US3435381A - Dispersive acoustic line using two-layer fluid media - Google Patents

Dispersive acoustic line using two-layer fluid media Download PDF

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Publication number
US3435381A
US3435381A US544785A US3435381DA US3435381A US 3435381 A US3435381 A US 3435381A US 544785 A US544785 A US 544785A US 3435381D A US3435381D A US 3435381DA US 3435381 A US3435381 A US 3435381A
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acoustic
layer
wave
dispersive
line
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English (en)
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Pierre Tournois
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Thales SA
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CSF Compagnie Generale de Telegraphie sans Fil SA
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/30—Time-delay networks
    • H03H9/40—Frequency dependent delay lines, e.g. dispersive delay lines

Definitions

  • the pulse compression technique is also applicable to sonar equipment; provided the dispersive lines are suitable for low frequencies. However, since the duration of the pulses to be compressed is then of an entirely different order of magnitude, lines of known types are generally unsuitable. Since the frequency excursion is reduced in this application and the pulses to be compressed have a substantially longer duration, the delays are insuflicient to produce effectivel the desired compression.
  • a dispersive acoustic line for compressing electrical signals frequency modulated on a carrier wave comprising: a hollow acoustic waveguide having two ends; first and second fluid media filling said guide; said media forming a first and a second superimposed layer extending from one of said ends to the other; said first fluid medium propagating acoustic waves with a higher phase velocity than said second fluid medium; first electroacoustic transducer means coupled to said second layer at one of said ends for generating in said first and second media an acoustic compressional wave in response to said electrical signals and second electroacoustic transducer means coupled to said second layer at said other end for receiving said compressional wave.
  • FIG. 1 shows a stratified medium as used in a delay line according to the invention
  • FIG. 2 is an explanatory drawing
  • FIG. 3 is a chart, showing the properties of the stratified medium of FIG. 1;
  • FIG. 4 shows the wave trains propagating between the ends of a dispersive line
  • FIG. 5 shows one embodiment of a dispersive line according to the invention
  • FIG. 6 is a further embodiment of a dispersive line according to the invention.
  • FIG. 7 is a partial view showing a coupling arrangement according to the invention.
  • a dispersive line according to the invention comprises a stratified medium in which acoustic compression waves are propagated.
  • a dispersive line comprises a stratified medium in which acoustic compression waves are propagated.
  • the waves propagating in such a medium will be hereinafter called D waves.
  • FIG. 1 shows a reference frame oxyz and a stratified medium portion comprising, from the bottom to the top, a first fluid medium 1, which is upwardly bounded by the plane zoz, a fluid layer 2 of constant thickness e, and a fiat and polished, rigid plate 3.
  • a first fluid medium 1 which is upwardly bounded by the plane zoz
  • a fluid layer 2 of constant thickness e and a fiat and polished, rigid plate 3.
  • the D wave characterized by the wave number k is propagated along ox, provided its phase velocity C is linked with the respective phase velocities of the media 1 and 2 by the inequalities It may be shown that this inequality can be satisfied for a plurality of types of D waves, since there exists an infinity of possible transverse distributions of pressure amplitudes along 03
  • the D wave presents in fact an amplitude distribution which varies harmonically within the layer 2 and which extends into the layer 1 with an exponential decay.
  • the acoustic pressure distribution at the abscissa x is propagated in the direction 0x with the phase velocity C.
  • FIG. 2 shows the depth distribution of acoustic pressures p for the D waves of the order 0 (a), order 1 (b) and order 2 (c). It can be seen that the acoustic pressure is at its maximum at the rigid wall 3 and decreases exponentially towards the base. If the thickness of medium 1 is defined by a base which is suificiently remote from the upper surface, no parasitic reflection can occur.
  • FIG. 3 summarizes graphically the properties of D waves transmitted through a stratified medium in accordance with the invention.
  • the curves have been plotted for a layer 2 of sulphur hexafluoride (SP and a layer 1 of air.
  • the variable along the abscissa is the parameter ef/C where e is the thickness of the layer 2, f the frequency of the D wave and C the phase velocity characteristic of the fluid of the layer 2.
  • the curves C/C and vg/C represent the frequency variations of the phase and group velocities of the Wave D.
  • the curve t /r shows how the ratio of the group varies delay time t to the delay time 1' of a plane wave passing 3 through the layer 2 with the velocity C
