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23 Acoustic Accessibility in the Classroom and Beyond
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Table 23.1 Mean speech recognition scores (% correct) by children
with normal hearing and children with SNHL for monosyllabic words
across various SNR and RT values
RT (seconds) SNR Normal
0.0 Quiet 94.5 83.0
+12 dB 89.2 70.0
+6 dB 79.7 59.5
0 dB 60.2 39.0
0.4 Quiet 92.5 74.0
+12 dB 82.8 60.2
+6 dB 71.3 52.2
0 dB 47.7 27.8
1.2 Quiet 76.5 45.0
+12 dB 68.8 41.2
+6 dB 54.2 27.0
0 dB 29.7 11.2
Abbreviations: RT, reverberation time; SNR, signal-to-noise ratio.
Source: Table adapted from Finitzo-Hieber and Tillman.
poorly (60%). As the SNR decreases or as the RT lengthens, speech
recognition decreases to the worst case studied (SNR = 0 dB; RT =
1.2 seconds), where children with normal hearing achieve a 30%
score and children with hearing loss recognize virtually none of
the speech (11%). Both of these listening conditions have been
reported in classroom environments. Imagine trying to succeed
in school perceiving only 11% of what the teacher presents, orally!
Another dramatic example of the interplay between classroom
acoustics and speech was reported by Leavitt and Flexer.13 Using
the Rapid Speech Transmission Index (RASTI), they demonstrated
that 83% of the speech energy, delivered in the front of a classroom, was available to a listener in the front row of a typical
classroom-sized environment. However, in the back row of the
same classroom, only ~ 50% of the speech energy was available.
RASTI is a measure of speech energy as it traverses a room and is
an index of the amount of energy available to be perceived when
inuenced by SNR and RT, not the amount actually perceived. Even
less of the signal will be available if the listener has hearing loss or
reduced auditory and language processing. These factors hamper
the student’s actual perception of the available speech energy.
That is, add the impact of the classroom acoustical environment
to the distortion imposed by a damaged auditory-linguistic or
cognitive system, and it becomes apparent why simply using a
hearing aid is not likely to result in satisfactory communication
in the classroom.
14
hearing
SNHL
4
23.5.1 Acoustic Guidelines for RT
As previously discussed, speech recognition in adults with
normal hearing is not significantly aected until the RT exceeds
~ 1.0 second. For listeners with SNHL, most investigators have
recommended that RTs for listening environments should not
exceed ~ 0.4 second (through the speech frequency range:
500, 1,000, and 2,000 Hz) to provide optimum communicative
eciency.14 For permanent buildings, the current American
National Standard for classroom acoustics,12 discussed in the
following section, recommends an RT60 of 0.6 seconds or less for
average-sized classrooms and 0.3 to 0.4 seconds if the classroom
contains children who are deaf or hard of hearing. A review of
the literature suggests that appropriate RTs for persons with
hearing loss are rarely achieved.
15,16
23.5.2 Status of the ANSI SI2.60 Acoustic
Standard
Compliance with the ANSI classroom performance criteria is,
at this writing, mostly voluntary. It is hoped that, in the future,
adequate acoustics will be thought of as a necessary requirement
in a learning environment, not a luxury.
The ANSI standards have been incorporated in the Green
Building Code and are pending in the latest revision of the universal building code, but are presented as model codes at this time.
These model revisions include good acoustics, making the classroom listening environment no less important than items already
the codes, such as adequate lighting and ventilation. Every eort
in
should be made to meet the background noise and reverberation
stipulations of the standard as a first step in improving listening
and learning environments for all children. Because of expense,
meeting the ANSI stipulations is often not possible by physical
room modification alone.
18
Pearl
Meeting the ANSI acoustic performance stipulations does not
guarantee adequate classroom acoustic accessibility for children
with special listening and learning needs. Other SNR-enhancing
methodologies and technologies must also be considered.
23.5 Classroom Acoustic Guidelines
and Standards
The American National Standards Institute (ANSI) has issued
Standard S12.60 for classroom acoustics, including performance
criteria, design requirements, and guidelines.12 This standard
recommends background noise level of no more than 35 dB(A).
The standard provides a phase-in of background noise level criteria for relocatable classrooms. Studies have reported that these
acoustic criteria are only infrequently achieved in the academic
15,16,17
setting.
