Relative importance of different spectral bands to consonant identification: relevance for frequency transposition in hearing aids.

Listeners with high-frequency dead regions (DRs) benefit from amplification of frequencies up to 1.7 times the edge frequency, f(e), of the DR. Better consonant identification might be achieved by replacing the band from f(e) to 1.7f(e) with a higher spectral band. We aimed to identify the optimal band, using simulations with normal-hearing listeners. In experiment 1, nonsense syllables were lowpass filtered to simulate DRs with f(e) of 0.5, 0.75, and 1.0 kHz. Identification was measured for each of these base bands alone and with a bandpass-filtered band added (but not transposed). The added band either extended from f(e) to 1.7f(e) or its center frequency was increased, keeping bandwidth fixed in ERB(N)-number. Performance improved with increasing center frequency and then reached an asymptote or declined. Experiment 2 used a mid-frequency base band, and a lower-frequency added band. The results also showed a beneficial effect of frequency separation of the added and base bands. Experiment 3 resembled experiment 1, but with bandwidth fixed in Hertz. For higher-frequency added bands, the benefit was lower than for experiment 1.

Evaluation of a frequency transposition algorithm using wearable hearing aids.

Transposition of acoustic information from higher to lower frequencies may help people with severe or profound high-frequency hearing loss, especially when a 'dead region' is present. Previously, we (Robinson et al, 2007 ) evaluated the benefit of an FFT-based transposition algorithm in a laboratory study. Although results were promising, we hypothesized that further training and exposure would be needed to gain the full benefit. This was tested here by implementing the algorithm in wearable digital hearing aids. Five subjects with high-frequency dead regions used the aids for five weeks. Performance on the transposing and control conditions was compared objectively using speech tests (vowel-consonant-vowel, 's' detection, and speech in noise) and subjectively using questionnaires. Overall, the results showed no benefit with the transposition even after experience. Subjective preference was generally for the control condition.

Using transposition to improve consonant discrimination and detection for listeners with severe high-frequency hearing loss.

Transposition of high-frequency information to lower frequencies may help people with high-frequency hearing loss associated with a 'dead region' (DR) to detect and identify certain consonants, such as 's'. Conventional high-frequency amplification is often not beneficial in such cases. We designed and evaluated a new transposition algorithm which was adapted to each subject's high-frequency DR. Frequency components from well within the DR were transposed to just within the DR without applying frequency compression. Low-frequency components were amplified, but unaffected by transposition. Transposition only occurred if there was significant high-frequency energy, preventing high-frequency background noise of moderate level from being transposed. Consonant discrimination was tested using vowel-consonant-vowel (VCV) stimuli, and the detection of word-final 's' and 'z' was assessed using word pairs. Seven subjects with high-frequency DRs were tested in quiet using a transposed and a control condition. Following transposition, two subjects improved significantly and none performed significantly worse on the VCV-test overall. The perception of affricates was consistently improved. Averaged across subjects, the detection of word-final 's' and 'z' was significantly improved, with five subjects improving significantly individually.