19th INTERNATIONAL CONGRESS ON ACOUSTICSMADRID, 2-7 SEPTEMBER 2007

NUMERICAL SIMULATION OF ATTENUATION CHARACTERISTICS OF
SOFT-TISSUE CONDUCTED SOUND ORIGINATED FROM VOCAL TRACT
PACS: 43.35.Mr
Otani, Makoto1; Hirahara, Tatsuya1; Adachi, Seiji2
1Faculty of Engineering, Toyama Prefectural University; Kurokawa 5180, Imizu, Toyama, 9390398, Japan; otani@pu-toyama.ac.jp
2Fraunhofer Institute for building physics; Nobelstrasse 12, 70569 Stuttgart, Germany;

ABSTRACT
A non-audible murmur (NAM), a very weak speech sound produced without vocal vibration,
can be detected by a special NAM microphone attached to the neck, thereby providing a newcommunication tool for use with functional speech disorders. The NAM microphone is acondenser microphone covered with soft-silicone impression material that provides good
impedance matching with the soft tissues of the neck. Because higher-frequency components
are suppressed severely, however, the NAM detected with this device can be insufficiently clear.
To improve NAM clarity, the mechanism of NAM production as well as the transfer
characteristics of the NAM in soft neck tissues must be clarified. We have investigated soundpropagation from the vocal tract to the neck surface, using a finite difference method and ahead model based on magnetic resonance imaging scans. Numerical results show that,compared to vocal tract transfer functions, soft-tissue-conducted sound attenuates 50 dB at 1kHz, which consists of 30 dB full-range attenuation due to air-to-soft-tissues transmission loss
and -10 dB/octave spectral decay due to a propagation loss in soft tissues. The decay agrees
well with the spectral characteristics of the measured NAM.

INTRODUCTION
The ordinary voice is air-conducted sound and is easily audible for listeners located near thespeaker. The mechanism of production of the ordinary voice is as follows: First, the vocal cords
are vibrated by an air stream from the lungs, generating the periodic glottal flow that is theprimary source of sound; the vocal tract is resonated due to the vocal cords vibration. Next, theresonated sound radiates from the mouth as an air-conducted sound. Besides this ordinaryvoice, there are various other types of vocal sounds, e.g., shouting, laughing, crying andwhispering. These vocal sounds are more or less audible for people near the speaker.
Additionally, there are much weaker murmured vocalizations that have very small energy andare consequently usually unheard even by listeners very nearby. Such a weakly murmured
vocalization, however, can be detected by using a special microphone attached to the surfaceof the skin close behind the ear. In other words, an “air-conducted” weak murmured voice isinaudible; however, a “body-conducted” one can be audible. Such body-conducted weakmurmured voice sounds are named non-audible murmur (NAM).
The NAM can be detected by using a special microphone, called a NAM microphone [1-3].
This device is a condenser microphone covered with soft impression material such as soft
silicon and urethane elastomer, which provides good impedance matching with the soft tissue ofthe neck. The NAM, which is inaudible even for people nearby, can be audible when detectedby the NAM microphone. This may enable the development of human-to-machine and humanto-human interfaces whose inputs are inaudible voice, thereby providing a “silent”
communication tool.
The NAM microphone also will be able to revive the speechcommunication of those with vocal cord problems caused by laryngeal cancer, nerve disorders
and muscle diseases.
The NAM has been recorded in recent years. Characterization of the NAM indicates that thesignal exhibits severely suppressed higher-frequency components, and does not have sufficientclarity. In order to improve the clarity of NAM detection, the mechanism of NAM production andthe sound transfer characteristics of the NAM in soft neck tissues must both be investigated. To
reveal transfer and attenuation characteristics of NAM propagation through soft tissues, weinvestigated sound propagation from the vocal tract to neck surface, using a finite differencetime domain (FDTD) method and a head model constructed on the basis of magnetic resonanceimaging scans.

HEAD MODEL
Three-dimensional geometrical data of a human head uttering a vowel /e/ NAM were obtainedusing phonation-synchronized magnetic resonance imaging scans [4]. A two-dimensional imageof the median sagittal plane was extracted (Fig. 1a). Subsequently, for simplicity, ahomogeneous head model was generated (Fig. 1b), i.e., the head is approximated as being
composed only of soft tissue. The vocal tract was replaced by a simplified model with arectangular cross-section 30 mm wide. This latter approximation was made because a scanned
vocal tract is extremely narrow in the vicinity of the vocal cords, and would require too fine of agrid to be simulated by the FDTD method.

(a)(b)
Fig. 1 – (a) MR image at median saggital plane; (b) Two-dimensional head model

NUMERICAL SIMULATION
Cyber Logic Wave 2000 Pro [5] was used as a two-dimensional FDTD solver. The simulated
region is a rectangular area, which includes the head model of the following dimensions: 414mm (height) x 292 mm (width), with a 2-mm grid. The edges of the region are an infinite
boundary, i.e. absorptive boundary. Physical parameters of soft tissue were set as follows;density ρ, 1,100 kg/m3; bulk modulus λ1, 2,600 MPa; bulk viscosity λ2, 0.001 Pa∙s; shear
modulus μ1, 0.025 MPa; shear viscosity μ2, 1,500 Pa∙s, according to the reference [6]. Note that
μ2 was 100 times larger than the value found in the reference, in order to avoid numerical
divergence. Also note that the value of λ2 isdetermined from those of other soft
materials, because the value of λ2 for soft
tissue is not known. Corresponding
parameters of air are as follows; ρ, 1.24kg/m3; λ1, 0.147 MPa; λ2, 0.13 Pa∙s; μ1 = μ2= 0.
Figure 2 shows the geometry of thesimulated region including the head, asound source, and the receivers. Ideally, thesound source would be located near thevocal cords, as in the real human head;
unfortunately, due to a limitation ofnumerical modelling, it cannot be locatedthere. Instead, the sound source is locatedin front of the mouth, assuming a reciprocal

