Electromagnetic Compatibility

Daniel D. Hoolihan

Forthcoming standard ANSI C63.19 ought to clarify issues of compatibility between hearing aids and wireless telephones used by hearing-aid wearers.

As U.S. baby boomers age, their use of personal hearing aids for sound amplification will surely increase. At the same time, the high-technology device called a cellular telephone has been increasing in popularity as its price comes down and its perceived usefulness grows. (The United States had more than 76 million cellular telephone subscribers in 1999, up from 61 million in 1998 and up 74 million in 10 years.) These developments have led the electromagnetic compatibility engineering community to begin to look at the interaction between the two electronic devices. Its investigation has been encouraged by two key U.S. agencies, the Food and Drug Administration and the Federal Communications Commission (FCC).

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 draft final version of a standard this year. The standard is out for vote by the full C63 committee at press time. Two objections have been received and are presently being resolved.

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

Draft Standard ANSI C63.19

The draft ANSI standard has been numbered C63.19 and is 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 telecommunication 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. Draft standard C63.19, in its "Overview," announces a frequency range of interest of 800–3000 MHz (800 MHz to 3 GHz), although the current version focuses on the two bands of 800–950 MHz and 1600–2000 MHz (or 1.6–2.0 GHz).

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 draft standard calls for wireless communications devices to be measured for

• RF electric-field emissions.
• RF magnetic-field emissions.
• T-coil (telecoil) mode, magnetic-signal strength in the audio band.
• T-coil mode, magnetic-signal frequency response through the audio band.
• T-coil mode, magnetic-signal and noise articulation index. The hearing aid must be measured for
• RF immunity in microphone mode.
• RF immunity in T-coil mode.

T-coil is an abbreviation for telecoil, an inductive coil used in some hearing aids to allow reception of a magnetic-field signal instead of an acoustic signal. The magnetic, or inductive, mode of reception is commonly employed in conjunction with telephones, auditorium loop systems, and other systems that provide the required magnetic-field output.

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. Draft 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.

Figure 1. Magnetic field measurement test setup.

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 draft 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.

• Annex A: Definitions of Reference Axes.
• Annex B: Acoustic Test Frequencies.
• Annex C: Equipment and Setup Calibration.
• Annex D: Test Equipment Specifications.
• Annex E: Sample Measurement Uncertainty Estimates.
• Annex F: Use of Helmholtz Coils for Calibration.
• Annex G: RF Envelope Comparison for U.S. WD Systems.
• Annex H: Explanation of Rationales Used in This Standard.
• Annex I: Alternative Test Method for RF Immunity.

Wireless Device—RF Emission Test. The first major technical section of proposed 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.

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.

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.

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

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

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."

Table I. Hearing aid and cell phone near-field categories as defined in draft ANSI C63.19. During testing, the hearing aid must maintain an input-referenced interference level of less than 55 dB and a gain compression of less than 6 dB.

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.

Acceptable Interoperability

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.

Table II. System classification by joint performance of hearing aid and wireless device, as defined in draft ANSI C63.19. The U-category sum is the sum of the numbers of the categories in which any two devices in question belong.

Conclusion

This draft of ANSI C63.19 represents more sthan four years of sustained effort by many individuals from the hearing-aid and wireless-communications device industries. 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.

Daniel D. Hoolihan was formerly vice president of Minnesota Operations for TÜV Product Service Inc. (New Brighton, MN). He is the founder of Hoolihan EMC Consulting (Lindstrom, MN). He can be e-mailed at hoolihan@emcxpert.com.

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