  • the drawing shows on the left the two sets of curves corresponding to the order and on the right to the mode order 1.
  • the D wave can be generated in the stratified medium by a plane acoustic Wave penetrating into the layer 2 at an incidence 0.
  • FIG. 3 shows in dotted line the angle 0 of incidence of the plane generating wave and of the zero order D wave.
  • the value of this angle of incidence is derived from the relation
  • the plane Wave generating the D wave undergoes alternating reflections on the rigid wall 3 and on the acoustic diopter formed by the flat junction between the two fluids.
  • the D wave is the sum of all waves reflected and refracted in this stratified medium.
  • FIG. shows an embodiment of the dispersive line according to the invention. It comprises an acoustic wave guide of rectangular cross-section shaped as a parallelepipedal box 4 and a cover 3 equipped with oblique inlets 5 and 6.
  • the diaphragm is an elastic film with a Youngs modulus of less than 20 kg./mm.
  • a film of polythene may be used with a thickness of 5-10 microns.
  • the end faces of the wave guide are provided on the insides with nonreflecting layers 8 and 9.
  • the inlets 5 and -6 are closed by electroacoustic transducers 10 and 11 of which only the radiating surface is represented by crosshatching.
  • the front wall has 'been removed for the sake of clarity.
  • the height of the vessel guide is so selected that the D wave is sufficiently damped on reaching the rigid base, and the lateral walls of the wave guide are spaced by a distance much greater than the thickness of the layer so as to minimize the disturbing action of the lateral walls (end effects).
  • an electric signal V such as that shown in FIG. 4, is applied to the electroacoustic transducer 10.
  • This signal creates in the stratified medium a D wave propataging parallel to 0x and emerging from the layer at the end of the line, where it is recovered by the transducer 11 which supplies an electric signal V
  • the suitable angle of incidence is obtained by means of the two inlet caps 5 and -6 which make possible the matching of the phase velocities between the transducers and the stratified medium.
  • the compressed signal V delivered by the dispersive line has for envelope the Fourier transform of the envelope of the input signal under the condition that the carrier of the input signal V is linearly modulated in frequency in the frequency range where the group delay time of the D wave varies linearly as a function of the frequency.
  • FIG. 4 relates to the compression of a rectangular pulse with the duration T linearly modulated in frequency in a band A about the centre frequency f
  • the compressed pulse has at midheight width a duration equal to the reciprocal of the modulation range A and its amplitude has been increased with respect to the input signal in the proportion x/TAf, which is the square root of the dispersion factor.
  • thickness e of the layer 2 can be obtained without difiiculty.
  • FIG. 6 shows a modification of the dispersion line according to the invention; it is formed by a rigid wave guide which is shaped as a spiral and consists of a vessel 4 and a cover 3.
  • An impermeable diaphragm 7 separates the superimposed channels filled with the fluids 1 and 2.
  • At the ends of the spiral are provided matched acoustic loads 8 and 9 as in the embodiment shown in FIG. 5 and inclined caps 5 and 6.
  • the caps are equipped with electroacoustic transducers 10 and 1.1.
  • This embodiment of the dispersion line is less bulky due to its shape.
  • the transducers used in the case of gaseous fluids are electrodynarnic loudspeakers.
  • the diaphragm separating the gases is a soft film having a high elasticity and negligible mass.
  • each end of the line has two symmetrical branches forming a V-shaped arrangement whose channels 5 and 13 are inclined at an angle of incidence 0 on either side of the normal N to the diaphragm 7.
  • the channel 5 is covered by the transducer 10 and the channel 13 terminates in an absorbent acoustic load 12.
  • This structure differs from that of FIG. 5 in that the wave reflected by the diaphragm 7 under the incidence 0 is absorbed so that it cannot interfere with the operative of the line.
  • a dispersive acoustic line for compressing electrical signals, frequency modulated on a carrier wave comprising: a hollow acoustic waveguide having two ends; first and second fluid media filling said guide; said media forming a first and a second superimposed layer extending from one of said ends to the other; said first fiuid medium propagating acoustic waves with a higher phase velocity than said second fluid medium; first electroacoustic transducer means coupled to said second layer at one of said ends for generating in said first and second media an acoustic compressional wave in response to said electrical signals and second electroacoustic transducer means coupled to said second layer at said other end for receiving said compressional wave.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Transducers For Ultrasonic Waves (AREA)
US544785A 1965-05-03 1966-04-25 Dispersive acoustic line using two-layer fluid media Expired - Lifetime US3435381A (en)

