## 4 Discussion  
Overall, masks decreased the Baidu Cloud ASR accuracy by 10.4 pp and Tencent Cloud ASR accuracy by 10.1 pp under noises at a recording distance of 0.6 m, which is roughly the distance between a car driver or machine operator and the ASR system. When people cannot get closer to the ASR system or raise their volume, masks could hinder ASR. Given the wide use of masks in daily life in the post-pandemic era, considering the influence of masks on speech is strongly recommended when training ASR models. There is no effect of masks on ASR accuracy in a quiet situation, with the ACC median values showing the ceiling effect for most situations as being 100%. These results are consistent with those from human subject studies of [PERSON] et al. and [PERSON] et al., who reported that the difference between the speech recognition scores with the surgical mask present and without the mask was less than 1 pp for normal-hearing listeners [16; 17]. Noting that the speech recognition scores were high in all conditions (i.e., higher than 99% for [PERSON] et al., and 96.9% for [PERSON] et al. ), the ceiling effect [9] might be a possible reason for the non-significant differences in the speech recognition scores in the two studies [16; 17]. Under the restaurant noise and speech-shaped noise, significant main effects of masks on ASR accuracy at a recording distance of 0.6 m were found (restaurant noise gave ASR\({}_{\text{D}}\), \(p\) = 0.005, ASR\({}_{\text{T}}\), \(p\) = 0.048, and speech-shaped noise gave ASR\({}_{\text{T}}\), \(p\) < 0.001). At the recording distance of 0.6 m and under noises, the activated-carbon mask (M2) impacted ASR accuracy most, followed by the headwear medical protective mask (M4) and KN95 mask (M5). The activated-carbon mask decreased the ASR accuracy by 18.5 pp, the headwear medical protective mask decreased it by 13.4 pp, and the KN95 mask decreased it by 10.6 pp at the recording distance of 0.6 m. The cloth mask (M6) has the least effect on ASR, reducing ASR accuracy by 0.9 pp. The surgical mask (M1) and hanging-ear medical protective mask (M3) performed similarly, decreasing the ASR accuracy by 9.1 pp and 8.9 pp. These results are consistent with those from human subject studies of [PERSON] et al., [9] and [PERSON] et al. [14]. [PERSON] et al., reported that the speech was 12 pp less intelligible with the surgical mask and 13 pp less intelligible with the N95 mask than for the unmasked condition [9]. The results of [PERSON] et al. showed that the surgical masks reduced the STI by 3% and 4%, and the N95 masks reduced the STI by 13% and 17% [14]. In this study, we established two recording distances, 0.2 m for mobile phone users and 0.6 m for machine operators (such as in vehicles and for robots), to evaluate the impacts of masks on ASR performance in standard human-machine interactions. For the 0.2 m distance, masks decreased the ASR accuracy by 3.6 pp on average under noises but without statistical significance (\(p\) > 0.054), whereas for the 0.6 m distance, masks significantly decreased the ASR accuracy by 10.2 pp on average under noises (restaurant noise gave ASR\({}_{\text{D}}\), \(p\) = 0.005, ASR\({}_{\text{T}}\), \(p\) = 0.048, and speech-shaped noise gave ASR\({}_{\text{T}}\), \(p\) < 0.001). There might be debate about whether the signals recorded at 0.6 m with lower voice amplitudes could be amplified easily by the speech enhancement algorithms of the ASR systems. However, results indicate that the ASR accuracy was affected by the absolute SPL of the speech signals given equal SNR (at least in an anechoic environment). For the medical protective masks, the headwear medical protective mask (M4) decreased the ASR accuracy under noises much more than the hanging-ear medical protective mask (M3), by an average of 5.2 pp. this can be explained by the fact that that the headwear medical protective masks had better airtightness and higher sound-transmission loss than hanging-ear medical protective masks [34; 35].