Sec. 68.316  Hearing aid compatibility magnetic field intensity 
          requirements: Technical standards.

    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

1  INTRODUCTION
2  SCOPE
3  DEFINITIONS
4  TECHNICAL REQUIREMENTS
4.1  General
4.2  Axial Field Intensity
4.3  Radial Field Intensity
4.4  Induced Voltage Frequency Response

Appendix A--Bibliography

                          List of Illustrations

                              Figure Number

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 Aids

    (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.)

                             1 Introduction

    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 this 
standard.
    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  Scope

    2.1  The purpose of this document is to establish formal criteria 
defining the magnetic field intensity presented by a telephone to

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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 
operating conditions.
    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.

                             3  Definitions

    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 
couple.
    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 induction-loop 
systems.
    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 
would coincide.

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                        4  Technical Requirements

    4.1  General.
    These criteria apply to handsets when tested as a constituent part 
of a telephone.
    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 4.4.1).

    4.3  Radial Field Intensity.
    When measured as specified in 4.1.2, radial components of the 
magnetic field as measured at four points 90 deg. 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 2).
    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.

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                        Appendix A--Bibliography

    (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 
Voiceband Applications.
    (A3) EIA Standard RS-470, Telephone Instruments with Loop Signaling 
for Voiceband Applications.
    (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]