2nd International Hearing Loop
Conference: Telecoil Panel
Linda Kozma-Spytek
Research Audiologist
Technology Access Program
Gallaudet University
RERC - Telecommunications Access
RERC – Hearing Enhancement
Telecoils
amplified
passive
A telecoil is a coil of wire around a
magnetic metal core that is sensitive to
magnetic fields.
Sensitivity is determined by its:
• size and geometry
• programming gain
(amplification) applied
Photo used with permission from Oticon, Inc.
Amplified Telecoils
• consist of a coil and preamplifier
• preamp increases sensitivity of the telecoil
• should have RF-interference protection
Approximately 60% of hearing aid fittings in the US include a
telecoil, and all cochlear implants have telecoils.
Telecoils are used in hearing devices to connect to:
Telephones (directly)
• Landline Phones (corded and cordless)
• Wireless Devices (basic cell phones, feature phones and smart
phones)
Hearing Assistive Technology (HAT)
• Induction Loop Systems (directly)
• Other HATs (FM/IR) via
• Neckloops
• Silhouettes
• Headsets
Audio Devices (telephones, music players, etc.) via
• Neckloops
• Silhouettes
• Headsets
Programming
Fully programmable memory dedicated for telecoil use:
• amplification characteristics can be independently adjusted by
the audiologist, including mixing input from the telecoil and
microphone
• amplification characteristics can also be preset
(manufacturer default telecoil program)
Manual:
Automatic:
Remote
Control
Touchless
Telecoil
Why use telecoil coupling?
+ Reduce acoustic background noise in all communication
settings
(- magnetic noise)
+ Reduce the effects of poor room acoustics (reverberation) in
all communication settings
+ Reduce the effects of distance (level of the speaker’s voice
remains constant regardless of the distance between the
hearing aid wearer and the speaker) in settings where the
telecoil is part of a hearing assistive technology system
(- microphone location)
+ Eliminate acoustic feedback during telephone coupling
Acoustic
Loudspeakers
T
Induction Loop (IL)
System
reflected
sound
noise
noise
SPEECH
Telecoil Orientation
For best
coupling with a
telecoil, a
magnetic field
should cut
across all turns
in the wire of the
telecoil by
aligning with
the axis of the
core.
Figures adapted with permission from HyperPhysics
by Rod Nave, www.phy-astr.gsu.edu
no
coupling
maximum
coupling
maximum
coupling
no
coupling
vertical telecoils couple
best with the weaker radial
(vertical) magnetic field
lines of a telephone
`
`
horizontal telecoils couple
best with axial (horizontal)
magnetic field lines of a
telephone
Magnetic Field
Orientation
vertical telecoils
couple best with the
vertical magnetic field
lines of a room loop
Source Magnetic Field Strengths
dB (SPL)
equivalent
dB (A/m)
90
0
82
-8
72
-18
70
-20
68 [53]
-22 [-37]
63 [48]
-27 [-42]
60
-30
60
-30
50
-40
46
-45
27
-63
highest axial field strength measured on cordless phone
samples
IEC 60118-4 specified speech peak field strength for induction
loop systems (400 mA/m)
C63.19 required minimum axial and radial field strength for
wireless devices
IEC 60118-4 resulting long-term average level of speech for
induction loop systems (100 mA/m)
RS-504 required minimum axial field strength for landline
phones [in-use levels are ~15 dB lower]
RS-504 required minimum radial field strength for landline
phones [in-use levels are ~15 dB lower]
lowest axial field strength measured on cordless phone samples
ANSI S3.22 (1996) - HA quality assurance standard – reference
for telecoil sensitivity measurement (31.6 mA/m)
TIA 1083 specified A-weighted magnetic noise maximum
field strength of ambient magnetic noise (A-weighted)
measured in a parking lot, under power lines
field strength of ambient magnetic noise (A-weighted)
measured in a living room
Net Frequency Response
Magnetic Source
Receiver
+
Telephone
=
+
Induction Loop
System
HA (amplified)
telecoil
Net
Frequency
Response
HA Signal
Processing
Comparison of Magnetic Frequency Responses for
Telephones and Audio Loops
Magnetic Source Frequency Response
100
HA Amplified Telecoil Frequency Response
100
1000
15
10
10
5
5
0
0
-5
-5
-10
-10
dB
dB
15
-15
-15
-20
-20
Audio Loop
Telephone
-25
-30
-25
-30
-35
Hz
Hz
Net Frequency Response
100
1000
15
10
5
0
-5
dB
-35
1000
-10
-15
Amplified Telecoil +
Audio Loop
Amplified Telecoil +
Telephone
-20
-25
-30
-35
Hz
10000
What needs to be considered for telecoil coupling capability?
Source (Magnetic)
Requirements:
Receiver (Telecoil)
Requirements:
• High enough signal strength
• Low enough noise/distortion
level
• High enough signal to noise ratio
• Usable field orientations
• Large enough coverage area
• Appropriate frequency response
• Adequate sensitivity
• Low enough noise/distortion
level
• High enough signal to noise ratio
• Usable telecoil orientation
• Standard frequency response
System Verification Goals:
To objectively quantify the real-ear performance of the hearing
aid in telecoil mode (when coupled to a telephone or hearing
assistive technology) in relation to a prescriptive target
Need: Level-controllable standard inductive signal sources
Acknowledgements
The contents of this PowerPoint presentation were developed
under grants from the Department of Education, NIDRR grant
numbers H133E080006 and H133E040013.
However, the opinions and content are those of the grantees
and do not necessarily represent the policy of the U.S.
Department of Education.