The American National Standards Institute (ANSI) Accredited Standards Committee C63 on Electromagnetic Compatibility was asked by representatives of the cellular phone and hearing aid industries to investigate the electromagnetic interaction between the electronic hearing aid and the wireless communications device (or cellular phone, referred to in this article as WD). The full committee voted to examine the situation and assigned the task to its Subcommittee 8 on EMC and Medical Devices. Subcommittee 8 formed a working group to take up the issue. Cochaired by Steve Berger of Siemens Business Communications Systems Inc. and Tom Victorian of Starkey Laboratories, the working group began meeting in 1996 and arrived at a final version of a standard in 2000. The standard was approved by the C63 committee and then published in the American National Standards Institute Standards Action for public review in November. No comments on the proposed standard were received during the 45-day public comment period. The standard is expected to be generally available in the second quarter of 2001.

This article itemizes elements of the standard and briefly describes its key technical sections.

The new ANSI standard is designated C63.19 and entitled "American National Standard for Methods of Measurement of Compatibility between Wireless Communications Devices and Hearing Aids." It encompasses both methods of measurement and definitions of limits for establishing hearing aid compatibility and the accessibility of wireless communications devices to wearers of hearing aids.

The document consists of an overview; a list of references; a glossary of definitions, acronyms, and abbreviations; three major technical sections entitled "Wireless Device—RF Emissions Test," "Hearing Aid RF Near-Field Immunity Test," and "Wireless Device Audio Band—Magnetic Signal Test"; sections on acceptable performance for interoperability, test equipment calibration and measurement uncertainty, and the test report; and a bibliography. Appended to the standard are four normative and five informative annexes, which are outlined in the discussion below.

Overview. ANSI standard C63.19, in its "Overview," announces a frequency range of interest of 800–3000 MHz (800 MHz to 3 GHz), although it focuses on the two bands of 800–950 MHz and 1600–2000 MHz (or 1.6–2.0 GHz). A revision already being planned will incorporate higher frequencies.

The primary purpose of the standard is to furnish the tests and parameters that will accurately predict the ability of a hearing-aid user to employ a wireless communications device. Its intent is to match hearing aids and WDs by category such that a desired level of performance is achieved. For example, a hearing aid with above-average radio-frequency (RF) immunity could be matched with a WD exhibiting below-average emission qualities, and the wearer of the hearing aid would be satisfied with the quality of hearing while enjoying the benefits of the WD.

The standard calls for wireless communications devices to be measured for

The standard is intended to apply to all types of hearing aids with acoustic output, including behind-the-ear, in-the-ear, in-the-canal, and completely-in-the-canal types.

Two principal conditions of exposure subject hearing aid users to undesirable RF emissions. A far-field condition reflects the type of field a hearing aid would experience if its wearer were standing next to someone using a WD. A near-field condition corresponds to the more intense fields that a hearing-aid wearer is susceptible to when using a cellular phone or other WD. ANSI C63.19 addresses the latter case, the near-field situation.

References. In this section, the standard lists 52 relevant documents ranging from other ANSI standards to International Electrotechnical Commission (IEC) standards to government standards developed by the FCC.

Definitions, Acronyms, and Abbreviations. Definitions of key words used in the standard appear in this section. Important acronyms pertinent to the cellular phone and hearing aid industries are also explained, including esoteric technical acronyms such as IRIL, which stands for input-referenced interference level, the equivalent acoustic input sound-pressure level—typically at 1 kHz—that would produce the same acoustic output in a hearing aid as that produced by an RF interference source.

Bibliography. Leaving aside for the moment the technical sections of the standard and skipping to the end, the standard concludes with a bibliography of 23 technical papers treating the challenges involved in measuring either hearing aids or WDs.

Annexes. The following nine annexes supplement the text of the standard. The first four are normative; that is, part of the standard's requirements. Annexes E–I are informative, which means that they add information but are not requirements of the standard. Together they constitute 45 pages of very valuable details regarding measurement of the devices concerned with reference to the parameters of interest in the standard.

Wireless Device—RF Emission Test. The first major technical section of ANSI standard C63.19 covers measurement of the near electric field and the near magnetic field generated by wireless communications devices. It is sufficient for the purposes of the standard to measure the devices at the frequencies at which each WD is capable of transmission as part of its normal operation. The measurements must take place in a laboratory that complies with ANSI C63.4, "American National Standard for Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz."

Two small probes (one E-field, one H-field) are used in conjunction with a probe-positioning system. The goal of this setup is a measurement uncertainty of ±2 dB. A 5 x 5-cm region, 1 cm from the surface of the WD, is controlled so as to be usable by a hearing aid (see Figure 1 for the test setup). To be usable, the hearing aid emissions must be minimized to avoid excessive self-interference. This 25-cm² area is searched via the probe-positioning system in order to find the highest RF emissions.

Figure 1. Magnetic field measurement test setup.

