## 1 Introduction
However, in mosaic speech, the blocks are static (except for the random fluctuation of noise) and have a frequency resolution suitable to simulate the auditory periphery. In brief, mosaic speech was made in the following way (for further details about the generation of mosaic speech, the reader can refer to Section 2.3.2). The initial procedure resembled the procedure to generate noise-vocoded speech [22; 23; 24; 25]. First, the original speech signal was separated into several frequency band-pass filters, following the concept of _critical bands_. The waveform of each frequency band was cut into temporal segments of the _original mosaic block duration_ (OMBD), for example of 40 ms, and the total amount of its sound energy was calculated by squaring and adding up instantaneous amplitudes. In the same frequency band-pass filters, a white noise of the same duration as that of the speech signal was generated and the waveforms in these filters were cut into the same temporal segments. Cosine-shaped rise and fall times were used for each of these temporal segments, and the total amount of its sound energy was calculated in the same way as for the speech signal. There was a time-frequency correspondence between the speech signal and the white noise. Each temporally-segmented band noise was amplitude-adjusted to make its total sound energy equal to that of its counterpart in the processed speech signal. By putting all these segmented band noises together on the time-frequency plane, mosaic speech was obtained. As in the previous study with Japanese mosaic speech [14], OMBD was varied, but only in two steps this time (20 and 40 ms). Although it had been shown that mosaic speech with an OMBD of 40 ms or shorter was almost perfectly intelligible (>95%), so far it was not known whether intelligibility would be similarly good when the OMBDs are compressed or stretched in time (i.e., when the mosaic blocks are made shorter or longer), while preserving the same acoustic information. For example, if we compress original mosaic blocks of 80 ms, which are not sufficiently understandable, into mosaic blocks of 20 ms, does this improve intelligibility? As an opposing example, if we stretch out original mosaic blocks of 20 ms, which are reasonably intelligible, into mosaic blocks of 80 ms, does intelligibility deteriorate? Thus, our primary goal was to measure the intelligibility of mosaic speech in which OMBD was either compressed, preserved, or stretched in time. Our secondary goal was to investigate whether the intelligibility of mosaic speech varies with the language background of the listener. Instead of Japanese, as used previously [14], English mosaic speech was used here. Since English is spoken all over the world, it is sometimes complicated to define English speech sounds, due to phonetic varieties in different geographic and social environments [26]. There are many different ways to pronounce sounds in English, depending on accent preferences or personal habits. In some cases, there can be many indistinguishable pronunciations of different consonants, such as between // and /f/, and /d/ and /\(\delta\)/. English allows a lot of allophonic variants of each phoneme, and this makes more confusion when the language is pronounced in two or more ways [27]. Differences in English speech sounds are even larger among non-native speakers of English, who often refer to pronunciation patterns in their own language accent [28]. For example, Indonesian speakers sometimes pronounce English words in a specific way since their native language, which is written with Roman letters, has a high degree of grapheme-phoneme correspondence, so that words are pronounced as they are written. English, however, is more non-phonemic [29]. Whereas near-perfect (>95%) intelligibility of mosaic speech with segments of 40 ms or shorter has been reported for the Japanese language [14], we here investigated whether English mosaic speech, with all its complexity in speech sounds, would also be near perfect at a similar segment duration or not.