Silent-speech enhancement using body-conducted vocal-tract resonance signals

Abstract

The physical characteristics of weak body-conducted vocal-tract resonance signals called non-audible murmur (NAM) and the acoustic characteristics of three sensors developed for detecting these signals have been investigated. NAM signals attenuate 50 dB at 1 kHz; this attenuation consists of 30-dB full-range attenuation due to air-to-body transmission loss and −10 dB/octave spectral decay due to a sound propagation loss within the body. These characteristics agree with the spectral characteristics of measured NAM signals. The sensors have a sensitivity of between −41 and −58 dB [V/Pa] at 1 kHz, and the mean signal-to-noise ratio of the detected signals was 15 dB. On the basis of these investigations, three types of silent-speech enhancement systems were developed: (1) simple, direct amplification of weak vocal-tract resonance signals using a wired urethane-elastomer NAM microphone, (2) simple, direct amplification using a wireless urethane-elastomer-duplex NAM microphone, and (3) transformation of the weak vocal-tract resonance signals sensed by a soft-silicone NAM microphone into whispered speech using statistical conversion. Field testing of the systems showed that they enable voice impaired people to communicate verbally using body-conducted vocal-tract resonance signals. Listening tests demonstrated that weak body-conducted vocal-tract resonance sounds can be transformed into intelligible whispered speech sounds. Using these systems, people with voice impairments can re-acquire speech communication with less effort. Keywords: non-audible murmur, body-conducted sound, voice conversion, talking aids, -2 - IntroductionWhile microphones have been used in many scientific fields to sense speech sounds, a recently developed device called a -non-audible murmur (NAM) microphone‖ is receiving increasing attention as a new means for picking up body-conducted speech (Heracleous et al., 2003, Nakajima et al., 2006, Toda et al., 2005b, Nakamura et al., 2006. Typically, the speech sound used is air-borne sound, which is small, fast vibration of the air. Vibration of the air column in the vocal tract vibrates the tract wall, and some of the sound energy generated passes through the tissues of the neck and chest. The body-conducted sound that travels through the neck tissue can be sensed using a sensor modified from a microphone. Actually, speech sounds propagate not only through the air and the bone but also through the body tissue, including the muscles. Nakajima, a pioneer in NAM development, found that murmured speech, which is usually unheard by people nearby, can be detected by using a sensor attached to the neck behind the ear (Nakajima et al., 2003a). This body-conducted weak murmur is called -non-audible murmur (NAM)‖, and the sensor is known as a NAM microphone.Nakajima originally detected NAM using a stethoscopic NAM microphone (Nakajima et al., 2003a(Nakajima et al., , 2003b, which is an electret condenser microphone (ECM) implanted into a standard medical-use stethoscope with the tubes removed. When this...

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“…In the literature, there are a few researches on learningbased speech enhancement [6][7][8][9][10][11][12][13]. Among them, learningbased speech enhancement approach using statistical spectral conversion has been proposed for alaryngeal speech caused by speech disorders [8,9], body-conducted speech [10], NAM-captured speech [11], and bone-conducted speech [12].…”

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“…The weak sound propagates through the body and it can be detected by using a sensor attached to the neck behind the ear, resulting in a NAM signal typically represented by voltage value. The NAM is inaudible, but, after applying appropriate signal amplification and processing to the detected NAM signal, it can be presented to a listener as an audible speech through an electroacoustic transducer, or to a speech recognition system, thus enabling silent-speech interfaces [1]- [3].…”

“…The weak sound propagates through the body and it can be detected by using a sensor attached to the neck behind the ear, resulting in a NAM signal typically represented by voltage value. The NAM is inaudible, but, after applying appropriate signal amplification and processing to the detected NAM signal, it can be presented to a listener as an audible speech through an electroacoustic transducer, or to a speech recognition system, thus enabling silent-speech interfaces [1]- [3].The speech sound we typically use is an air-borne sound, which is a small and fast vibration of the air. The vibration of the air column in the vocal-tract vibrates the wall of the vocal tract, and some sound energies pass through the tissues of the neck or chest.…”