Hearing Aid Research 

 

green marblePhase IIIB: Subjective Validation Study
 
          redballObjectives
                              - The overall objective of the current research was to relate subjective ratings of speech intelligibility, usability, and annoyance to acoustic measurements of the interference between wireless phones and hearing aids.

Evaluation of the Interaction Between
Wireless Phones and Hearing Aids
Phase IIIB: Subjective Validation Study


Executive Summary

Background 

In April 1996, ANSI ASC C63 established a task group under its subcommittee on medical devices (SC 8) to develop a standard documenting the methods of measurement and defining the limits for electromagnetic compatibility between wireless phones and hearing aids. In 1999, the committee provided a draft document C63.19-xxxx, American National Standard for Methods of Measurement of Compatibility between Wireless Communications Devices and Hearing Aids. In previous work, the University of Oklahoma Center for the Study of Wireless Electromagnetic Compatibility conducted an acoustic measurement-based evaluation of the standard (commonly referred to as the objective study). The purpose of the objective study was to determine correspondence of hearing aid immunity and wireless phone emission levels measured according to the standard with actual interference obtained with specific combinations of hearing aids and phones. 

The overall objective of the current research was to relate subjective ratings of speech intelligibility, usability, and annoyance to acoustic measurements of the interference between wireless phones and hearing aids. The C63.19 subjective validation study is subdivided into two parts: (1) subjective ratings of speech intelligibility, annoyance, and usability, and (2) objective acoustic measurements of hearing aid response. 

Methodology 

Hearing aids were custom manufactured for eighteen hearing-impaired participants. The aids spanned a range of immunity levels from no interference to severe interference. The participants rated the effects of the interference experienced when using five digital wireless phone technologies (CDMA at 800 and 1900 MHz, TDMA-50 Hz at 800 and 1900 MHz, and TDMA-217 Hz at 1900 MHz) at five transmission power levels (0, 6, 12, 18, and 24 dBm). 

Conclusions 

Based on acoustic measurements with three wireless phones, the aids for six participants demonstrated little or no interference at either 800 MHz or 1900 MHz. The data for one of these participants was not usable due to a moderately severe hearing loss coupled with major feedback from the hearing aid making the phones unusable. Unrelated to wireless phone RF interference, this person had difficulty understanding any speech through the phone. The remaining five of these six participants appropriately reported no annoying interference and 100% speech intelligibility. 

For the remaining twelve aids, there was often a difference in the level of immunity between the two RF carrier frequency bands. The aids showing interference were categorized as producing either moderate or severe interference in each band. In general, the level of correspondence between the subjective ratings and the measured interference was primarily a function of the immunity of the aid. Seven participants demonstrated high correspondence between the ratings and the interference as a function of changes in phone power level. These participants experienced moderate to severe interference in their aids at either 800 MHz or 1900 MHz or both. Three of these aids had telecoil capability. In this mode, interference was greater and seriously affected speech intelligibility and annoyance. Another participant, whose aid produced minimal interference in microphone mode and substantial interference in telecoil mode, provided constant ratings across power levels when tested in microphone mode and appropriately poor ratings when in telecoil mode. Another participant, whose aid produced significant interference in both modes at 1900 MHz, had a severe hearing loss and was not able to provide usable data due to poor speech intelligibility and high feedback with all phones. 

The final three participants demonstrated little or no change in rating response as a function of power level. This was believed to be due to low levels of interference at 800 MHz or 1900 MHz (but not both) or high levels of feedback as the phone is coupled to the ear. 

Thus, thirteen of the eighteen participants responded as predicted by the acoustic measurements made with actual wireless phones. The remaining participants experienced difficulties unrelated to wireless phone interference (severe hearing loss or excessive feedback). While all aids having a high immunity classification based on the C63.19 standard (TUV data) were included in this group, it is important to point out that other aids with a lower TUV immunity classification also performed well. This points to the need for further refinement of the immunity measurement and classification procedures. 

A proposed alternative to the existing C63.19 performance criteria and categories is presented in the following table. 

 
Hearing Aid ILM 55
WD Emissions
Category
E-Field
CW dB V/m
H-Field
CW dB A/m
E-Field
CW dB V/m
H-Field
CW dB A/m
C0
< 30
< -25
> 45
> -10
C1
30 to 40
-25 to -15
35 to 45
-20 to -10
C0
> 40
> -15
< 35
< -20
Although the resulting summation of assigned category values will range from 0 through 4, scores can be combined with the interpretation shown below. 
Sum
System Classification
0,1
Bad Performance
2
Intermediate Performance
3,4
Good Performance
There are three primary advantages to the use of the proposed categorization scheme: 
  •  There are fewer classification categories for hearing aids and for wireless devices. This simplifies the assignment process and the system performance classification for manufacturers and consumers. 
  •  The proposed split points, which employ a 10 dB spread rather than 5 dB, more accurately reflect the precision that is readily achievable in both the acoustic and subjective measurements (in the absence of any reliability data to the contrary).
  •  The proposed categories provide no worse agreement (and in some cases better agreement) among the C63.19 measurements (TUV data), the measured acoustic data from our laboratory measurements, and the subjective data on annoyance, speech intelligibility, and usability. In other words, there is no loss in predictive accuracy by moving to the proposed categorization scheme using only three categories. Details of the comparisons are provided in the body of the report.
  • In summary, the results of this study support the use of acoustic measurements of immunity as the basis for the ANSI C63.19 standard. The results also demonstrate the existence of hearing aids that can be used with success with a number of digital phones. Six of the eighteen aids demonstrated no interference or very slight interference at the highest power level when used with both 800 MHz and 1900 MHz phones. These conclusions provide an encouraging forecast of substantially improved access to digital wireless service by those individuals with impaired hearing.
       
    Copyright © 2010 The Center for Study of Wireless Electromagnetic Compatibility, University of Oklahoma. All Rights Reserved.