2nd International Hearing Loop Conference: Telecoil Panel

Linda Kozma-SpytekResearch Audiologist

Technology Access ProgramGallaudet University

RERC - Telecommunications Access RERC – Hearing Enhancement

passive

amplified

Photo used with permission from Oticon, Inc.

Sensitivity is determined by its:• size and geometry• programming gain (amplification)

applied

Amplified Telecoils • consist of a coil and preamplifier • preamp increases sensitivity of the telecoil• should have RF-interference protection

TelecoilsA telecoil is a coil of wire around a magnetic metal core that is sensitive to magnetic fields.

Approximately 60% of hearing aid fittings in the US include a telecoil, and all cochlear implants have telecoils.

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

Telecoils are used in hearing devices to connect to:

ProgrammingFully 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:Remote Control

Touchless Telecoil

Automatic:

+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

Why use telecoil coupling?

SPEECH

SPEECH

noise noise

reflected sound

Induction Loop (IL) System

T

SPEECH

Acoustic Loudspeakers

Telecoil Orientation

Figures adapted with permission from HyperPhysics by Rod Nave, www.phy-astr.gsu.edu

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.

maximum coupling

maximum coupling

no coupling

no coupling

horizontal telecoils couple best with axial (horizontal) magnetic field lines of a telephone

vertical telecoils couple best with the weaker radial (vertical) magnetic field lines of a telephone

``

vertical telecoils couple best with the vertical magnetic field lines of a room loop

``

Magnetic FieldOrientation

dB (SPL) equivalent

dB (A/m)  

90 0highest axial field strength measured on cordless phone samples

82 - 8IEC 60118-4 specified speech peak field strength for induction loop systems (400 mA/m)

72 -18C63.19 required minimum axial and radial field strength for wireless devices

70 -20IEC 60118-4 resulting long-term average level of speech for induction loop systems (100 mA/m)

68 [53] -22 [-37]RS-504 required minimum axial field strength for landline phones [in-use levels are ~15 dB lower]

63 [48] -27 [-42]RS-504 required minimum radial field strength for landline phones [in-use levels are ~15 dB lower]

60 -30 lowest axial field strength measured on cordless phone samples

60 -30 ANSI S3.22 (1996) - HA quality assurance standard – reference for telecoil sensitivity measurement (31.6 mA/m)

50 -40 TIA 1083 specified A-weighted magnetic noise maximum

46 -45field strength of ambient magnetic noise (A-weighted) measured in a parking lot, under power lines

27 -63field strength of ambient magnetic noise (A-weighted) measured in a living room

Source Magnetic Field Strengths

Receiver

HA (amplified) telecoil

+

+

Net Frequency Response

HA Signal Processing

=

Net Frequency Response

Magnetic Source

Induction Loop System

Telephone

Comparison of Magnetic Frequency Responses for Telephones and Audio Loops

100 1000 10000

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

HA Amplified Telecoil Frequency Response

Hz

dB

100 1000

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

Magnetic Source Frequency Response

Audio LoopTelephone

Hz

dB

100 1000

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

Net Frequency Response

Amplified Telecoil + Audio LoopAmplified Telecoil + Telephone

Hz

dB

What needs to be considered for telecoil coupling capability?Source (Magnetic)

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

Receiver (Telecoil) Requirements:

• 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

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.

Acknowledgements