## 4 Discussion
In our test, cloth masks had little effect on the ASR accuracy, only reducing the ACC by 0.9 pp at the recording distance of 0.6 m under noises, which is inconsistent with the results of [PERSON] et al. [36] The results of [PERSON] et al., showed that the fitted cloth mask impaired human speech recognition most [36]. The tightness of the cloth mask weave may account for the difference in results. To delve deeper into the mechanisms by which masks affect ASR accuracy, we measured the sound-transmission losses of these six masks. We used the mouth simulator to play the sweep signal with or without a mask and recorded it at a distance of 0.6 m. The sound-transmission loss was calculated using Equation (2), and the results are shown in Figure 7.  
\[TL(\omega)=10 lg\frac{|p_{i}(\omega)|^{2}}{|p_{t}(\omega)|^{2}} \tag{2}\]  
where '\(p_{i}\)' is the input sound pressure and '\(p_{t}\)' is the transmitted sound pressure. All masks gave a large attenuation in the frequency band centered at about 3.15 kHz and a smaller attenuation at around 250 Hz, consistent with the results of [PERSON] et al. [10] In general, the sound-transmission loss of masks was consistent with their influence on ASR accuracy: the cloth mask (M6), which produced the least acoustic attenuation, yielded the least effect on ASR accuracy; the activated-carbon mask (M2), which attenuated sounds most at around 3.15 kHz, impaired ASR accuracy the most. Furthermore, we analyzed the correlation between the sound-transmission loss of masks and their effect on ASR accuracy at the recording distance of 0.6 m. The results showed that there was a significant positive correlation between the declined values in the ACC with wearing masks and the sound-transmission loss of masks at 3.15 kHz (\(p\) = 0.943, \(p\) = 0.005), but no correlation between those at 250 Hz (\(p\) = 0.200, \(p\) = 0.704). The results indicated that the effects of masks on ASR accuracy can be attributed to the acoustic attenuation of masks on the high-frequency band of the speech signal. The low-pass-filtering characteristic of masks seems to greatly attenuate the energy of clear consonants and impair the ASR accuracy. Although masks attenuate the sound power at 250 Hz, which is around the fundamental monophthong frequency of Mandarin, the attenuation of low-frequency sound contributed little to the decline in ASR accuracy. Figure 7: The sound-transmission loss of masks. Post-hoc test results showed that masks similarly affected ASR accuracy for male and female speakers. On average, masks decreased the ASR accuracy for male speakers by 7.8 pp and for female speakers by 9.4 pp under noises at a recording distance of 0.6 m. However, it is worth noting that the cloth mask (M6) increased the ASR accuracy of male speakers for all noise conditions and ASR systems at a recording distance of 0.6 m by an average of 7.3 pp. There is no such anomaly for female speakers. Possibly, the cloth mask could enhance the feature extraction of ASR systems for male voices. It seems that the effects of masks on ASR are mediated through a complex mechanism related to the physical properties of masks (mainly affecting the sound-transmission loss), the voice characteristics, and the ASR algorithm. Future studies could further explore this issue by analyzing the effects of masks on the spectrum of speech signals and speech feature extraction of ASR systems for male speakers. The results showed that speech-shaped noise had a larger effect on ASR accuracy than the restaurant noise at the same SNR. The ASR accuracy under speech-shaped noise was 22.7 pp less than that under restaurant noise at the recording distance of 0.6 m. The average spectra of the two noises and the speech material are shown in Figure 8. Compared to restaurant noise, the spectrum of speech-shaped noise is more similar to that of the speech material, possibly resulting in a more substantial energetic masking effect [37] and informational masking [38].