these bodyworn receivers used linear amplification circuits with
limited frequency-response adjustability. Prior to 1982,
the local microphone was always on the body-worn
receiver, which resulted in its placement at chest level
(P. Henry, personal communication, May 30, 2008).
One of the most significant changes in wireless
technology has been the introduction of smaller and
more flexible FM receiver options (Figure 26–1).
Unlike the previous body-worn FM systems that
had fixed receiver channels, today’s FM receivers have
a large number of channels that can be easily changed
or synchronized with different transmitters. Federal
Communications Commission approval of additional
FM channels in the 216- to 217-MHz bands (Federal
Communications Commission, 1996) reduced the need
for large antennae on the receiver. This, along with
increased miniaturization of integrated circuitry, has
allowed smaller FM receivers to be built. FM receivers
are now designed to be ear-level devices that can be
worn with a child’s personal hearing instruments or
cochlear implants. Because of the change to ear-level
FM receivers, the hearing instrument or cochlear
implant microphone acts as the local microphone for
the system. One advantage of ear-level FM systems is
that the advanced circuitry and signal processing of
digital hearing instruments or cochlear implants can
be available to the child at the same time as the benefits of a remote (FM) microphone system. With the
availability of ear-level receivers, the FM system has
moved away from serving only as a classroom “auditory trainer” and now is utilized as hearing assistance
technology (HAT) in many areas of the child’s home
and family life. In any listening situation where background noise and reverberation or distance from the
FIGURE 26–1. Examples of ear-level FM receivers coupled to behindthe-ear hearing instruments.
FM Systems and Communication Access for Children
talker makes understanding difficult, an FM system
can be used. Conversations in the car, family trips to
the zoo and recreational activities can all be enhanced.
Current state-of-the-art FM receivers allow the
audiologist to adjust or program the output level of
the FM signal in relation to the local microphone signal to accommodate an individual child’s needs. In
addition, a teacher or parent can quickly change or
synchronize FM receiver channels as the child moves
from classroom to classroom or from classroom to home.
A range of integration options are available, including
FM receivers fully integrated into the hearing instrument, modular or dedicated receivers for specific hearing
instruments or cochlear implant models, and universal
receivers that can be switched between hearing instrument and cochlear implant models and manufacturers.
Ear-level FM-only devices also are now available for
children with normal hearing who have special listening needs and may benefit from an improved signalto-noise ratio, such as those populations previously
listed in this chapter.
Changes in FM transmitters also have improved
their flexibility. Current FM transmitters incorporate
features such as directional microphones, cheek or boom
microphone placement options, digital noise reduction
circuitry and Bluetooth wireless compatibility for integration with cell phones, computers, and MP3 (e.g.,
iPod™) players. The choice of directional microphone
settings allows flexibility in using the FM system in
a variety of settings outside traditional classrooms.
Some examples include using the transmitter as a conference microphone to hear a number of talkers in a
group activity, or choosing a narrow directional setting
to use the transmitter as a hand-held “pointer” microphone in a family gathering or party setting, where the
main talker frequently changes. Recent advances in
FM system technology include transmitters that incorporate adaptive FM levels to maintain ideal signal-tonoise ratios, as well as technology to adjust for varying
voice levels and microphone placements. Talker networks can be activated to improve team-teaching with
multiple FM transmitters active in a classroom. More
functions are becoming available to assist teachers and
families with troubleshooting and daily monitoring of
FM system use. Audiologists who evaluate FM systems
should be knowledgeable about current transmitter
features and how those features impact daily use.
Developmental Aspects of FM System Use
Although FM systems have been utilized outside the
classroom for a number of years (Benoit, 1989; Madell,
1992; Moeller, Donaghy, Beauchaine, Lewis, & Stelma-
chowicz, 1996), body-worn FM systems were considered bulky and cumbersome, limiting their use beyond
classroom environments. The introduction of the first
integrated FM/hearing instrument combination in 1992
(Sonovation, 2008) and, more significantly, the miniaturization of FM receivers coupled directly to hearing
instruments (Phonak, 2008) had a tremendous impact
on the extension of FM system use beyond classroom
environments and on their use with infants and young
children.