Applications Claiming Priority (1)

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FR15518A FR1452050A (fr) 1965-05-03 1965-05-03 Lignes acoustiques dispersives

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US (1) US3435381A (xx)
DE (1) DE1267762B (xx)
FR (1) FR1452050A (xx)
GB (1) GB1131442A (xx)
NL (1) NL6605950A (xx)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3581248A (en) * 1969-03-26 1971-05-25 Zenith Radio Corp Acoustic filters
US3760204A (en) * 1972-03-01 1973-09-18 Motorola Inc Acoustic surface wave resonator
USB501482I5 (xx) * 1973-09-04 1976-01-13
US3980904A (en) * 1973-10-26 1976-09-14 Tokyo Shibaura Electric Co., Ltd. Elastic surface wave device
US4649392A (en) * 1983-01-24 1987-03-10 Sanders Associates, Inc. Two dimensional transform utilizing ultrasonic dispersive delay line
US6072813A (en) * 1996-07-09 2000-06-06 Thomson-Csf Device for controlling light pulses by a programmable acoustooptic device
US20040045356A1 (en) * 2000-08-29 2004-03-11 Dwyer-Joyce Robert Sean Method and apparatus for determining thickness of a lubricant film

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3353120A (en) * 1964-01-15 1967-11-14 Csf Acoustic propagation line for compressing trains of electric waves

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3353120A (en) * 1964-01-15 1967-11-14 Csf Acoustic propagation line for compressing trains of electric waves

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3581248A (en) * 1969-03-26 1971-05-25 Zenith Radio Corp Acoustic filters
US3760204A (en) * 1972-03-01 1973-09-18 Motorola Inc Acoustic surface wave resonator
USB501482I5 (xx) * 1973-09-04 1976-01-13
US4012650A (en) * 1973-09-04 1977-03-15 U.S. Philips Corporation Diced substrate S.A.W. device for bulk wave attenuation
US3980904A (en) * 1973-10-26 1976-09-14 Tokyo Shibaura Electric Co., Ltd. Elastic surface wave device
US4649392A (en) * 1983-01-24 1987-03-10 Sanders Associates, Inc. Two dimensional transform utilizing ultrasonic dispersive delay line
US6072813A (en) * 1996-07-09 2000-06-06 Thomson-Csf Device for controlling light pulses by a programmable acoustooptic device
US20040045356A1 (en) * 2000-08-29 2004-03-11 Dwyer-Joyce Robert Sean Method and apparatus for determining thickness of a lubricant film
US7066027B2 (en) * 2000-08-29 2006-06-27 University Of Sheffield Method and apparatus for determining thickness of a lubricant film

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Publication number Publication date
NL6605950A (xx) 1966-11-04
GB1131442A (en) 1968-10-23
DE1267762B (de) 1968-05-09
FR1452050A (fr) 1966-02-25

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