Programming the Telecoil: A Case Study

A hearing aid telecoil enables a hearing aid user to listen on the telephone without acoustic feedback by taking advantage of the magnetic field associated with the signal emanating from the telephone.[1] Although clinicians carefully adjust the acoustic characteristics of hearing aids in the microphone mode in an attempt to improve face-to-face speech understanding, very little attention has been given to the output characteristics of telecoils and how well individuals with hearing loss understand speech over the telephone when using telecoils.

Fikret-Pasa and Garstecki[2] used probe microphone measures to examine the frequency response of various telephone amplifiers in an ear simulator. They found that the frequency response of the telephone amplifiers varied dramatically in terms of spectrum and input-output functions. Their study did not use hearing aids but rather separate telephone amplifiers or amplified telephones. Mueller and Bryant[3] described a method using probe microphone measurements to evaluate the telecoil response of the hearing aid in the ear canal. They suggested using either an area induction loop installed in a room or a neck loop or silhouette receiver. As the authors noted, however, for the method to have face validity, the strength of the magnetic field produced by the induction loop, neck loop, or silhouette receiver would need to be similar to that produced by the device used by the hearing aid user. The electromagnetic signal emanating from a telephone may or may not be similar to that from a loop or silhouette receiver.

Tannahill[4] reported poorer word recognition with telecoil signals compared with acoustic signals. He presented recorded words delivered to two different hearing aids and two different telephone handsets to normal-hearing listeners. The frequency response of the telecoil signal was narrower than the acoustic telephone response and had lower output when a standard telephone receiver was used. Tannahill recommended that when deciding on a hearing aid, clinicians should make sure that there is sufficient reserve gain in the hearing aid so that the volume can be turned up for use on the telephone. Plyler and associates[5] compared word recognition using acoustic versus electromagnetic coupling and found no significant difference between the two types of coupling. Subjects were allowed to adjust the volume on their hearing aid in each coupling condition. Ideally, hearing aid users should be able to pick up a telephone and not have to adjust the volume on their hearing aid to be able to understand speech over the telephone.

Until recently, there was no way to program the telecoil response independently from the acoustic response of a hearing aid. Hearing aid users had to switch their hearing aids manually to the telecoil mode and often needed to increase the volume setting on their hearing aids to receive an adequate signal.[1] Many newer hearing aids now allow programmable adjustment of the level and frequency response of the telecoil. In addition, some manufacturers offer a telecoil that enables the hearing aid user to access the telecoil without having to manually switch to the telecoil mode. The magnet within the telephone receiver serves to switch the hearing aid to the telecoil mode automatically.

Although the capability of programming the overall gain and frequency response of the telecoil is available on some hearing aids, there is no standardized method of setting these parameters. In addition, setting the telecoil response is often problematic in a clinic because there are many factors that influence real-world speech communication via the telephone, such as the characteristics of the listener’s telephone as well as the talker’s telephone, variability in transmission line characteristics, variability in the position of the telephone handset relative to the telecoil, and interference from other electromagnetic sources. Despite these challenges, it may be valuable to program a patient’s telecoil response based on coupler measurements. The data from one such individual are presented below.

REFERENCES

• 1 Skinner M W. Hearing Aid Evaluation. Englewood Cliffs. NJ; Prentice Hall 1988: 222-223
• 2 Fikret-Pasa S, Garstecki D C. Real-ear measures in evaluation of frequency response and volume control characteristics of telephone amplifiers.  J Am Acad Audiol. 1993;  4 5-12
• 3 Mueller H G, Bryant M P. Some commonly overlooked uses of probe microphone measures.  Semin Hear. 1991;  12 73-91
• 4 Tannahill J C. Performance characteristics for hearing aid microphone versus telephone and telephone/telecoil reception modes.  J Speech Hear Res. 1983;  26 195-201
• 5 Plyler P N, Burchfield S B, Thelin J W. Telephone communication with in-the-ear hearing aids using acoustic and electromagnetic coupling.  J Am Acad Audiol. 1998;  9 434-443
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• 8 Byrne D, Parkinson A, Newall P. Modified hearing aid selection procedures for severe/profound hearing losses. In: Studebaker GA, Bess FH, Beck LB The Vanderbilt Hearing-Aid Report II Parkton, MD; York Press 1991: 295-300
• 9 American National Standards Institute .Specification of Hearing Aid Characteristics (ANSI S3.22-1996) New York, NY; ANSI 1996: 1-21
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