With the miniaturization of FM receivers, it has
become more common for audiologists to recommend
FM systems for infants and young children with hearing loss. Gabbard (2005) reported preliminary findings
from a loaner FM project in the state of Colorado.
Parents of nine children between the ages of 15 and
30 months completed a questionnaire designed to provide information regarding use and benefit from hearing instruments and FM systems. On average, hearing
instruments were used 10 hours per day and FM systems were used four hours per day. Both were rated as
easy to operate and comfortable for the children. Comments from parents suggested that the FM system benefits were seen in the expected areas of distance and
noise and that they helped the children attend to
speech in those environments.
As stated earlier in this chapter, while very young
infants spend much of their time in close proximity to
talkers, there will be many situations where noise, distance, and reverberation become factors in their ability
to hear and understand speech. Once the child is
mobile, these situations become even more common.
Also, unlike adults, children (especially young children)
are in the process of learning speech and language. As
such, they require good audibility to develop adequate
speech and language skills. Thus, a primary reason for
using FM systems with infants and young children is to
provide them with a consistently audible signal during
this crucial period of speech and language development.
Anecdotally, concerns occasionally are expressed
regarding a possible negative impact of FM system use
on the development of localization abilities in infants
and young children as well as on their ability to learn
to understand speech in noise. Currently, no research
exists to support either of these concerns. There also
are practical reasons to question such concerns. It is
important to remember that FM systems typically are
not worn 100% of the time. There will be many situations in which infants and young children will be using
only their hearing instruments. During those times,
they will be receiving inputs from many directions,
providing information about the location of the sound
source. In addition, the signal of interest will arrive
at the ears embedded within any background noises
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within the environment. Even when the infant uses the
FM system, it is most common that both the FM system and hearing instrument microphones are active
simultaneously. Thus, the only input signal that would
not be providing information about sound-source
location would be that of the person wearing the FM
microphone. Although the signal from the FM microphone will be received at a better SNR than it would
through the hearing instruments alone, background
noise is not eliminated completely. It also is important
to remember that there are many adverse listening
environments where practice will not be able to improve
speech understanding for an individual with hearing
loss. Recall the Leavitt and Flexer (1991) data presented previously in this chapter. If the signal reaching
the ear has been severely degraded by noise, distance,
and reverberation, even a hearing instrument that perfectly reproduces the signal will be presenting that
severely degraded signal to the listener.
As children grow, and before they reach school
age, the potential benefit of FM system use enlarges as
their listening experiences expand. Adverse listening
environments where FM systems may improve audibility include car trips, family outings and group activities (e.g., dance lessons, children’s sports teams). As
parents of children seen in our clinics have reported,
the FM system increases the language learning opportunities the child experiences in daily life.
When children reach school age, listening environments continue to expand. Although the ability to
hear and understand verbal information is critical
to learning in most classrooms, acoustics may impede
audibility for many children with hearing loss and
some groups of children with normal hearing. FM systems can improve audibility of the teacher’s voice, the
voices of other students if they use the FM microphone, and audibility of the audio signal from televisions, videos, and computers. FM systems also may be
used in extracurricular activities such as sports, dance,
or drama. As they reach adolescence, students may be
involved in after-school jobs or in volunteer organizations where utilizing FM systems can improve communication access in a variety of adverse listening
environments. Especially in adolescence, the ability to
couple a small FM receiver to a personal hearing
instrument or to have the FM receiver incorporated
into a BTE hearing instrument case makes utilization
in a variety of listening environments much more feasible and attractive to users.
