An evaluation of target speech for a nonaudible murmur enhancement system in noisy environments

Nonaudible murmur (NAM) is a soft whispered voice recorded with NAM microphone through body conduction. NAM allows for silent speech communication as it makes it possible for the speaker to convey their message in a nonaudible voice. However, its intelligibility and naturalness are significantly degraded compared to those of natural speech owing to acoustic changes caused by body conduction. To address this issue, statistical voice conversion (VC) methods from NAM to normal speech (NAM-to-Speech) and to a whispered voice (NAM-to-Whisper) have been proposed. It has been reported that these NAM enhancement methods significantly improve speech quality and intelligibility of NAM, and NAM-to-Whisper is more effective than NAM-to-Speech. However, it is still not obvious which method is more effective if a listener listens to the enhanced speech in noisy environments, a situation that often happens in silent speech communication. In this paper, assuming a typical situation in which NAM is uttered by a speaker in a quiet environment and conveyed to a listener in noisy environments, we investigate what kinds of target speech are more effective for NAM enhancement. We also propose NAM enhancement methods for converting NAM to other types of target voiced speech. Experiments show that the conversion process into voiced speech is more effective than that into unvoiced speech for generating more intelligible speech in noisy environments.