Effects of frequency compression and frequency transposition on fricative and affricate perception in listeners with normal hearing and mild to moderate hearing loss

Abstract

Objectives: The authors have demonstrated that the limited bandwidth associated with conventional hearing aid amplification prevents useful high-frequency speech information from being transmitted. The purpose of this study was to examine the efficacy of two popular frequency-lowering algorithms and one novel algorithm (spectral envelope decimation) in adults with mild to moderate sensorineural hearing loss and in normal-hearing controls.

Design: Participants listened monaurally through headphones to recordings of nine fricatives and affricates spoken by three women in a vowel-consonant context. Stimuli were mixed with speech-shaped noise at 10 dB SNR and recorded through a Widex Inteo IN-9 and a Phonak Naída UP V behind-the-ear (BTE) hearing aid. Frequency transposition (FT) is used in the Inteo and nonlinear frequency compression (NFC) used in the Naída. Both devices were programmed to lower frequencies above 4 kHz, but neither device could lower frequencies above 6 to 7 kHz. Each device was tested under four conditions: frequency lowering deactivated (FT-off and NFC-off), frequency lowering activated (FT and NFC), wideband (WB), and a fourth condition unique to each hearing aid. The WB condition was constructed by mixing recordings from the first condition with high-pass filtered versions of the source stimuli. For the Inteo, the fourth condition consisted of recordings made with the same settings as the first, but with the noise-reduction feature activated (FT-off). For the Naída, the fourth condition was the same as the first condition except that source stimuli were preprocessed by a novel frequency compression algorithm, spectral envelope decimation (SED), designed in MATLAB, which allowed for a more complete lowering of the 4 to 10 kHz input band. A follow-up experiment with NFC used Phonak's Naída SP V BTE, which could also lower a greater range of input frequencies.

Results: For normal-hearing and hearing-impaired listeners, performance with FT was significantly worse compared with that in the other conditions. Consistent with previous findings, performance for the hearing-impaired listeners in the WB condition was significantly better than in the FT-off condition. In addition, performance in the SED and WB conditions were both significantly better than in the NFC-off condition and the NFC condition with 6 kHz input bandwidth. There were no significant differences between SED and WB, indicating that improvements in fricative identification obtained by increasing bandwidth can also be obtained using this form of frequency compression. Significant differences between most conditions could be largely attributed to an increase or decrease in confusions for the phonemes /s/ and /z/. In the follow-up experiment, performance in the NFC condition with 10 kHz input bandwidth was significantly better than NFC-off, replicating the results obtained with SED. Furthermore, listeners who performed poorly with NFC-off tended to show the most improvement with NFC.

Conclusions: Improvements in the identification of stimuli chosen to be sensitive to the effects of frequency lowering have been demonstrated using two forms of frequency compression (NFC and SED) in individuals with mild to moderate high-frequency sensorineural hearing loss. However, negative results caution against using FT for this population. Results also indicate that the advantage of an extended bandwidth as reported here and elsewhere applies to the input bandwidth for frequency compression (NFC/SED) when the start frequency is ≥4 kHz.

Figure 1

Mean 1/3-octave spectra displayed in terms of SNR for the unprocessed stimuli after mixing with speech-shaped noise at 10 dB SNR. Spectra are averaged over 15 exemplars per VC (3 female talkers times 5 renditions).

Figure 2

Average gain applied to the stimuli for each condition (different symbols) compared to the prescribed DSL targets (solid line). FT-off (NR) corresponds to the recordings made through the Inteo with frequency transposition deactivated and noise reduction activated. Symbols for NFC-off (+) and SED (○) overlap because SED stimuli were processed offline and recorded through the hearing aid with NFC-off.

Figure 3

Average pure tone thresholds of the HI listeners measured using the Sennheiser headphones (filled circles) are plotted alongside the distribution of 1/3-octave band levels for amplified speech energy in the wideband conditions. The solid and dashed lines represent conditions generated using recordings from the Inteo and Naída hearing aids, respectively. The thick line in the center of each distribution corresponds to the amplified long-term average speech spectrum of the VCs and the top and bottom thin lines correspond to the peaks (1^st percentile) and the valleys (70^th percentile), respectively.

Figure 4

A spectrogram of the carrier sentence “children like strawberries” and the stimulus “eeSH” before (top) and after (bottom) processing with frequency transposition. Arrows highlight the phonemes with significant high-frequency energy, which are most likely to be affected by the frequency-lowering settings used in this study. See text for details.

Figure 5

A spectrogram of the carrier sentence “children like strawberries” and the stimulus “eeSH” before (top) and after (bottom) processing with nonlinear frequency compression. See text for details.

Figure 6

A comparison of the frequency input-output functions from the nonlinear frequency compression (NFC, solid line) and spectral envelope decimation (SED, dotted line) algorithms.

Figure 7

Same as Fig. 5, but for spectral envelope decimation (SED).

Figure 8

An example of the hearing aid response for the Phonak Naída UP with NFC-off when analyzed with a 256-point FFT (HA, solid line) and the response for the high-pass filtered source (HP, dashed line) for the concatenated carriers and VCs for the stimulus /is/ that together formed the wideband stimulus.

Figure 9

Mean proportion correct for the NH listeners (light-gray bars) and HI listeners (dark-gray bars) for conditions involving the Widex Inteo hearing aid with frequency transposition activated (FT) and deactivated (FT-off) as well as with noise reduction activated (NR) or deactivated (No NR). By default, NR was activated when FT was activated. The wideband (WB) control was created by mixing the recordings from the FT-off condition and the original source signal after high-pass filtering. Error bars represent the standard error of the mean.

Figure 10

Mean proportion correct for the NH listeners (light-gray bars) and HI listeners (dark-gray bars) for conditions involving the Phonak Naída V UP hearing aid with nonlinear frequency compression activated (NFC) and deactivated (NFC-off). Spectral envelope decimation (SED) was processed offline and recorded through the hearing aid in its NFC-off setting. Whereas NFC lowered frequencies up to 6 kHz, SED lowered frequencies up to ~8.6 kHz in the band pass range of the hearing aid. The Wideband (WB) control was created by mixing the recordings from the NFC-off condition and the source signal after high-pass filtering. Error bars represent the standard error of the mean.

Figure 11

Scatterplot showing the proportion of correct responses for HI listeners in the follow up experiment where stimuli were recorded through a Phonak Naída V SP hearing aid with NFC activated (NFC) and deactivated (NFC-off). The slope of the linear least squares regression fit to the data (solid line) is significantly less than 1.0, indicating that listeners who performed more poorly with NFC-off tended to show the most improvement with NFC.