Recognition performance for four combinations of FM system and hearing aid microphone signals in adverse listening conditions

Objective: Children with moderate to severe hearing loss routinely use personal frequency modulated (FM) systems in the classroom to improve the signal to noise ratio of teacher-directed speech with notable success. Attention is now being given to the ability of these children to hear other students via the hearing aid (HA) microphone while using an FM system. As a result, a variety of FM system and HA microphone combinations have been recommended for classroom use. To date, there are no studies regarding the efficacy of these FM/HA combinations. The purpose of this study was to evaluate recognition performance using four FM/HA combinations and to characterize that performance for stimuli received primarily through FM system and HA microphone transmission. Design: Recognition performance for FM system and HA microphone signals was evaluated for two symmetrical and two asymmetrical FM/HA combinations using two commercially available FM systems (one conventional and one FM-precedence circuit). Eleven children (ages 9 to 12) with moderate to severe sensorineural hearing loss and eight children (ages 10 to 11) with normal hearing served as subjects. The two symmetrical FM/HA combinations included: 1) binaural FM system and HA microphone input using the conventional FM system, and 2) binaural FM and HA input using the FM-precedence circuit. The conventional FM system was used for the two asymmetrical combinations and included: 1) binaural FM input and monaural HA input, and 2) FM input to one ear and HA input to the other. Stimuli were 33 consonants presented in the form of nonsense syllables. The stimuli were presented through three loudspeakers representing a teacher and two fellow students in a classroom environment. Speech shaped noise was presented through two additional loudspeakers. Results: In general, no statistically significant differences in recognition performance were found between any of the FM/HA combinations. Mean recognition scores for HA microphone transmission (55%) were significantly poorer than those for FM system transmission (75%). As expected, initial consonants were more easily recognized than final consonants via FM system and HA microphone transmission. However, voiceless consonants were more easily recognized than voiced consonants via HA microphone transmission, which was not predicted on the basis of previous research. Conclusions: These results suggest that a certain amount of flexibility is present when choosing an FM/HA combination. However, recognition performance via the HA microphones was consistently poorer than performance via FM transmission. Because relevant material also originates from fellow students (e.g., answering teacher-directed questions), input via the HAs is often as important as information originating from the teacher. The results suggest that attempts to improve performance for signals transmitted through the HA microphones in a classroom setting would benefit children with hearing loss.