B i oe lectro m a g n etic Mea s u re m e n ts

Impedance Characteristics of The Vocal Tract Soft Tissues

Vojtech Misun, Anna RyndovaFaculty of Mechanical Engineering, TU Brno

Technicka 2, 61 669 Brno, CZECH REPUBLIC

L45ME

Abstract: This paper presents results of experimentalanalysis of acoustic properties of the pig's and cow's vocaltract soft parts (tongue, mucotts membrane) which are, withregard to the acoustic properties in focus, related to thehuman vocal tract properties.

INTRODUCTION

The human voice is produced in the larynx and passes

through the vocal tract of specific acoustic properties. Theseproperties may affect both propagation of the sound wavesthrough the vocal tract and its spectral and modalproperties.The following basic problems were to be considered :

o what are the values and ranges of the mentionedacoustic characteristics ?

r do they affect the generation of the voice frequencycomponents, that means the vowel formants generation?

o do they influence the spectral and modal properties ofthe vocal trackt cavities ?

Results of the acoustic properties to be obtained will furtherbe used for solution and analysis of the human vocal tractspectral properties. The FE model of the vocal cavities isgiven on the basis of MRI data.

EXPERIMENTAL ANALYSIS

The measurement set used for measuring of acousticproperties of the followed samples consists of the followingdevices :

. impedance tube (interferometer) Brtiel&Kjaer type4202, 29mm and 99mm diameters, (250 to 5000)Hz frequency ranges

o Bri.iel&Kjaer Wpe2l 12 frequency analyzer.

Figure I shows the impedance tube diagram. Theloudspeaker generates the acoustic wave of a chosenfrequency f and the acoustic pressure amplitude A. Thewave extends through the tube and is reflected back by thetested sample. Both these wave motions mutually add up -thev interfere.

As the measured sample absorbs parts of acousticenergy, incident and reflected waves have different acousticpressure amplitudes.

Acoustic pressure amplitudes of interference waves innodes and antinodes are measured with a microphoneconnected to a trolley moving along the measuring ruler. Itmust be noted that only the resulting values of the standingwaves in the pipe can be measured, that means values(A+B) and (A-B).

At a set f [Hz] wave frequency in the tube, the followingparameters are gradually measured : (A+B), (A-B) and thenkr , kz - positions of the first and second interference wavenodes - Figure 2.

Figure 2: Course of acoustic pressures in standing waves

We introduce the standins wave coefficient

A+BA_B

The reflection coefficient of the sample is thenthe relation

B n-lR. _I_-

A n+land the absorption coefficient by the relation

CI,=1-R2- 4n- (3)

(n+ 1)2

Specific acoustic impedance consists of the following twocomponents :

o real part

(1)

defined by

(2)

x I zo| -lBl'

Figure l: Impedance Tube Diagram r +lBl' - zl1lcosy

3*o IntenNATroNAL ConrrnrncE oN BroructRoMAGNETrsM8-12 October 2000, Bled - SLOVENIA

(4)

t46

. imaginary part

and the air wave resistance z4:410 Pa.s/m .

Figure 3 demonstrates evaluation and course of the

sound absorption coefficients and components of the

specific acoustic impedances of sample of the pig's tongue,

depending on the wave frequency.

a) Absuflinco&irr

Bioel ectro ma g n etic Mea s u re m e n ts

are found within the value range of 0,03 ( cr ( 0,15 in the

given frequency range.

The courses of specific acoustic impedances within the

whole frequency range of all the measured samples may be

considered very similar to each other. The real components

fall within a value range of 0,3 1 xlzo < 8,0 while

imaginary components fall within a range of -20,0 Syla 310,0. The impedance courses are almost identical for both

non-moistened and moistened samples.

CONCLUSION

The measurements of acoustic properties of the soft

parts of the vocal tract have confirmed the expected values

based on the presumption that their internal surfaces have

very low acoustic absorption coefficients.The low values of absorption coefficients reflect the

necessary property of the vocal tract since the acoustic

wave must pass through it with the smallest possible

acoustic losses. It is necessary that the acoustic powergenerated by the vocal folds and speech organ is taken out

of the vocal tract into the surrounding outer space withoutany diffrculties.

ACKNOWLEDGMENT

This contribution was produced within the work on the

grant project GA CR No. 106/98/K019 entitledMathemotical-physical modelling of vibroacoustic systems

in the voice and hearing biomechanics with the focus on

development of substitution materials and aids, and withinthe scientific- research project No. CEZ : J22198:

262100001.

REFERENCES

V. Misun, "Modeling and analysis of the human vocal

cavities," Proc. of the EMBEC '99. Vienna, 1999,pp. l146-1147V. Misun and A. Ryndova, "Absorption properties ofsoft tissues of vocal tract," Proc. of Interactions ofdynamic systems, Prague, 1999, pp. 127 -134

V. Misun, "Investigation of resonant cavities in the

voice biomechanics," Proc. of Interactions of dynamic

systems, Prague, 1998, pp. 35-42

rlBlsiny

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with a phase parameter

, -.( -'n' - -t)' [(r-1, )

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(6)

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Figure 3 : Acoustic properties of the pig's tongue [3]

ANALYSIS OF RESULTS

The results of the experimental analysis of acousticproperties of the tested samples' indicate that internalsurfaces of vocal tracts reach very low values of sound

absorption coefficients almost over the whole monitoredfrequency range (250 - 5000)Hz. The results include bothsoft parts of the vocal tract - the tongue and mucous

membrane covering internal bone parts of the vocal tract.

The measurements prove that acoustic characteristics ofthe soft parts only change slightly within the human speech

or singing frequency range, that is within the frequency

range (S0 - 2048)Hz. It can be stated that coefficients ofacoustic absorption of all the soft tissues of the vocal tract

tll

t2l

3"o I ntrnNATroNAL Cot re nerucE oN Bro ructRoMAG N ETrsM

8-12 October 2000, Bled - SLOVENIA