23.5.3 Noise Eects on Student and
Teacher Performance
In addition to aecting speech recognition directly, poor acoustic
environments can also compromise sound-to-meaning mapping, academic achievement, literacy, and attendant listening
and learning behaviors.19 Everyone has diculty communicating
in noise, but children have special problems in noise because
their language skills are still developing, and their brains are
not yet tuned to extract meaningful sounds automatically from
noise. Kraus and White-Schwoch19 contend that background
251
III Hearing Access Technologies for Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
noise disrupts the brain mechanisms important for language
development.
Pearl
Background noise disrupts the brain mechanisms important for
language development and can also result in cognitive fatigue.
An additional problem caused by noise in the classroom
concerns listening fatigue experienced by all children, and
by children with hearing loss in particular.
fatigue from mental exertion during listening tasks is very problematic for children with hearing loss in classrooms.21 Because
the children have to allocate more of their cognitive resources
to listening tasks, fewer resources are available for higher-level
processes such as problem solving and cognitive integration of
new information.
In addition, noisy classrooms exert negative consequences for
teachers as well. Specifically, teachers experience a significantly
higher incidence of vocal problems than the general population; it
is reasonable to assume that these vocal diculties are caused, at
least in part, by their having to increase vocal output to overcome
the eects of classroom noise during the school day.
20
In fact, cognitive
22
23.6 Identication of Acoustic
23.6.1 Assessing Compliance with ANSI
Acoustic Standards
Recall that the two primary performance criteria oered in the
ANSI standards are background noise level (dBA) and rever-
beration time (RT60). The following is a suggested method for
conducting a preliminary classroom acoustics survey based on
these criteria. Sound level meters are typically used to measure
background noise and reverberation time. These require some
training and experience to be used eectively, and they are
expensive.
Assessing Classroom Acoustic Accessibility
While many audiologists regularly survey classrooms for compliance with the ANSI performance criteria, many do not. The
importance of desirable classroom acoustics for listening and
learning in the classroom cannot be overstated, and it behooves
each and every audiologist to advocate for good acoustic environments in every classroom. Because some of the ANSI criteria
just described require expensive instrumentation, many audiologists without this instrumentation have not been able to make
the acoustic measurements required to identify classrooms in
need of strong advocacy for change or intervention. Fortunately,
technology has made it possible and inexpensive enough for each
and every audiologist to be able to screen for classroom acoustic
adequacy. The next paragraphs will describe how this screening
might be accomplished.
Problems
It is fairly easy to identify acoustic problems in a room. Use of
the observational guide in Table 23.2 may help identify acoustic
problems that can interfere with communication and instruction. To use the guide, simply observe whether any of the listed
conditions for background noise and reverberation exist in a
room. If the answer is “Yes” to any of the conditions cited on the
form, further acoustic analysis is warranted.
When noise and/or reverberation levels are suspected of
exceeding those recommended by ANSI,12 the observations are a
handy indicator that further assessment is advisable.
Table 23.2 Guide for observing classroom features that impact acoustics
Classroom observation: Background noise Yes No
Audible heating, air conditioning, and ventilation system
Equipment must be turned o during instruction
Exterior noise is audible during instruction
Internally generated noise during instruction (aquarium air pump, student activities)
Classroom observation: Reverberation Yes No
Hard surface ceiling without acoustic tiles
Ceilings are higher than 11 feet
Ceilings tiles are present but painted over
Floor constructed of hard reective material such as wood or tiles
Walls constructed of hard reective material such as plasterboard or concrete blocks
Screening Acoustical Measurements
“There’s an app for that!” There are few who have not either heard
or uttered that phrase at some point over the last few years. The
range and scope of applications for smartphones, tablets, and
even laptop and desktop computers continues to grow at a rapid
rate.
We are interested in accurate, cost-eective, and easy-to-use
approaches to measure the acoustic characteristics of classrooms.
Specifically, as per the current ANSI standards, we would like
to be able to measure the unoccupied noise levels and octave
252
23 Acoustic Accessibility in the Classroom and Beyond
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
band reverberation times that are present in a given classroom
accurately and reliably. Obtaining accurate data regarding these
parameters enables documentation of room compliance (or lack
of compliance) with the ANSI standard mentioned earlier in the
chapter.
It is critical to state that these screening measures cannot
replace full acoustic analysis by certified acousticians or acoustics specialists, nor are they intended to do so. They are tools
for audiologists to obtain essential information related to the
acoustical challenges students face daily in their typical learning
environments.