The standard describes detailed pretest and test procedures that include both a manual scanning method and an automatic scanning method.

Hearing Aid RF Near-Field Immunity Test. This part of the standard treats the method of measuring the level of immunity of a hearing aid to radiated electromagnetic fields originating from a wireless communications device. It specifies a near-field illumination technique because that is a more realistic simulation of the near-field condition in actual interaction between the hearing aid and the WD. The microphone-mode evaluation of the interference effect of RF emissions from WDs is procedurally very similar to that employed with the T-coil mode. However, in the T-coil mode, an evaluation of the effects of base-band interference sources must be made in order to fully appraise the signal quality a user would receive.

The test facility must have a reasonably low ambient for acoustic noise and must be an environment of moderate temperature and humidity. The hearing aid must be tested with a fresh battery installed, and the battery must be within ±5% of its rated voltage in a no-load condition.

The necessary test equipment consists of the same set of two dipoles that was used to measure the WD, plus a signal generator, a power amplifier, an RF directional coupler, an RF power meter, and microphones. Also needed are a microphone preamplifier, an ear coupler, a microphone calibrator, an audio signal generator, an acoustic transmission line, a hearing aid–immunity test fixture, and RF cables.

The standard provides a detailed test setup and a procedure for validating the experimental setup in order to ensure accuracy of results. The validation procedure includes a check of RF interference to test equipment, a characterization of tubing attenuation and resonances, an audio input-source setup (see Figure 2), and selected pretests taken from ANSI S3.22, "American National Standard, Specification of Hearing Aid Characteristics."

Figure 2. Setup of the near-field immunity test for hearing aids.

The planned revision of ANSI C63.19 would aim to improve the correlation between this test method and the use of a gigahertz transverse electromagnetic (GTEM) cell for immunity testing hearing aids.

Wireless Device Audio Band—Magnetic Signal Test. In this chapter, the standard describes the method for measuring the audio-band magnetic signals from the WD. Three quantities are measured and evaluated: first, the field intensity of the desired signal at the center of the audio band; second, the frequency response of the desired signal measured across the audio band; and third, the signal quality, which is defined as the difference between the desired and undesired magnetic field levels.

The equipment used is similar to that characterized in the previous section of the standard. The basic test configuration is shown in Figure 3.

Figure 3. Setup of the magnetic-field test for the WD audio band. The optional power supply is permitted for base-band signal measurements only; it is not allowed for noise measurements.

Calibration and Measurement Uncertainty. Under this heading, the standard describes the baseline conditions necessary for the measuring equipment and the ambient conditions of measurement. For example, the measuring instruments should be marked with the date of last calibration, date of next calibration, and validation initials or source and location of calibration records. The measurement uncertainty is calculated with reference to NIST Technical Note 1297, "Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results."

Test Report. The test report is the means by which the test results are presented to the appropriate procuring or regulatory agency or to the testing laboratory for archival purposes. The report is expected to include the test plan, the applicable standards, identification of the unit of equipment tested, the test configuration, a list of test equipment used (including calibration dates), units of measurement, location of the test site, measurement procedures employed, measurement data obtained, general and special test conditions, a summary of test results, required signatures, and test report annexes. The document is to be maintained by the testing organization for a period of at least three years following the date of the test.

The "Performance" chapter of the standard presents the performance requirements for acceptable interoperability of hearing aids with WDs. When these parameters are met, as determined by the tests described in the standard, a hearing aid operates acceptably in close proximity to a wireless communications device.

Research has shown that a signal-to-interference ratio of 20 dB provides a signal quality that is acceptable for normal operation of the hearing aid. A 10-dB degradation (that is, a signal-to-interference ratio of 10 dB) puts the performance into an area of unacceptability. A 10-dB improvement (from 20 to 30 dB) removes almost all the interference. Using these research data, the standard provides tables that categorize hearing aids and cellular phones and systems of the two on the basis of performance. The information in these tables is reproduced here as Tables I and II.

It can be seen from Table II, for example, that combining a low-immunity hearing aid in Category U0 with a Category U3 telephone that has low emissions produces a numerical score of 3, which means the combination is acceptable for normal use.

More than four years of sustained effort by many individuals from the hearing aid and wireless communications device industries have culminated in the publication of ANSI C63.19. The measurement techniques provided by the standard are quite complicated and exacting; they will challenge both captive laboratories of manufacturing companies and independent testing laboratories. But the end result of the application of this standard should be improved accessibility to modern communications devices for anyone wearing any type of hearing aid who wishes to communicate wirelessly.

A final note of interest: In December 2000, FCC released a public notice entitled "Wireless Telecommunications Bureau seeks comment on request to reopen the petition for rulemaking regarding hearing-aid-compatible telephones." The C63 Committee of ANSI commented on the notice by making the C63.19 standard its proposed answer to the challenge of the successful coexistence of cellular phones and hearing aids.