The effect of crossover frequency on aided speech perception in the presence of environmental sounds

Since its introduction several years ago, multichannel signal processing has become a nearly ubiquitous component of programmable and digital hearing aids. Rapid development of sophisticated multichannel circuits has proceeded well ahead of sound clinical techniques to implement this new technology. Splitting the incoming acoustic signal into as few as two independent high and low frequency bandpass filters (channels) can provide significant perceptual benefits for some hearing aid wearers but no empirically derived relationship has ever been found between a given set of bandpass filter settings and improved speech perception in noise. More specifically, adjustments to the crossover frequency at which the bandpass filters intersect, has never conclusively been shown to improve speech perception in noise. This might have been because the area of crossover frequency settings in and of itself has never received a great deal of attention.; The primary purpose of this study was to determine whether speech perception is significantly affected by changing the crossover frequency of a two-channel hearing aid across different sound environments. Those environmental sounds included: the steady state low frequency engine of a jet in flight, the slowly modulating wideband energy of ocean waves breaking on a beach and the high frequency transient bursts of rain hitting a tin roof. Nine participants were given the Hearing in Noise Test (HINT) and the Four Alternative Auditory Features (FAAF) test in the presence of each environmental sound. They were all tested wearing pairs of two-channel digital hearing aids with the crossover frequency set each of four ways: wideband, 800 Hz, 1600 Hz or 3200 Hz.; Statistically significant group differences were found for both the 1600 Hz and 3200 Hz settings over the wideband condition on the HINT in the rain. Comparison of the HINT scores for each of the nine participants indicated that the 1600 Hz setting was superior to all others in both the rain and jet sounds. The wideband setting led to the poorest results in the rain and the 800 Hz crossover was poorest in the jet environment. This experiment showed that speech perception is significantly affected by the interaction between crossover frequency settings and listening environments.