A recent online search of basic sound level meter (SLM) apps
available for Apple iOS and Android platforms yielded several
that could be useful for the measurement of classroom acoustics.
Of this small group, few appeared to have been designed by
audio professionals for use by professionals or could possibly be
considered equivalent to stand-alone Type II sound level meters
(specified in the ANSI standard). The others seem to be designed
more for casual estimation purposes only, because they lack
features such as A-weighting, spectral analysis, or measures of
reverberation time.
Pearl
All family members (and the child, too, if he or she has a smartphone or tablet) and professionals who work with children with
hearing loss should be encouraged to download an SLM app,
even a casual one, for their smartphone or tablet. The purpose
of the app is to be ever mindful of the noise in an environment;
noise is a particular barrier to communication for children with
hearing loss.
One such application is AudioTools, designed by Studio Six Digital
(Boulder, CO).
What about reverberation measures? In the AudioTools appli-
cation is an “Energy Time Curve (ETC)” and an “Impulse Response
(IR)” feature. The ETC feature can be used to obtain an overall RT60,
and the IR can be used to obtain octave band RT60 measures.
Critical Distance
Once the RT60 of a classroom has been determined, another very
useful metric can be calculated based on the volume of the room.
The first sounds that emanate from a speaker to a listener in a
room are referred to as direct sound. As time progress, the direct
sound has an opportunity to be reected from more and more
surfaces until the sound is dissipated. These later reverberations
add distortion and noise to the direct sound and, if significant,
can add to the background noise level and impair reception of
the signal.
Critical distance is defined as the point in a room where the
direct and late reected sound are equal in intensity. Distances
less than the critical distance result in the direct sound being
louder than the later reverberant sound, and this distance would
be desirable for listening and learning.
23.7 Remedies for Acoustic
Problems
Remedies for acoustic problems are many and can be implemented in sequence from least costly and simple to most costly
and professionally installed. Some will help an individual student; most will help all students.
While the options available may be limited, the well-designed
application can be extremely useful and may also be bundled
with sophisticated features that go far beyond the performance
criteria specified in the ANSI standard. It should be noted that all
SLM applications, from the casual to the sophisticated, are simply
manipulating and displaying data that have been input directly
via the internal microphone. The internal microphone’s frequency
response must be considered when making measurements. The
frequency response of the current iPhone’s built-in microphone,
for example, while quite consistent from unit to unit, has a steep
low-frequency rollo beginning at about 250 Hz of nearly 24 dB/
octave. This low-frequency rollo minimizes wind noise when
the phone is being used for its intended purpose, as a telephone.
Acoustic measurement designers must compensate for this feature
and will generally state that low-frequency accuracy is somewhat
more compromised than accuracy at higher frequencies.
Another important consideration is whether the application
one chooses provides a mechanism for calibration. Be mindful
of the location of the smartphone microphone and how you are
holding the device when using it as a measurement tool because
of the diering location of the device’s internal microphone. The
better-designed applications provide a way to match the reading
of the application to a Type II sound level meter for calibration.
Pitfall
Typical acoustic panels and other surface materials absorb high
frequencies better than low frequencies, resulting in reduced
speech intelligibility. A classroom audio distribution system
(CADS) can add back critical high frequencies (consonants)
that have been unintentionally removed by sound-absorbent
materials.
Ambient classroom noise can originate from several possible
sources. These sources include external noise (noise that is
generated from outside the school); internal noise (noise that
originates from within the school building, but outside the
classroom) and noise generated within the classroom. In order
to conduct the most appropriate remedy for a classroom, it must
first be determined which specific noise source, or sources, need
to be reduced.
Acoustic modification of the classroom environment must be
implemented in order for children to obtain the maximal benefit
from their hearing aids, cochlear implants, or wireless micro-
phone systems. Actually, the acoustic modi
fication suggestions
253
III Hearing Access Technologies for Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
discussed in the following paragraphs would help every child
and teacher in the building, whether or not they have a hearing
3
loss.
Pearl
Remedies for acoustic problems in classrooms will help all
students.
Table 23.3, Table 23.4, and Table 23.5 provide some simple
suggestions to help remedy specific sources of background noise
and reverberation. While these suggestions may help improve
the acoustic characteristics of the room, they must be considered
first-order interventions; the room may require more extensive
remedies if the conditions warrant them.