Fig.2 – Simulated region including the head,vibrating plate, and receivers
219th INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRID

theorem. The sound source is a pulse-driven vibrating plate. The receivers are located at theinlet and outlet of the vocal tract and at the surface of the neck close behind the ear, i.e., wherethe NAM microphone would be attached. These receivers are referred to as “out”, “in”, and“nam”, respectively. Note that nam is located inside the head in 2-D modelling, although it islocated on the neck surface in the real human situation.

RESULTS
Figure 3 shows the sound intensity level observed at each receiver. We approximated thesound intensity level is by dividing the square of sound pressure at each receiver by acoustical
impedance at each medium. Iout, Iin, and Inam denote the sound intensity levels at out, in, andnam, respectively. Iout and Iin are relatively suppressed at lower frequencies, which reflect thefrequency characteristics of the particle velocity pulse used as a sound source signal. Inam,however, is flat. Iout is larger at higher frequencies than Iin due to characteristics of soundradiation from the mouth. It is also shown that Iout and Iin have some spectral peaks that areformants. Inam has spectral peaks at 6.3 and 9.7 kHz. These peaks can be attributed toeigenfrequencies of the head [7].

Eigenfrequencies ofthe headFig. 3 – Sound intensity level at outlet and inlet of vocal tract, and NAM microphone

Formants-50 dB@ 1 kHz

Vocal tract
transfer function

Spectrumat NAM microphone-10 dB/octave

Fig. 4 – Attenuation characteristics at NAM microphone

In order to clarify a transmission loss of sound from air to soft tissue and a propagation loss
through soft tissues, Iin / Iout and Inam / Iout were calculated (Fig. 4). Iin / Iout corresponds to thevocal tract transfer functions, showing spectral peaks, i.e. formants, at 0.5, 1.4, 2.1, and 3.0 kHz.
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19th INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRIDInam / Iout corresponds to the sound intensity level observed at nam if a sound radiated from themouth has flat characteristics. Compared to the vocal tract transfer function, the sound intensity
level at nam (Inam / Iout) is approximately 50 dB lower at 1 kHz. Furthermore, the sound intensity
level at nam shows -10 dB per octave spectral decay at higher frequencies.

DISCUSSION
Sound energy originating in the vocal tract propagates from the air inside the vocal tract to thesoft tissues. Subsequently, it propagates inside the soft tissues and reaches to the NAM
microphone, traveling in soft tissues. Within the soft tissues in the current 2D numerical model,
the shortest propagation distance is 70 mm. The acoustic impedance of air (Zair) and soft
tissues (Ztissue) are approximately 408 and 1,630,000 kg/m2s, respectively. According to thesevalues, transmission loss from air to soft tissues (4 Zair Ztissue / (Zair + Ztissue )2) corresponds toabout 30 dB attenuation in sound intensity level at full band. Furthermore, a sound decays
during propagation in the soft tissues. It is likely that this propagation loss results in the spectral
decay of -10 dB/octave observed in the numerical result. These phenomena can reasonably
explain the attenuation characteristics of the NAM detected at the neck surface. The acoustic
analysis of the measured NAM shows a spectral decay of -23 dB/octave, which involves glottal
sound-source characteristics having a spectral decay of -12 dB/octave [8]. This result indicatesthat the measured NAM involves -11 dB/octave spectral decay due to propagation loss in thesoft-tissue, which is roughly parallel to the spectral decay observed in the current numericalresult, i.e. -10 dB/octave. However, the frequency dependency of the soft-tissue propagation
loss is inconsistent with the spectral decay derived from an absorption coefficient of soft tissues
in the ultrasonic range, i.e. 0.5–2 dB/cm/MHz [9]. According to this value, there would be
virtually no spectral decay due to soft-tissue propagation in the audible range.

CONCLUSIONSIn order to clarify the attenuation characteristics of the NAM detected by a NAM microphone,
we numerically simulated a sound that originates in the vocal tract and propagates to the necksurface. Compared to the vocal tract transfer function, the sound detected at the NAM
microphone attenuates by 50 dB at 1 kHz and has a spectral decay of -10 dB/octave at higher
frequencies. The attenuation of 50 dB at 1 kHz consists of the air-to-soft-tissue transmission
loss of 30 dB and the spectral decay of -10 dB/octave due to propagation in the soft tissue,
showing good agreement with the acoustic analysis of the measured NAM.
This work is supported by SCOPE-S of the Ministry of Internal Affairs and Communications of Japan.

References[1] Y. Nakajima et al., “Non-Audible Murmur Recognition Input Interface Using Stethoscopic Microphone
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[5] http://www.cyberlogic.org/[6] H.L. Oestreicher, “Field and Impedance of an Oscillating Sphere in a Viscoelastic Medium with anApplication to Biophysics”, J. Acoust. Soc. Am., 23(6), 707–714, (1951).
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[9] D.E. Goldman and T.F. Hueter, “Tabular data of the velocity and absorption of high-frequency sound inmammalian tissues”, J. Acoust. Soc. Am., 28(1), 35–37, (1956).

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19th INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRID