A functional measure of front-to-back ratio

Monaural hearing aid users (aged 26-75 yrs; mn: 52 yrs) with moderate to severe bilateral sensorineural losses tracked free field sweep-frequency Bekesy thresholds from .25-4 kc/s in a typical audiological test booth. Each S was fitted, on the ear on which S usually wore an aid, with successively 3 directional microphone hearing aids (DMHAs) now commonly issued, with similar frequency response and gain characteristics but with a range, as shown by the manikin KEMAR in an anechoic chamber, of 8 db in FBR. Warbled tones were presented from a loudspeaker either 1 m directly ahead of or 1 m directly behind S's head. The difference in thresholds (180 degrees data subtracted from 0 degree data) in db was taken as functional front-to-back ratio (FBR) at selected frequencies. These were compared with FDRs from KEMAR. The functional FBR was shown to be sensitive enough (p less than .001) to differentiate among DMHAs, and the aids were on the average ordered similarly from highest to lowest by both KEMAR and functional FBRs. It was found that the data at 1 kc/s were predominately useful to test. However, individual differences were such that it was concluded that more reliance should be placed on a functional test of FBR. In addition, a significant Pearson r of 0.42 (p less than .01) was obtained between functional FBR and monosyllabic DS (S/N: 0 db). These results suggest that the functional FBR may play an important role in measuring hearing aid selection.