Table 23.3 Suggestions for reduction of external room noise
1. Locate classrooms away from high noise sources such as automobile highways.
2. Ensure external walls have a sound transmission loss (STL; refers to the amount of noise that is attenuated as it passes through a material) of at least
45–50 dB.
3. Make sure exterior walls are free of openings that could reduce STL.
4. Design, install, and seal windows in exterior walls to block sound entry.
5. Install building landscaping, such as earthen berm, to reduce exterior sound transmission.
6. Provide concrete sound barriers around building with an STL of 30–35 dB.
Pearls
Because “adequate acoustics” is an invisible and ambiguous
•
concept, the necessity of creating a favorable acoustic environment may be questioned by school personnel.
About 95% of today’s population of children who are identied
•
with hearing loss at birth likely will go directly into general
education classrooms by 5 or 6 years of age. Those classrooms
must be acoustically ready for them.
“Preferential seating” may improve visual accessibility to
•
speech in some situations but does not control the background
noise and reverberation in the classroom, stabilize teacher
and pupil position, or provide for an even and consistent SNR.
Rather, think “strategic seating” for best acoustic access to the
entire classroom.
Table 23.4 Suggestions to reduce internal room noise levels
1. Position children away from high noise sources during instruction.
2. Loud heating, ventilation, and air conditioning systems should be acoustically treated or replaced.
3. Installation of sound-absorptive material on oor to dampen noise of student movement and movement of desks and chairs. Rubber tips can be added
to chairs and desks if oor does not have sound-absorptive material.
4. Add acoustic paneling to walls (but beware of high-frequency sound absorption, which can reduce speech intelligibility).
5. Add acoustic tile to ceiling.
6. Hang sound-absorptive curtains or blinds on windows.
7. Avoid open-plan classrooms.
8. Ensure that partitions between adjacent classrooms are sucient to block sound transfer between rooms.
9. Position noisy equipment, such as printers, outside of instructional space.
10. Ensure that noisy equipment is either acoustically enclosed or turned o during instructional periods.
11. Encourage children/teachers to wear soft-soled shoes.
Table 23.5 Suggestions to reduce room reverberation
1. Add acoustically absorbent material to oor and ceiling.
2. Add sound-absorptive panels to hard-surfaced walls (but beware of high-frequency absorption, which can reduce speech intelligibility).
3. Add acoustically absorbent materials such as drapes and blinds to cover the hard reective surface of windows.
4. Avoid placing mobile hard-surfaced boards parallel to walls to reduce reections between surfaces.
5. As room size increases, so does reverberation time; strive for smaller classrooms and ceiling heights less than 11 feet.
6. Observe critical distances during teacher–student instruction and interactions.
254
23 Acoustic Accessibility in the Classroom and Beyond
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
23.8 Conclusion
Improving listening and learning environments is a primary
focus of pediatric and educational audiologists. This focus should
include home, day care, and classroom environments.
The ANSI classroom acoustic stipulations are an important goal
for every classroom. Acoustical modifications of learning spaces
and also use of personal remote microphone (RM) systems or CADS
(RM or IR) as SNR-enhancing technologies are viable and cost-effective means for ensuring signal saliency and auditory focus for
children, whether or not they have diagnosed hearing problems.
Numerous studies suggest that every classroom ought to have
well-installed and used CADS as a necessary learning condition
for all children. See Chapter 24 for more information.
Dedication
Carl Crandell was a dear friend and colleague to both of us.
Although he did not physically contribute to this chapter, his
thoughts and spirit run through every word. Carl was a passionate
proponent of acoustic accessibility for all children. His wish was
for others to become advocates for children, and we hope that by
reading this chapter you will understand that that is our wish too.
Discussion Questions
1. Compliance with ANSI classroom acoustics standard currently
is voluntary. Why should the standard be compulsory for all
classrooms?
2. What modications can be made to improve acoustic accessi-
bility in learning and living environments?
3. What function do apps have in measuring and managing
acoustic accessibility in school and at home?
