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§ 68.316 Hearing aid compatibility: Technical requirements.
A telephone handset is hearing aid compatible for the purposes of this
section if it complies with the following standard, published by the
Telecommunications Industry Association, copyright 1983, and reproduced
by permission of the Telecommunications Industry Association:
Electronic Industries Association Recommended Standard RS-504 Magnetic Field
Intensity Criteria for Telephone Compatibility With Hearing Aids
[Prepared by EIA Engineering Committee TR-41 and the Hearing Industries
Association's Standards and Technical Committee]
Table of Contents
List of Illustrations
4 TECHNICAL REQUIREMENTS
4.2 Axial Field Intensity
4.3 Radial Field Intensity
4.4 Induced Voltage Frequency Response
List of Illustrations
1 Reference and Measurement Planes and Axes
2 Measurement Block Diagram
3 Probe Coil Parameters
4A Induced Voltage Frequency Response for receivers with an axial
field that exceeds -19 dB
4B Induced Voltage Frequency Response for receivers with an axial
field that exceeds -22 dB but is less than -19 dB
Magnetic Field Intensity Criteria for Telephone Compatibility With Hearing
(From EIA Standards Proposal No. 1652, formulated under the cognizance
of EIA TR-41 Committee on Voice Telephone Terminals and the Hearing
Industries Association's Standards and Technical Committee.)
Hearing-aid users have used magnetic coupling to enable them to
participate in telephone communications since the 1940's. Magnetic
pick-ups in hearing-aids have provided for coupling to many, but not
all, types of telephone handsets. A major reason for incompatibility
has been the lack of handset magnetic field intensity requirements.
Typically, whatever field existed had been provided fortuitously rather
than by design. More recently, special handset designs, e.g., blue
grommet handsets associated with public telephones, have been
introduced to provide hearing-aid coupling and trials were conducted to
demonstrate the acceptability of such designs. It is anticipated that
there will be an increase in the number of new handset designs in the
future. A standard definition of the magnetic field intensity emanating
from telephone handsets intended to provide hearing-aid coupling is
needed so that hearing-aid manufacturers can design their product to
use this field, which will be guaranteed in handsets which comply with
1.1 This standard is one of a series of technical standards on voice
telephone terminal equipment prepared by EIA Engineering Committee
TR-41. This document, with its companion standards on Private Branch
Exchanges (PBX), Key Telephone Systems (KTS), Telephones and
Environmental and Safety Considerations (Refs: A1, A2, A3 and A4) fills
a recognized need in the telephone industry brought about by the
increasing use in the public telephone network of equipment supplied by
numerous manufacturers. It will be useful to anyone engaged in the
manufacture of telephone terminal equipment and hearing-aids and to
those purchasing, operating or using such equipment or devices.
1.2 This standard is intended to be a living document, subject to
revision and updating as warranted by advances in network and terminal
equipment technology and changes in the FCC Rules and Regulations.
2.1 The purpose of this document is to establish formal criteria
defining the magnetic field intensity presented by a telephone to which
hearing aids can couple. The requirements are based on present
telecommunications plant characteristics at the telephone interface.
The telephone will also be subject to the applicable requirements of
EIA RS-470, Telephone Instruments with Loop Signaling for Voiceband
Applications (Ref: A3) and the environmental requirements specified in
EIA Standards Project PN-1361, Environmental and Safety Considerations
for Voice Telephone Terminals, when published (Ref: A4).
Telephones which meet these requirements should ensure satisfactory
service to users of magnetically coupled hearing-aids in a high
percentage of installations, both initially and over some period of
time, as the network grows and changes occur in telephone serving
equipment. However, due to the wide range of customer apparatus and
loop plant and dependent on the environment in which the telephone and
hearing aid are used, conformance with this standard does not guarantee
acceptable performance or interface compatibility under all possible
2.2 A telephone complies with this standard if it meets the
requirements in this standard when manufactured and can be expected to
continue to meet these requirements when properly used and maintained.
For satisfactory service a telephone needs to be capable, through the
proper selection of equipment options, of satisfying the requirements
applicable to its marketing area.
2.3 The standard is intended to be in conformance with part 68 of the
FCC Rules and Regulations, but it is not limited to the scope of those
rules (Ref: A5).
2.4 The signal level and method of measurement in this standard have
been chosen to ensure reproducible results and permit comparison of
evaluations. The measured magnetic field intensity will be
approximately 15 dB above the average level encountered in the field
and the measured high-end frequency response will be greater than that
encountered in the field.
2.5 The basic accuracy and reproducibility of measurements made in
accordance with this standard will depend primarily upon the accuracy
of the test equipment used, the care with which the measurements are
conducted, and the inherent stability of the devices under test.
This section contains definitions of terms needed for proper
understanding and application of this standard which are not believed
to be adequately treated elsewhere. A glossary of telephone
terminology, which will be published as a companion volume to the
series of technical standards on Telephone Terminals For Voiceband
Applications, is recommended as a general reference and for definitions
not covered in this section.
