Effects of nonlinear frequency compression on the acoustic properties and recognition of speech sounds in Mandarin Chinese

Abstract

The present study examined the change in spectral properties of Mandarin vowels and fricatives caused by nonlinear frequency compression (NLFC) used in hearing instruments and how these changes affect the perception of speech sounds in normal-hearing listeners. Speech materials, including a list of Mandarin monosyllables in the form of /dV/ (12 vowels) and /Ca/ (five fricatives), were recorded from 20 normal-hearing, native Mandarin-speaking adults (ten males and ten females). NLFC was based on Phonak SoundRecover algorithms. The speech materials were processed with six different NLFC parameter settings. Detailed acoustic analysis revealed that the high front vowel /i/ and certain compound vowels containing /i/ demonstrated positional deviation in certain processed conditions in comparison to the unprocessed condition. All five fricatives showed acoustic changes in spectral features in all processed conditions. Fourteen Mandarin-speaking, normal-hearing adult listeners performed phoneme recognition with the six NLFC processing conditions. When the cut-off frequency was set relatively low, recognition of /s/ was detrimentally affected, whereas none of the NLFC processing configurations affected the other phonemes. The discrepancy between the considerable acoustic changes and the negligible adverse effects on perceptual outcomes is partially accounted for by the phonology system and phonotactic constraints in Mandarin.

Speech Perception and Sound-Quality Rating with an Adaptive Nonlinear Frequency Compression Algorithm in Mandarin-Speaking Hearing Aid Users

“…It is noteworthy that even though the participants could detect different phonemes after compression, some phonemes such as /s/ and /ɕ/ become similar to /ʂ/ due to the lowered spectral prominences and become difficult to discriminate. 7 Therefore, in a future study, more participants should be tested on different aspects of speech perception including detection, distinction, and recognition. Also, the perceptual outcome with different types of NLFC algorithms in comparison to NLFC-off should be evaluated at different times to examine the factor of acclimatization in speech perception with NLFC-fitted hearing aids.…”

“…Through disproportionally compressing high-frequency components to the low-frequency regions, the inaudible high frequencies become audible to the hearing aid users. [4][5][6][7] SoundRecover (SR) by Phonak Naída is an NLFC algorithm commercially used in modern hearing aids. 8 While NLFC enables hearing-impaired listeners to gain access to inaudible high frequencies, to those listeners with severe hearing loss who have very limited output bandwidth and require a more aggressive parameter setting (e.g., a lower CT and a higher CR), low-frequency sounds (e.g., vowels and sonorant consonants) might be adversely affected due to potential distortions of the spectral structure.…”

“…8 While NLFC enables hearing-impaired listeners to gain access to inaudible high frequencies, to those listeners with severe hearing loss who have very limited output bandwidth and require a more aggressive parameter setting (e.g., a lower CT and a higher CR), low-frequency sounds (e.g., vowels and sonorant consonants) might be adversely affected due to potential distortions of the spectral structure. 7 To further improve the audibility for listeners with severe hearing impairment and maintain the spectral shape of vowels and low-frequency speech signals, an adaptive NLFC algorithm (known as Soun-dRecover2 or SR2) was developed. 9 The algorithm uses two cut-offs, one lower cut-off (CT1) and one higher cut-off (CT2).…”

“…This adaptive selection of the CTs maintains the vowel formant structures and maximizes the access to high-frequency sounds. 7,9 Clinically, the settings of CT1, CT2, and CR depend on the severity of the hearing loss. The range of CT1 is between 800 and 7,000 Hz and that of CT2 is between 1,600 and 7,700 Hz.…”

Effects of Adaptive Non-linear Frequency Compression in Hearing Aids on Mandarin Speech and Sound-Quality Perception

“…For people with severe-to-profound hearing loss, more aggressive settings with a lower CT and a higher CR are required because the patients have a narrower audible frequency bandwidth and the inaudible frequencies start at a lower frequency point in comparison to people with mild or moderate hearing loss. While the use of a lower CT ensures that a wider range of high frequencies can be shifted down so that they become audible to hearing aid users, it may also introduce unwanted detrimental effects to consonant and vowel perception (Alexander, 2016;Yang et al, 2018) and sound-quality perception (McDermott, 2011;Parsa et al, 2013;Souza et al, 2013). Therefore, to achieve a balance between audibility (lower CT) and fidelity (higher CT), Phonak introduced a new adaptive NLFC algorithm (known as SoundRecover 2 or SR2) in which the CT is switched between a low cut-off (CT1) and a high cut-off (CT2) based on the short-term energy distribution of the input signal (Rehmann et al, 2016).…”