References
[1] Shannon C. A mathematical theory of communication. Bell Syst Tech J
1948;27:379–423, 623–656
[2] Crandell CC, Smaldino JJ, Flexer C. Sound-field Amplification: Applications to
Speech Perception and Classroom Acoustics. 2nd ed. New York, NY: Thomson
Delmar Learning; 2005
[3]
Smaldino JJ, F
Listening, Learning and Literacy in the Classroom. New York, NY: Thieme; 2012
[4] Finitzo-Hieber T, Tillman TW. Room acoustics eects on monosyllabic word
discrimination ability for normal and hearing-impaired children. J Speech Hear
Res 1978;21(3):440–458
[5] Bradley JS. Speech intelligibility studies in classrooms. J Acoust Soc Am
1986;80(3):846–854
[6]
Boo
Audiology: Diagnosis, Technology and Management. 2nd ed. New York, NY:
Thieme; 2014:201–208
[7] Killion M. SNR loss: I can hear what people say, but I can’t understand them.
Hear Rev 1997;4(12):8, 10, 12, 14
[8] Hawkins DB, Yacullo WS. Signal-to-noise ratio advantage of binaural hearing aids
and directional microphones under dierent levels of reverberation. J Speech
Hear Disord 1984;49(3):278–286
[9] Valente DL, Plevinsky HM, Franco JM, Heinrichs-Graham EC, Lewis DE. Exper-
imental investigation of the eects of the acoustical conditions in a simulated
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lassroom on speech recognition and learning in children. J Acoust Soc Am
2012;131(1):232–246
[10] French NR, Steinberg JC. Factors governing the intelligibility of speech sounds. J
Acoust Soc Am 1947;19:90–119
[11] Crandell CC, Kreisman BM, Smaldino JJ, Kreisman NV. Room acoustics interven-
tion ecacy measures. Semin Hear 2004;25(2):201–206
Acoustical Society of America. ANSI/ASA S12.60.2010 American National Stan-
[12]
dard Acoustical Performance Criteria, Design Requirements, and Guidelines for
Schools, Part 1: Permanent Schools, and Part 2: Relocatable Classrooms Factors.
http://acousticalsociety.org/search/site/s12.60.2010. 2009/2010. Accessed
December 31, 2017
[13] Leavitt R, Flexer C. Speech degradation as measured by the Rapid Speech Trans-
mission Index (RASTI). Ear Hear 1991;12(2):115–118
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Wróblewski M, Lewis DE, Valente DL, Stelmachowicz PG. Eects of reverbera-
tion on speech recognition in stationary and modulated noise by school-aged
children and young adults. Ear Hear 2012;33(6):731–744
[15] Crandell C, Smaldino JJ. An update of classroom acoustics for children with
hearing impairment. Volta Review 1994;96(4):291–306
[16] Knecht HA, Nelson PB, Whitelaw GM, Feth LL. Background noise levels and
reverberation times in unoccupied classrooms: predictions and measurements.
Am J Audiol 2002;11(2):65–71
[17] Nelson E, Smaldino JJ, Erler S, Garstecki D. Background noise levels and reverber-
ation times in old and new elementary school classrooms. J Educ Audiol Assoc
2007/2008;14:16–22
[18] American Speech-Language-Hearing Association. Building-Code Committee
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asha.org/article.aspx?articleid=2468403&resultClick=3. Accessed December 31,
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[19] Kraus N, White-Schwoch T. Listening in the din: A factor in learning disabilities?
Hear J 2015;68(9):38,40
[20] McCreery R. For children with hearing loss, listening can be exhausting work.
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[21] Bess RH, Gustafson SJ, Hornsby BWY. How hard can it be to listen? Fatigue in
school-age children with hearing loss. J Educ Audiol 2014;20:34–47
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lexer C, eds. Handbook of Acoustic Accessibility: Best Practices for
throyd A. The acoustic speech signal. In: Madell JR, Flexer C, eds. Pediatric
255
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copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
24 Remote Microphone Technologies
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Erin C. Schafer and Jace Wolfe
24 Remote Microphone Technologies
Summary
This chapter summarizes types of remote microphone systems,
advanced signal processing, and benefits of these systems for
children with hearing loss or other auditory disorders. There
are multiple advantages and disadvantages to the three types of
remote microphone systems (personal, soundfield, and personal
soundfield) as well as to the various types of transmission.
Careful consideration of each child’s unique hearing needs, frequent listening environments, and current hearing aid, cochlear
implant, and other technology will help to ensure appropriate
selection, fitting, and management of remote microphone systems for children.
Keywords
remote microphone, frequency modulation, transmitter,
receiver, pediatric
Key Points
Remote microphone technology signicantly improves
•
speech recognition in noise and reverberation as well as when
the target signal originates at a distance from the primary
talker.
Modes of wireless transmission from the transmitter
•
to the receiver include frequency modulation, digital
radiofrequency, electromagnetic induction, and infrared
transmission.
