Clinical measures of hearing aid directivity: assumption, accuracy, and reliability

A number of clinical measures of directivity, including the front-to-back ratio (FBR) and front-to-side ratio (FSR), have been suggested to audiologists to monitor the functionality of hearing aids with directional microphones. These suggestions, however, are based on the assumption that directivity measured clinically changes monotonically when compared with changes measured using the directivity index (DI) and perceptual directional benefit. The objective of the present study was to empirically examine this assumption. In addition, the reliability of the clinical directivity measure was estimated to establish a referral threshold for defective directional microphone hearing aids.

The directivity of the directional microphones of two behind-the-ear hearing aids was systematically degraded by plugging the microphone ports. The directivity was measured using four clinical measures: the FBR and FSR performed in the test chamber of a hearing aid analyzer and in sound field. Each measure was repeated four times in each directivity-degraded condition. The degraded directivity was also assessed using the DI measure in an anechoic chamber. The perceptual directional benefit in each directivity-degraded condition was obtained by testing 10 hearing-impaired adults in a sound field with diffuse noise using the Hearing in Noise Test (HINT).

The results of the DI and HINT measures showed strong correlation between the two FSRs (test chamber and sound field), while the two FBRs showed no correlation. The directivity generated by the FBRs could remain unchanged even when the directional microphone had lost more than 50% of its directivity. The results further indicated that the measures performed in the sound field were more reliable than those performed in the test chamber. Based on the results of the reliability measures, a 30% change in directivity was suggested as the referral threshold signifying defective directional systems.

Because the FSR predicts the DI and HINT measurements more accurately than does the FBR, it is suggested that clinicians use the FSR to monitor hearing aid directivity. By using the FSR measure and informed by the suggested referral threshold, clinicians would be able to correctly identify defective directional microphone hearing aids at an early stage, rather than at a point when the directivity has been diminished completely or even reversed.