Study of the Interaction of 
Wireless Phones and Hearing Aids


Executive Summary



Background

The Center for the Study of Wireless Electromagnetic Compatibility was established at the University of Oklahoma, School of Industrial Engineering on the Norman Campus in Fall 1994. Its charter is to work with industry and government to investigate and resolve interaction issues between wireless phones and other electronic devices. The Center began its initial examination of the interaction issues between wireless phones and hearing aids in May 1995. The Hearing Aid Clinical Study reported here was conducted in cooperation with the Hough Ear Institute in Oklahoma City, Oklahoma.

This report presents the results of Phase I (clinical trials) of the Hearing Aid - Wireless Phone Interaction Study completed by the University of Oklahoma Wireless EMC Center. Phase I results were presented on April 29, 1996 to representatives of hearing aid user groups, wireless service providers and manufacturers, hearing aid manufacturers and the Federal Communications Commission. To date in the United States, this clinical study is the most comprehensive scientific effort to involve a diverse group of hearing aid users to determine the degree of interaction between hearing aids and wireless phones. This study has identified several factors that contribute to the interaction and has also demonstrated the complexity of the problem.

Scope of the Clinical Trials

Phase I of the Hearing Aid Study focused on testing 78 people (68 hearing aid wearers and 10 people with unimpaired hearing) to (1) evaluate the degree of interaction between wireless phones and hearing aids, (2) document the existence and relative severity of the interaction as a function of hearing aid type, hearing loss configuration and wireless phone technology, and (3) determine the effectiveness of proposed solutions such as shielding the hearing aid and shielding the phone antenna.

Three phone technologies were studied during the Phase I clinical trials: (1) 1900 MHz PCS (J-STD-007), (2) 800 MHz TDMA (IS-54), and (3) 800 MHz CDMA (IS-95), where the numbers in parentheses refer to the specific industry standard for the modulation scheme used.

Two interference measures, speech recognition (words identified correctly from a standard audio-taped word list) and annoyance rating (0 to 5 scale; 0-no interference to 5-unbearable), were used to determine the degree of interference when hearing aid wearers were exposed to a digital phone at a 2 cm distance (less than 1 in). Two additional measures, detection threshold (the distance at which a hearing aid user detects interference, not necessarily annoying) and annoyance ratings at fixed distances between 25 and 300 cm (10 in to 10 ft), were used to determine the interference to a hearing aid wearer due to bystander use of a wireless phone.

Results

All phones were tested in their worst-case interference mode (highest operating power and CDMA with a variable vocoder rate) to determine the maximum potential interference, realizing that phones operate at varying power levels, all of which are less severe as an interference source compared to full-power operations. Caution must be exercised in using these results to directly contrast one phone technology with another, due to differences in the frequency bands used and differences in the implementation of these technologies. Hence, the results should be interpreted carefully.

A statistical analysis of the clinical data was performed and the following general conclusions can be drawn from the Phase I results:

1. All three phone technologies tested (J-STD-007, IS-54, IS-95) interfered in many, but not all, instances with hearing aids with respect to all four interference measures -- speech recognition, annoyance rating at 2 cm, bystander detection threshold, and bystander annoyance ratings (25 to 300 cm).

2. Hearing aid wearers did not report any interference while using analog cellular phones.

3. Unimpaired hearing participants responded in a very different fashion from hearing aid users in the speech recognition and annoyance tests. Hence, their use as test subjects may be inappropriate in examining hearing aid interference, developing standards, and evaluating solutions.

4. The following factors were found to generate statistically reliable differences in interference:

  • phone technology (J-STD-007, IS-54, IS-95)
  • hearing aid type (behind-the-ear (BTE), in-the-ear full shell (ITE), in-the-ear half shell (HS), in-the-canal (ITC), and completely-in-canal (CIC))
  • hearing loss configuration (flat, sloping, ski slope, rising)
  • hearing loss severity (mild, moderate, moderately severe, severe, and profound).

5. In more than 80 percent of the tests, hearing aid users did not experience any interference unless the phone was within one meter (3.3 ft). In fact, in 16 percent of the tests, participants reported no interference even when the phone was at a distance of less than 4 inches (10 cm).

6. On the average, hearing aid users in the study did not experience any annoyance unless the phones were within two feet of the hearing aid. Only 2 percent of the tests at one meter (3.3 ft) and 12 percent of the tests at a half meter (1.6 ft) resulted in annoying interference (rating of 3 or greater). However, the results varied by hearing aid type, hearing loss configuration and phone technology.

7. Among the three phone technologies tested in the maximum interference configuration (i.e., full power which is not typical of their operation in normal use), CDMA-800 phones (IS-95) tested with a variable vocoder rate resulted in lower interference across all measures.

8. Among hearing aid types, BTE users experienced the most interference, while ITC users experienced the least interference.

9. Among hearing loss configurations, ski slope hearing loss participants experienced the least interference from RF signals.

10. Shielding the BTE hearing aids with a metallic coating effectively reduced bystander interference at all distances.

11. Placing a copper shield between the phone antenna and the hearing aid reduced interference. The technical feasibility and manufacturability of any shielding and its impact on phone and system performance have not been evaluated.

12. The perception of interference increased with an increase in hearing loss severity.

13. In addition to the factors identified in conclusion 4, the following interaction (combination) effects were also statistically significant:

  • phone technology and hearing aid type
  • hearing loss configuration and hearing aid type.

For example, participants with BTE hearing aids noticed comparatively greater interference from PCS-1900 (J-STD-007) phone signals. Similarly, CIC users reported greater than expected interference from TDMA-800 (IS-54) phone signals.

Future Research

While the clinical study is complete, the data should be viewed as “first phase” results and not typical of actual phone use. The research focused on evaluating worst-case interaction conditions for hearing aids and phone technologies. Additional investigation is required to test these three phone technologies and others under “normal” operating conditions which users might experience.

Phase II of the Hearing Aid - Wireless Phone Interaction Study involves both instrument-based and hearing aid user testing with an emphasis on identifying the mechanism of the interaction and leading to the development of standards for hearing aid immunity and phone emissions, and the evaluation of solutions. Phase II also involves quantification of the subjective annoyance ratings of hearing aid users in terms of objective acoustic measurements. When completed, the study will have evaluated most North American digital phone technologies and types of hearing aids.

   
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