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					XVI Session of the Russian Acoustical Society                                 Moscow, November 14-18, 2005




                              N.N. Bochkarev
      PLASMA SOURCE of ACOUSTIC WAVES - REFERENCE ACOUSTIC SOURCE

      Institute of Atmospheric Optics SB RAS
      1 Akademicheskiy Ave., Tomsk, 634055 Russia
      Phone: (+7-382-2) 492-738; Fax: (+7-382-2) 492-086
      E-mail: bonic@iao.ru

The description of the developed and made plasma source of acoustic waves is given. The source working on a
principle of modulation of the one-electrode high-frequency category in temperature ~ 4500 K, has a fidelity,
unattainable for electrodynamic sources, of transfer of a broadband acoustic signal. The peak frequency
characteristic of a source absolutely smooth in a range of frequencies up to frequency of modulation (some tens
MHz), that allows to consider such source reference.

        Recently special interest is shown to nonconventional acoustic sources based on use of
discharge plasma (electroarc, high-frequency), existing in air at atmospheric pressure and having
direct connection with air. The electrospark acoustic sources provide the large levels and high stability
of sound pressure, but have significant erosion of electrodes, have a characteristic edge spectrum of a
source and essential nonlinear distortions.
        The large opportunities of qualitative reproduction of high sound frequencies are opened by
application of a plasma acoustic source based on the one-electrode high-frequency category. The
broadband plasma acoustic source working on a principle of peak modulation of the high-frequency
category of limited volume with temperature 4000 ÷ 4600 K, has the minimal nonlinear distortions.
        At the same time decisions of a number of scientific and technical tasks in the field of acoustics
frequently need use of non-standard recording gauges, which characteristics are necessary for
determining. For example, calibration and measurement of the peak and phase frequency
characteristics of the acoustic gauge made from piezoceramic and having a working frequency strip in
a little some MHz. The standard calibrators for such range of frequencies are not present, and use of a
various sort electrodynamic, piezo- or others sources for the purposes of calibration is inexpedient, as
they have by significant heterogeneity of the characteristics, in view of the resonant properties.
        With the purpose of the decision of this technical task it is offered to use as the calibrator a
plasma source of acoustic waves, the principle of which work consists in the following. At submission
on an electrode of a high-frequency voltage with frequency 13 or 27 MHz from the free end of an
electrode is raised high-frequency the category, which is formed under action of ascending flows and
burns unlimited time. Due to low temperature of plasma, the erosion of an electrode low and
significally does not influence stability of burning of the category.
        The high-frequency category is more higher the order and than intensity of an electrical field of
electroarc plasma. If a high-frequency voltage to simulate on amplitude, volume of the channel
conducting a current, high-frequency category will vary with frequency of a modulating voltage. The
change of volume of plasma results in change of pressure in air with frequency of modulation. If the
high-frequency generator to simulate by a voltage of sound frequency, in an environment the sound
fluctuations will appear. In view of high mobility of particles of ions the plasma source of acoustic
waves is capable to reproduce without appreciable frequency and transitive distortions not only sound
fluctuations, but also ultrasound of a range of frequencies in MHz.
        In a fig. 1 the appearance of a plasma source of acoustic waves, and in a fig. 2 - his simplified
basic circuit is shown.
        The high-frequency category by capacity ~20 W is raised in open space from a thin rod
electrode from wolfram Through concentric tubular fider the electrode is connected to the lamp high-
frequency generator, which is created on the basis of a generating lamp GMI-6. Working frequency –
13 MHz. On same lamp the modulator is constructed also. The opportunity of smooth change of
capacity of the high-frequency category, raised from an electrode is stipulated. The longitudinal size of


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XVI Session of the Russian Acoustical Society                                  Moscow, November 14-18, 2005

the channel of the category conducting a current, makes 5 ÷ 10 diameters of the channel and is
approximately equal 1 ÷1,5 sm.




                                  Fig. 1. A plasma source of acoustic waves




                   Fig. 2. The simplified basic circuit of a plasma source of acoustic waves

       In a fig. 2 the peak frequency characteristic of a plasma source of acoustic waves measured by a
method "of sliding frequency" in a sound range with use of a measuring complete set of firm Robotron
is given. For comparison in a fig. 3 the peak frequency characteristic is given in the same frequency
range and measured by the specified method for high-quality acoustic system of industrial
manufacturing 100АС-060, which non-uniformity reaches 26 dB, is especial in the field of frequencies
1 ÷ 5 kHz. The non-uniformity of the characteristic is caused by resonant properties of electrodynamic
sources of acoustic system.
       According to a fig. 2, the plasma source of acoustic waves, has the smooth peak frequency
characteristic which is not having of resonances. If necessary by introduction of an adjusting RC-chain
in the circuit of modulation of a high-frequency voltage it is possible to remove smooth rise of the
peak frequency characteristic (~30 dB), having transformed it in flat. To remove resonant character of
the peak frequency characteristic shown in a fig. 3, for electrodynamic sources of acoustic systems it
is impossible.
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XVI Session of the Russian Acoustical Society                                   Moscow, November 14-18, 2005


       Efficiency of the created source makes some 100-th shares of percent and can be increased by
selection of special design features. The ratings show, that at use powerful diffusion of the category
with speed of a plasma flow 10 m/c and area of section of the category 18 см2 at temperature of
plasma 2000 K probably essential increase of efficiency of a similar source, and the prospective
acoustic capacity will make more than 800 W.

                         Р, дБ   100



                                 80



                                 60



                                 40



                                 20
                                                                           R=1м

                                  0
                                            1                              10
                                                                                 f, кГц

                    Fig. 3. The peak frequency characteristic of a plasma source of acoustic waves




       Fig. 4. The peak frequency characteristic of acoustic system 100АС-060. Scale on a vertical - 50dB. On a
                                         horizontal - frequency in Hz

      Thus, use of a plasma source of acoustic waves allows to execute peak frequency calibration on
sound pressure of the non-standard broadband equipment, as such source has unattainable for known
ways of generation width of a strip of reproduced frequencies at insignificant nonlinear and peak
frequency distortions.




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