Types of systems include personal, classroom audio distribu-
•
tion, and personal soundeld systems,
24.1 Introduction to Remote
Microphone Technologies
Although hearing aids and cochlear implants are the primary
hearing technologies for most infants and children with hearing loss, they cannot address all listening needs, particularly
in noisy or reverberant environments or when the child is at a
distance from the talker of interest.
overview of the eects of noise and reverberation on children’s
hearing performance. A clear and audible speech signal is a
critical precursor to auditory brain development and typical
speech-language progress; therefore, a favorable signal-to-noise
ratio (SNR) must be provided in all of a child’s listening and
learning environments.
The most eective approach to improve speech recognition in
environments with noise, reverberation, and distance from the
talker is the use of hearing assistive technology (HAT), which
includes remote microphone (RM) technologies.
Regardless of manufacturer or type of system, all RM technologies
aim to improve the SNR at the listener’s ear through wireless
transmission of the primary talker’s voice with a microphone
(worn 3–8 inches from mouth) and transmitter to the receiver
used by the listener. Over the past 10 years, RM technologies have
evolved into smaller, more technologically advanced devices with
multiple modes of transmission, sizes/types of receivers, and
signal processing characteristics. In addition, there are lower-cost
personal RM technology options, which provide greater opportunities for schools and families to purchase these devices for use at
home, in social situations, and for sports. The greatest commonality among RM systems is the microphone. As shown in Fig. 24.1,
typically used microphones include (1) a lavalier that is clipped to
4, 5,6 ,7,8
1,2, 3
Chapter 23 provides an
9,10,11,12,13,14,15
b d
Fig. 24.1 Types of microphones including (a)
a hand-held microphone, (b) a lavalier, (c) a
boom microphone, (d) a microphone built into
a
c e
the transmitter on a clip, and (e) a microphone
built into the transmitter pen.
257
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copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Table 24.1 Overview of types of transmission
Description How it works Advantages Disadvantages
Electromagnetic Induction loop system
Near-eld
magnetic induction
(NFMI)
Digital radiofrequency
(RF)
FM Frequency-modulated (FM)
Infrared (IR) Delivers infrared light signal
Abbreviations: DM, digitally modulated; RF, radiofrequency; RM, remote microphone.
delivers signal to telecoil of
personal hearing technology
Digital audio streaming
accessory delivers wireless
signal to user’s hearing
technology
Audio signal to RM converted
to a digital code and
delivered to user’s hearing
technology
radio wave delivers wireless
signal to receiver connected
to user’s hearing technology
or loudspeakers
to the receiver coupled to the
user’s hearing technology,
earphones, or loudspeakers
Signal from transmitter
converted to electrical
current, amplied, and
delivered to conducting wire
(i.e., loop) around room
or around neck. Magnetic
ux travels through loop,
is detected by telecoil in
personal device, which
converts and processes the
signal
Receiver captures Bluetooth
signal from transmitter,
converts it to DM signal,
and delivers it as electrical
pulses to neckloop. Loop
creates magnetic ux that is
detected by NFMI receiver in
user’s hearing device
Uses ultra-high-frequency
carrier (2.45 GHz or 900
MHz) waves to deliver
digital information via
RF transmission using
frequency/channel-hopping;
bi-directional exchange
occurs once paired
Electromagnetic radio wave
with a carrier frequency
delivered from transmitter
and captured by receiver,
which demodulates signal
and delivers it to the
user’s hearing device or to
loudspeakers
Transmitter converts
audio signal to infrared
light, sends it to stationary
receiver, which converts it
to an electrical signal that
is processed and delivered
to listener through hearing
technology, earphones, or
loudspeakers
Relatively inexpensive
•
Room: does not require
•
individual receivers
Available to all with
•
telecoil
Lower power
•
consumption
Allows for RM, binaural
•
streaming, and phone
applications
Minimal interference
•
Low power consumption
•
Immune to interference
•
Enhanced precision and
•
sound quality of audio
signal
Wider bandwidth than FM
•
Connectivity to phones,
•
tablets, computers, etc.