As high school students transition to college or
full-time employment, FM systems can continue to play
a role in communication access. Those individuals who
have benefited from FM system use for many years
may be more likely to continue using the devices into
adulthood, viewing them as an integral part of their
hearing assistance technology. Audiologists can remain
a vital resource in helping to examine technology options
and funding sources for changing needs.
FM Verification
The development of ear-level FM receivers has meant
that the hearing instrument microphone acts as the
local microphone of the FM system. This allows the FM
system user to hear other talkers and sounds in the
environment and monitor one’s own voice, while still
receiving a consistent input from the main talker using
the FM microphone/transmitter. With most current
hearing instrument circuits incorporating some type of
nonlinear compression processing, verification of FM
systems has been significantly impacted. Any verification of the function of the FM system is dependent on
accurate evaluation of the hearing instrument processing. Hearing instrument processing options may include
multiple channels, frequency compression/transposition and active noise-reduction. Previous FM system
verification guidelines (ASHA, 1994, 2002) assumed
local microphones functioned with single-channel linear circuitry. It has become clear in recent years that
the rapid advances in hearing instrument and FM
technology require a re-examination of FM system fitting and verification procedures.
Another technological advance that has impacted
FM system verification is the availability of hearing
instrument test systems that offer speech-mapping
using real speech inputs, rather than broadband noise
or pure-tone inputs. As an input signal, real speech
allows more accurate evaluation of how hearing instruments and FM systems respond to typical inputs.
Speech is one of the primary signals that both FM systems and hearing instruments are designed to transmit
and amplify. Advanced compression and noise reduction processing react differently to rapidly changing
speech than they do to constant-level signals. Current
guidelines recommend the use of calibrated real-speech
inputs for the evaluation of hearing instrument and
FM microphone responses.
Electroacoustic FM System Verification
There are three assumptions that guide current verification processes and assist in prioritizing testing (American Academy of Audiology [AAA], 2008a, 2008b).
FM Systems and Communication Access for Children
1. Ear-level FM systems use the microphone of
the hearing instrument as the local microphone, and
the input from the FM microphone is processed by the
hearing instrument circuitry. All verification measures
of the relationship between the FM and hearing instrument microphones are based on the assumption that
the hearing instrument processing has been adjusted
to provide appropriate audibility and output for the
individual user (AAA, 2004). Before verifying FM system performance, the audiologist must first verify that
the hearing instrument is set appropriately and is meeting chosen prescriptive targets for a variety of speech
input levels. True estimates of maximum output are
obtained from input to the hearing instrument microphone, not the FM microphone. High compression
ratios in the FM microphone/transmitter likely will
prevent the hearing instrument from reaching its maximum output in response to FM input. The audiologist
must also confirm that the hearing instrument is able
to accept an FM input.
2. Most FM systems are worn with both the FM
microphone and hearing instrument microphone(s)
active at the same time. Therefore, all verification should
be completed with the FM system and hearing instrument microphones active simultaneously. The FM system output level can then be adjusted in relationship
to the hearing instrument response so as to preserve as
much of a speech-to-noise benefit as possible. Traditionally, it has been recommended that the signal from
the FM system maintain a 10 dB advantage over other
signals coming to the local microphone.
3. Because of the nonlinear characteristics of current hearing instrument and FM system circuitry, different input levels to the two microphones will result in
changing compression and gain results. This will create
problems during electroacoustic verification measures
as testing only can be completed sequentially, meaning
that the hearing instrument microphone response is
evaluated, followed by the FM microphone response.
Current test systems do not allow for simultaneous
inputs to both the hearing instrument and FM system
microphones with separate outputs for the two inputs.
Therefore, if the FM microphone response is evaluated
using input levels that are typical for close microphone
placements (80–95 dB SPL) and the hearing instrument
microphone response is evaluated separately for conversational input levels (60–65 dB SPL), the compression parameters in response to these input signals will
be significantly different than they will be when those
signals reach both microphones simultaneously. Consequently, no valid comparisons could then be made
between the FM system and hearing instrument
responses if different input levels were tested sequen-
tially. A thorough discussion of the impact of nonlinear
compression characteristics and the use of sequential
versus simultaneous measurements can be found in
Platz (2004, 2006).
