  Speech communication systems based on Voice-over-IP technology are frequently
used by native as well as non-native speakers of a target language, e.g. in
international phone calls or telemeetings. Frequently, such calls also occur in
a noisy environment, making noise suppression modules necessary to increase
perceived quality of experience. Whereas standard tests for assessing perceived
quality make use of native listeners, we assume that noise-reduced speech and
residual noise may affect native and non-native listeners of a target language
in different ways. To test this assumption, we report results of two subjective
tests conducted with English and German native listeners who judge the quality
of speech samples recorded by native English, German, and Mandarin speakers,
which are degraded with different background noise levels and noise suppression
effects. The experiments were conducted following the standardized ITU-T Rec.
P.835 approach, however implemented in a crowdsourcing setting according to
ITU-T Rec. P.808. Our results show a significant influence of language on
speech signal ratings and, consequently, on the overall perceived quality in
specific conditions.
