# A Design Method for Gammachirp Filterbank for Loudness Compensation in Hearing Aids
## 4 Experiments and Discussion
### Comparison of Speech Quality of the Reconstructed Signals  
To objectively evaluate the performance of the gammachirp filterbank, this study first designed a 16-channel gammachirp filterbank, and combined the channels to obtain 8-channel and 4-channel filterbanks according to an age-related hearing-loss audiogram. The speech test signals passed through the gammachirp filterbank and gammatone filterbanks, and we compared the quality of those output speech test signals.  
The test speeches were selected from the CASIA Chinese emotional corpus, which was recorded by the Institute of Automation, Chinese Academy of Sciences. It included four professional speakers and five kinds of emotion: angry, happy, sad, surprised, and neutral, with a total of 9600 different pronunciations. We randomly selected a total of 300 recordings from the corpus as experimental data, all of which were recorded by a speaker in a neutral emotional state. The sampling rate of the test signal was 16 kHz, the sampling precision was 16-bit, and the duration of each file was approximately 2 s.  
Two objective speech quality indices, PESQ and STOI, were applied to assess the quality of the output signals from the gammachirp and gammatone filterbanks with different numbers of channels, and those indices are compared in this section. STOI and PESQ are both closely related to human auditory perception and are widely used as evaluation criteria of voice quality. A higher value represents the higher intelligibility of the processed speech.  
Figures 4 and 5 are the comparisons of the PESQ and STOI speech quality indices of the 8-channel and 4-channel gammatone filterbanks and gammachirp filterbanks. The index values of the different signals are shown on the vertical \(y\)-axis, and the serial numbers of the test speeches are shown on the horizontal \(x\)-axis. It can be observed from the comparison that the performance of the 8-channel and 4-channel gammachirp filterbanks obtained by considering the patients' audiogram characteristics is improved, and the PESQ and STOI values are better than for those of the gammatone filterbank. The indices of the 8-channel and 4-channel gammatone filterbanks and gammachirp filterbanks are compared in Table 2. Compared with the 8-channel filterbanks, the PESQ and STOI of the 4-channel gammachirp filterbanks are close to those of the 8-channel filterbanks, and the output signal has comparable intelligibility. By all of these comparisons, the gammachirp filterbank designed based on patients' hearing characteristics can better simulate the filtering properties of the human ear, the output speech has higher intelligibility, and the signal can be approximately and accurately reconstructed.  
Figure 4: Speech quality index comparisons of 8-channel filterbanks. (**a**) Comparison of PESQ; (**b**) comparison of STOI.  
Figure 3: Amplitude–frequency response of various filterbanks. (**a**) Amplitude–frequency response of gammachirp filterbank when c = 0; (**b**) amplitude–frequency response of gammachirp filterbank when c = 2; (**c**) amplitude–frequency response of gammachirp filterbank when c = -2; (**d**) amplitude–frequency response of gammachirp filterbank with various chirp factors.