Documented benet with
•
personal hearing devices
Good transmission range
•
Can be used outside
•
Does not require
•
direct line of sight with
transmitter
Benet to 1–2 children
•
in class
Dicult to gain optimal
•
placement in every
situation
Cumbersome to move to
•
another classroom
Cannot use for most
•
extracurricular activities
Installation expense
•
Requires telecoil
•
Interference
•
Telecoil frequency
•
response may dier from
personal device
Signal strength may vary:
•
listener location, telecoil
orientation, and distance
Requires a neck- or body-
•
worn interface
No long-range
•
transmission with NFMI
(requires Bluetooth)
Transmission delays
•
Not universally available
•
in public settings like
induction
May have signal dropouts
•
outside and around
reective surfaces
Phone connectivity
•
limited to certain models
of phones
Susceptible to
•
interference
High power consumption
•
Larger antenna than DM
•
receivers
Signal dropouts in bright
•
environments
Requires direct line of
•
sight between transmitter
and receiver, making
it dicult to use in
classrooms
clothing or a lanyard, (2) a boom worn on the head, (3) hand-held
microphones that can be held, passed around, or placed on a table
near the talker, and (4) a microphone built into the transmitter.
The following sections of this chapter will outline the dierent
modes of signal delivery, types of RM systems, characteristics of
current transmitters and receivers, and verification strategies.
24.2 Types of Signal Delivery
The wireless transmission between the RM transmitter and
receiver occurs through electromagnetic induction, digital
radiofrequency (RF) transmission, frequency modulation (FM)
radio transmission, near-field magnetic induction (NFMI), or
infrared (IR) transmission. A summary of each transmission
type and how it works,16 with its advantages
9,10,11,13 ,14,17
and
disadvantages, is provided in Table 24.1. For more information
on digital RM, the reader is referred to Wolfe et al.
At this time, the most common devices used by children employ
either electromagnetic induction, FM, or digital RF transmission,
with the latter becoming the most commonly used of these
technologies. Regardless of the transmission mode, these RM
technologies are often paired with advanced noise-management
signal processing, which will be discussed later in this chapter. It
is possible, however, that some children who are mainstreamed
in their classrooms will use a device with NFMI transmission.
Because NFMI can be used for only one child, this device would
not be feasible when multiple children with hearing loss are in the
same class. IR transmission is not commonly used for personally
worn devices in classrooms, given the need for a direct line of
sight between the transmitter and receiver.
12,16 ,18
258
24 Remote Microphone Technologies
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Table 24.2 Overview of types of RM systems
Personal Soundeld/CADS Personal soundeld
Description Transmitter delivers signal to the child’s
Modes of delivery Electromagnetic induction, digital RF,
SNR improvements ~15–25 dB ~ 2–5 dB ~ 5–10 dB
Advantages Greatest SNR improvements Benet to all children in class Likely better SNR improvement than
Disadvantages
Abbreviations: CADS, classroom audio distribution systems; FM, frequency modulation; HA, hearing aid; IR, infrared; NFMI, near-eld magnetic induction; RF,
radiofrequency.
personal hearing technology (e.g., HA)
FM, NFMI IR
Most expensive
•
Benet for only one child
•
Transmitter delivers signal to
loudspeaker to provide uniform
distribution of sound
FM, digital RF, IR, hard-wired FM, IR
Least SNR improvement
•
Cumbersome or impossible to move
•
to another classroom
Cannot use for most extracurricular
•
activities
Transmitter delivers signal to a small
loudspeaker that is placed on a desk or
close to the child
CADS
Benet to 1–2 children in class
•
Dicult to gain optimal placement
•
in every situation
Cumbersome to move to another
•
classroom
Cannot use for most extracurricular
•
activities
24.3 Types of RM Systems
The types of RM systems include personal, soundfield or
classroom audio distribution systems (CADS), and personal
soundfield. An overview of the types of RM systems, describing
the SNR performance,
disadvantages of each, is provided in Table 24.2.
24.3.1 Personal Systems
Personal systems are designed to provide SNR improvements
to one child via a transmitter and receivers that are electrically
coupled to hearing aids, cochlear implants, or bone conduction
hearing devices. The benefits of personal RM systems are well
documented and typically provide improvements in SNR rang-
ing from 5 to 25 dB SNR over a hearing aid or cochlear implant
14,19,20,21
alone.
microphone on the personal hearing device is disabled; however,
this practice is not recommended for children who frequently
participate in group discussions and projects at school. Modes
of transmission for personal systems may include all of the types
listed in Table 24.1.