Because current test systems do not allow simultaneous measures of hearing instrument and FM system microphones, electroacoustic verification protocols
require that input levels to the FM microphone be less
than actual use inputs. This is required to ensure that
the hearing instrument response to an FM system input
has the same compression characteristics as its response
to input from the hearing instrument microphone. Verification of the relationship between FM system and
hearing instrument responses can be completed with
equal inputs to the FM system and hearing instrument
microphones. The input level to both systems must
be sufficiently low so that input compression in the
FM transmitter is not activated, and compression in
the hearing instrument will act equally on both the FM
and hearing instrument test signals. The goal is to
achieve equal outputs for equal inputs to the two
microphones. This new approach to FM system verification is defined as transparency.
With the FM receiver set at its default FM system
gain position of +10 dB, the AAA Guidelines (AAA,
2008b) recommend a 65 dB SPL input to the hearing
instrument microphone and a 65 dB SPL input to the
FM microphone (in the HA+FM position) to evaluate
for transparency (Figure 26–2).
If the FM system output at this +10 dB setting is
more than ±2 dB different from the hearing instrument
output, adjustments (offsets) are made to the FM system level to achieve equal outputs, or transparency.
When verified in this manner, the FM system has been
shown to maintain a 10 dB signal-to-noise advantage
in relationship to the hearing instrument when typical
use inputs are presented simultaneously to both microphones (Platz, 2006). This 10 dB recommendation is
offered as a general starting point for selecting the FM
system level. It is based on the relationship between
typical use inputs to the FM and hearing instrument
microphones and the kneepoint for compression in the
FM microphone/transmitter. For step-by-step verification procedures, see AAA (2008b).
Because of the nonlinear nature of current hearing
instrument and FM system technology, functional gain
or amplified sound field threshold (ASFT) testing
should never be used to verify FM system function.
With the changing compression characteristics of both
FM and hearing instrument microphones, no predictable relationship exists between amplified threshold information and FM system performance at typical
use input levels.
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FIGURE 26–2. SPLogram graph illustrating transparency measures with 65 dB SPL input to
the hearing instrument (test 1) and 65 dB SPL input to the FM microphone (test 2). Circles are
hearing thresholds, crosses are long-term average speech spectrum targets, and asterisks
are maximum output targets.
Real-Ear FM Verification
Real-ear verification of FM systems can be completed
for any hearing instrument + FM or FM-only combination. Due to the number of measures and possible
adjustments needed when verifying FM receivers coupled to hearing instruments, electroacoustic verification
in a test box is typically the more efficient approach.
However, verification of ear-level FM-only systems
requires special considerations. Ear-level FM-only fittings are primarily used with children/students who
have normal or near-normal hearing. The child’s ability to hear oneself and others needs to be preserved;
therefore, ear-level FM-only fittings are designed to be
nonoccluding. Electroacoustic verification in a test box
with the FM receiver coupled to a 2 cc coupler does not
provide accurate information about how the system
functions on the child’s ear when open-ear acoustics
are preserved. In addition to open-ear acoustic considerations, the FM-only receiver is not coupled to personal
hearing instruments and has no local microphone that
amplifies other talkers. Using the same transparency
approach previously described for FM receivers coupled to hearing instruments is not appropriate when
verifying ear-level FM-only receivers.
When verifying any nonoccluding fitting, two
sound pathways must be considered (Figure 26–3):
(1) the amplified sound pathway, which is the transmitted signal from the FM system. This is the primary
signal of interest for verification; and (2) the direct or
unamplified sound pathway, which includes unamplified portions of the main talker’s voice, other talkers,
the FM user’s own voice and background noise. In a
nonoccluding fitting, unamplified signals can move
freely into and out of the ear canal. At the same time,
much of the low-frequency regions of the amplified
input signal leak out of the open ear canal (Hoover,
Stelmachowicz, & Lewis, 2000). This will affect the
expectations and targets for the FM system response in
the child’s nonoccluded ear (Figure 26–4).