3.1 A telephone is a terminal instrument which permits two-way,
real-time voice communication with a distant party over a network or
customer premises connection. It converts real-time voice and voiceband
acoustic signals into electrical signals suitable for transmission over
the telephone network and converts received electrical signals into
acoustic signals. A telephone which meets the requirements of this
standard also generates a magnetic field to which hearing-aids may
3.2 The telephone boundaries are the electrical interface with the
network, PBX or KTS and the acoustic, magnetic and mechanical
interfaces with the user. The telephone may also have an electrical
interface with commercial power.
3.3 A hearing aid is a personal electronic amplifying device,
intended to increase the loudness of sound and worn to compensate for
impaired hearing. When equipped with an optional inductive pick-up coil
(commonly called a telecoil), a hearing aid can be used to amplify
magnetic fields such as those from telephone receivers or
3.4 The reference plane is the planar area containing points of the
receiver-end of the handset which, in normal handset use, rest against
the ear (see Fig 1).
3.5 The measurement plane is parallel to, and 10 mm in front of, the
reference plane (see Fig 1).
3.6 The reference axis is normal to the reference plane and passes
through the center of the receiver cap (or the center of the hole
array, for handset types that do not have receiver caps).
3.7 The measurement axis is parallel to the reference axis but may be
displaced from that axis, by a maximum of 10 mm (see Fig 1). Within
this constraint, the measurement axis may be located where the axial
and radial field intensity measurements, are optimum with regard to the
requirements. In a handset with a centered receiver and a circularly
symmetrical magnetic field, the measurement axis and the reference axis
eCFR graphic ec02jn91.027.gif
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4 Technical Requirements
These criteria apply to handsets when tested as a constituent part of a
4.1.1 Three parameters descriptive of the magnetic field at points in
the measurement plane shall be used to ascertain adequacy for magnetic
coupling. These three parameters are intensity, direction and frequency
response, associated with the field vector.
4.1.2 The procedures for determining the parameter values are defined
in the IEEE Standard Method For Measuring The Magnetic Field Intensity
Around A Telephone Receiver (Ref: A6), with the exception that this EIA
Recommended Standard does not require that the measurements be made
using an equivalent loop of 2.75 km of No. 26 AWG cable, but uses a
1250-ohm resistor in series with the battery feed instead (see Fig 2).
4.1.3 When testing other than general purpose analog telephones,
e.g., proprietary or digital telephones, an appropriate feed circuit
and termination shall be used that produces equivalent test conditions.
4.2 Axial Field Intensity.
When measured as specified in 4.1.2, the axial component of the
magnetic field directed along the measurement axis and located at the
measurement plane, shall be greater than -22 dB relative to 1 A/m, for
an input of -10 dBV at 1000 Hz (see Fig 2).
Note: If the magnitude of the axial component exceeds -19 dB relative
to 1 A/m, some relaxation in the frequency response is permitted (See
4.3 Radial Field Intensity.
When measured as specified in 4.1.2, radial components of the magnetic
field as measured at four points 90DEG apart, and at a distance >=16 mm
from the measurement axis (as selected in 4.2), shall be greater than
-27 dB relative to 1 A/m, for an input of -10 dBV at 1000 Hz (see Fig
4.4 Induced Voltage Frequency Response.
The frequency response of the voltage induced in the probe coil by the
axial component of the magnetic field as measured in 4.2, shall fall
within the acceptable region of Fig 4A or Fig 4B (see 4.4.1 and 4.4.2),
over the frequency range 300-to-3300 Hz.
4.4.1 For receivers with an axial component which exceeds -19 dB
relative to 1 A/m, when measured as specified in 4.1.2, the frequency
response shall fall within the acceptable region of Fig 4A.
4.4.2 For receivers with an axial component which is less than -19 dB
but greater than -22 dB relative to 1 A/m, when measured as specified
in 4.1.2, the frequency response shall fall within the acceptable
region of Fig 4B.
eCFR graphic ec02jn91.028.gif
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eCFR graphic ec02jn91.029.gif
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eCFR graphic ec02jn91.030.gif
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eCFR graphic ec02jn91.031.gif
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(A1) EIA Standard RS-464, Private Branch Exchange (PBX) Switching
Equipment for Voiceband Applications.
(A2) EIA Standard RS-478, Multi-Line Key Telephone Systems (KTS) for
(A3) EIA Standard RS-470, Telephone Instruments with Loop Signaling for
(A4) EIA Project Number PN-1361, Environmental and Safety
Considerations for Voice Telephone Terminals.
(A5) Federal Communications Commission Rules and Regulations, part 68,
Connection of Terminal Equipment to the Telephone Network.
(A6) IEEE Standard, Method for Measuring the Magnetic Field arould a
Telephone Receiver. (to be published)
[ 49 FR 1363 , Jan. 11, 1984, as amended at 61 FR 42187 , Aug. 14, 1996]
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