(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 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.

2.1The 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 operating conditions.

2.2A 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.

2.3The 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).

2.4The 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.

2.5The 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.

3.1A 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.

3.2The 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.

3.3A 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.

3.4The 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).

3.5The measurement plane is parallel to, and 10 mm in front of, the reference plane (see Fig 1).

3.6The 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).

3.7The 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.

4.1General.

These criteria apply to handsets when tested as a constituent part of a telephone.

4.1.1Three 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.

4.1.2The 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).

4.1.3When 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.

4.2Axial 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).

4.3Radial Field Intensity.

When measured as specified in 4.1.2, radial components of the magnetic field as measured at four points 90° 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).

4.4Induced 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.

4.4.1For 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.

4.4.2For 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.