FM Systems and Communication Access for Children
FIGURE 26–3. Two sound paths into the ear when wearing a nonoccluding
FM system.
FIGURE 26–4. Graphic representation of amplified (gray arrows) and direct (black arrows) sound as
it enters (left) and leaves (right) the nonoccluded ear canal.
Verification priorities for ear-level FM-only fittings are two-fold. First, maximum output should be
confirmed in the child’s ear. Because most FM-only
devices are fitted on normal-hearing ears, it is critical
to verify the maximum output of the system using a
high-level pure-tone input. Output or volume control
adjustments may be needed to prevent exposure to
excessive sound pressure levels. Second, the FM system
should maintain consistent audibility and comfort for
the main talker’s voice. Recommended use settings are
determined based on the response of the FM-only system to close speech inputs (1-6” from talker’s mouth).
Specific procedures for real-ear verification of FMonly receivers will vary depending on the real-ear test
system that is utilized. These procedures will be influenced by how the real-ear reference microphone is
used during testing and whether FM specific inputs
and targets are available in the test system (AAA,
2008b; Eiten, 2008; Eiten & Lewis, 2008). For step-by-step
real-ear verification procedures, see AAA (2008b).
Behavioral FM System Verification
Behavioral verification of FM systems using speech
perception measures is a recommended option when
an appropriate sound-field test environment is available (AAA, 2008b). Although behavioral verification
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is not a substitute for electroacoustic verification for
HA+FM and FM-only systems, it remains the primary
method of verification for FM systems coupled to
cochlear implants. When using speech perception tasks
to verify FM system performance, testing should be
completed under listening conditions that are representative of the child’s typical listening environments.
Priority is given to testing in noise. The child’s performance when the FM microphone is not active (unaided,
hearing instrument only, cochlear implant only) is compared to performance with the FM microphone active
under the same noise conditions. The vocabulary and
language level of the child must be considered when
choosing this verification option.
Testing with adaptive or variable noise and/or
speech levels is not recommended as part of the
behavioral verification of FM systems. When testing
is conducted with varying speech levels and a fixed
noise level, the input to the FM microphone will most
likely be significantly lower than that found in normal
use (Boothroyd & Iglehart, 1998). Similarly, when testing is conducted with varying noise levels and a fixed
speech level at the FM microphone, the resulting noise
levels may exceed typical classroom noise levels. For
these reasons, the use of such testing to determine a
threshold signal-to-noise ratio is not recommended for
behavioral FM system verification. These types of procedures are appropriate when evaluating performance
with personal hearing instruments or cochlear implants
alone for determining HAT candidacy. Some currently
available speech-in-noise tests that use adaptive or
variable speech and/or noise levels include the BKBSIN (Bamford-Kowal-Bench Speech-In-Noise test; Etymotic Research, 2005) and HINT-C (Hearing-In-Noise
Test-Child; Nilsson, Soli, & Gelnett, 1996).
Conclusions
Research continues to support the need for hearing
assistance technology in poor acoustic environments
for children with hearing loss and for children with
normal hearing who have special listening needs. Developments in FM system technology have expanded their
use and now provide audiologists with many options
for addressing communication access needs. Miniature,
ear-level FM receivers are appropriate for children of
all ages both in and out of school. Fitting and verification techniques have evolved to keep pace with technological developments in both hearing instruments
and FM systems. It is important for audiologists who
work with children to understand current methods for
selecting and fitting FM systems in order to address
the specific communication needs of each child. Audiologists must also incorporate appropriate verification
guidelines into their clinical practice and be aware of
future advances as they become available.
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