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Introduction
Introduction
This workbook contains procedures for testing FM systems with the 6500-CX hearing aid
analyzer using coupler and real-ear measurement methods. An FM system consists of two
main parts: a transmitter and a receiver. The transmitter microphone can come in many dif-
ferent sizes and shapes, and it can be omni-directional or directional. The body-worn FM
receiver can be coupled to a button earphone or interfaced with the personal hearing aid via
a direct audio input boot, silhouette coil, or via a neck loop. The ear-level FM receivers are
housed within the BTE aid or attached to the base of the BTE via a boot connection.
No matter the combination, all FM systems can be tested. These test procedures should be
performed on an FM system at least once a year. For children under the age of 5, these pro-
cedures should be performed every six months.
As with most hearing aids, both the gain and the maximum output (SSPL) of an FM system
should be adjusted to meet the listener’s auditory requirements.
There are several issues to consider when comparing the hearing aid and FM performance.
• With FM systems, the pickup microphone is normally worn at the chest, 6-8 inches
(15-20 cm) under the mouth of the talker, whereas with hearing aids, the pickup
microphone is worn by the listener, typically at a minimum of 3 ft. (1 m) from the
talker. This difference affects both the level and the frequency content of typical
input signals. With FM systems, typical speech input levels range from 75 to 85 dB
SPL, as compared with 60 to 70 dB SPL for hearing aids. With the chest-worn loca-
tion of the pickup microphone, the high frequencies are relatively lower in level
(about 5 dB at 5 kHz) and the low frequencies are relatively higher in level (about 5
dB at 500 Hz) as compared with directly in front of the talker’s mouth.
•
The increased “vocal effort” by teachers in a classroom situation may create further
changes to the speech spectrum at the input to an FM system, as compared with the
speech spectrum at the input of a hearing aid (Cornelisse et al., 1991).
•
FM systems often have an automatic gain control (AGC) or other nonlinear charac-
teristic that could interact with that of the hearing aid being used to deliver the FM
signal—if such a hearing aid is being used in this fashion.
•
The frequency response of an FM system is often not as adjustable as that of a hear-
ing aid, limiting the range of electroacoustic modifications available.
The procedures recommended in this chapter are based on the ASHA guidelines for fitting
and monitoring FM systems (1999), available at:
www.asha.org/audiology/fm_guidelines.htm.