Personal FM/Digital RF Systems
In these systems, the signal is delivered via RF transmission to a
radio antenna integrated into the child’s personal hearing device
or to a radio receiver coupled to the direct auditory input of the
personal hearing device (Fig. 24.2). More specifically, when
using FM or digital RF transmission, the receiver often couples to
the device with a special adapter, battery door, earhook, or audio
shoe. Dedicated receivers or built-in receivers, made specifically
for some hearing aids and cochlear implants, are also available
from some manufacturers. Dedicated receivers are designed
to have seamless and inconspicuous connections between the
receiver and personal device.
Greater improvement in SNR is achieved when the
10,14,15,19,20,21,22,23,24
advantages,
9,10,13 ,20
and
Digital RF systems may be subcategorized into two general
classes: personal FM systems and accessory systems. Personal
systems are generally intended to function universally with
hearing aids, cochlear implants, and bone conduction processors
developed by most hearing technology manufacturers. Typically,
the receivers of these systems include a three-pronged Europlug
(Fig. 24.3), which possesses a standard design meant for use across
multiple makes and models of hearing technology. Personal systems may also be coupled to induction neckloops for use with the
t
elecoil of the wearer’s hearing technology. In contrast, accessory
systems are designed to function with the manufacturer’s hearing
aids for which the accessory device was developed. Accessory
systems deliver a signal to a receiver that is included inside the
wearer’s hearing technology (e.g., a 2.4-GHz radio antenna, an
NFMI receiver). Accessory remote microphones will not transmit
signals to hearing aids, cochlear implants, or bone conduction
processors made by other manufacturers.
Accessory systems are typically less expensive than personal
systems. When initially introduced to the market, accessory systems had fewer signal processing options and were generally less
ticated than personal systems (e.g., no directional micro-
sophis
phone technology, no noise reduction processing, no adaptive
gain changes). However, some manufacturers are now producing
accessory systems that possess sophisticated, automatically
switching directional microphones. The authors anticipate that
manufacturers will continue to improve the sophistication of the
signal processing included in RM accessories. It is also possible
that the price dierence between personal and accessory systems
may diminish over time. As a result, the most inclusive terminol-
ogy for all digital systems is “digital RM system.”
In addit
ion to receivers that connect to personal devices, some
manufacturers oer a transparent (i.e., limited-gain) receiver that
is designed for children with auditory processing issues, such as
auditory processing disorder (APD), attention-deficit/hyperactivity
disorder (ADHD), and autism spectrum disorder (ASD). The radio
antenna for these devices is built into the device, which often looks
like a small behind-the-ear (BTE) hearing aid. These transparent
receivers do not include a built-in microphone and only provide the
259
III Hearing Access Technologies for Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Fig. 24.2 Types of personal FM and digital RF systems for hearing aids and cochlear implant sound processors.
signal from the transmitter. According to one study, children with
ASD and ADHD showed significantly improved speech recognition
in noise, on-task behavior, and teacher-rated listening behaviors
when using the RM technology versus a no-device condition.25
Most personal FM or digital RF transmitters have audio input
ports to allow the teacher or parent to connect the transmitter to
other audio sources, including the television, smart board tablets,
computers, and other audio devices.
Electromagnetic/NFMI Systems
Personal systems that utilize analog electromagnetic induction
or NFMI require the use of an induction loop (or hearing loop)
worn around the neck (neckloop) as well as a specialized receiver
or intermediary device (NFMI) to receive the audio signal (Fig.
24.4). In addition, the child’s personal hearing technology
must contain a telecoil or NFMI capability. Transmitters that
are coupled to receivers using electromagnetic induction may
use FM, digital RF, or infrared transmission to send the signal
to the receiver. In addition to the signal from the RM, many of
these transmitters also contain an audio input port to allow for
audio input from other electronics including mobile phones,
laptop computers, tablets, and televisions. For devices with
NFMI, Bluetooth is typically used to transmit the signal from an
audio streaming accessory to the intermediary device, which
in turn sends the signal via NFMI through the neckloop. Many
hearing aids and cochlear implants that utilize NFMI also allow
connections with Bluetooth-enabled personal electronic devices
including mobile telephones, tablets, and computers.
24.3.2 Soundeld and Classroom Audio
Distribution Systems
Soundfield systems or CADS deliver the signal from RM via
Fig. 24.3 Three-pronged Europlug in most universal personal
receivers.
FM, digital RF, IR, or hard-wired transmission to one or more
loudspeakers with the goal of uniform distribution of the sound
260
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