EP1002311B1 - Method and apparatus for attenuating sound - Google Patents

Method and apparatus for attenuating sound Download PDF

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Publication number
EP1002311B1
EP1002311B1 EP97913207A EP97913207A EP1002311B1 EP 1002311 B1 EP1002311 B1 EP 1002311B1 EP 97913207 A EP97913207 A EP 97913207A EP 97913207 A EP97913207 A EP 97913207A EP 1002311 B1 EP1002311 B1 EP 1002311B1
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EP
European Patent Office
Prior art keywords
sound
actuator
sound pressure
approximation
particle velocity
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP97913207A
Other languages
German (de)
French (fr)
Other versions
EP1002311A1 (en
Inventor
Kari Kirjavainen
Jukka Lekkala
Hannu NYKÄNEN
Seppo Uosukainen
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VTT
VTT Technical Research Centre of Finland Ltd
Original Assignee
VTT
VTT Technical Research Centre of Finland Ltd
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Publication of EP1002311A1 publication Critical patent/EP1002311A1/en
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
    • G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
    • G10K11/1787—General system configurations
    • G10K11/17879—General system configurations using both a reference signal and an error signal
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
    • G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
    • G10K11/1785—Methods, e.g. algorithms; Devices
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3026—Feedback
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3027—Feedforward

Definitions

  • the invention as it is defined in the appended claims relates to a method of attenuating sound, comprising measuring at least one component of a sound field and producing attenuation sound by means of at least one actuator on the basis of the measurement result, and adjusting the measurement result by a coefficient.
  • the invention further relates to an apparatus for attenuating sound, comprising at least one device measuring a component of a sound field and at least one actuator producing attenuation sound on the basis of the measurement result.
  • JMC elements It is previously known to attenuate sound by using elements known as JMC elements.
  • the idea of the method is to measure the sound pressure and particle velocity of a sound field and on the basis of the measurements to control actuators, i.e. means for producing attenuation sound, so as to allow a sound field to be produced which is exactly similar to the original sound field but of the opposite sign. In that case, the sound field can be completely eliminated by means of the actuators.
  • a dipole actuator is driven by means of the measured sound pressure, a monopole actuator by means of the normal component of particle velocity, and a quadripole actuator by means of the tangential component of particle velocity. This method is, however, complicated and difficult to implement.
  • An object of the present invention is to provide a method and apparatus by means of which sound attenuation can be implemented in a reasonably effective and simple way.
  • the method of the invention is characterized in that a measurement result is adjusted with the approximation of the impedance or admittance of a sound field, and the approximation is adjusted on the basis of the measured residual sound pressure.
  • the apparatus of the invention is characterized in that the apparatus comprises at least one device measuring residual sound pressure and a device producing an error signal, said apparatus being arranged to adjust the attenuation sound to be produced on the basis of the approximation of the impedance or admittance of a sound field, which has been adjusted on the basis of the measured residual sound pressure.
  • the basic idea of the invention is that at least one component of the sound field is measured, and at least one actuator producing a sound field of the opposite sign is controlled on the basis of the measurement, and the result is adjusted by utilizing the approximation of the impedance or admittance of the sound field, and the approximation can be estimated and adjusted by measuring the residual sound pressure.
  • Any detector or actuator can be left out of the apparatus consisting of two detectors and two actuators and replaced with the approximation of the impedance or admittance of the sound field in question.
  • the idea of one preferred embodiment is that the value of the impedance or admittance is adjusted substantially continuously by measuring how high the level of the residual sound pressure is.
  • An advantage of the invention is that with respect to sound attenuation very good and reliable results are achieved with a relatively simple method and apparatus.
  • Figure 1 illustrates a sound attenuation element 1.
  • the sound attenuation element 1 comprises a detector 2 for the sound pressure p.
  • the sound attenuation element 1 further comprises a detector 3 for the particle velocity.
  • the device is used for indicating the normal component u x of particle velocity.
  • the sound pressure detector 2 controls a dipole actuator 4 directly via a filter 5 on the basis of the measurement result of the sound pressure p.
  • the dipole actuator 4 is used for producing a sound opposite to the original sound field.
  • the intensity of the sound produced by the dipole actuator 4 is described with the strength f s of the dipole.
  • the normal component u x of particle velocity measured with the particle velocity detector 3 is also utilized in controlling the dipole actuator 4.
  • the effect of the particle velocity is taken into account by the device 6 producing a correction signal.
  • the velocity component is multiplied by the approximation of the impedance H z of the sound field.
  • H z ⁇ x c can be considered the initial value, where ⁇ is the density of the medium and c is the sound velocity in the medium.
  • the approximation of the impedance H z is adjusted by measuring the remaining sound pressure, i.e. the residual sound pressure p res , which is sent to the device 6 producing a correction signal.
  • control can be implemented with substantially no delay, and furthermore, by utilizing the velocity signal u x adjusted with the approximation of the impedance H z , an accurate result can be achieved.
  • There may be several measuring devices for the residual sound pressure p res whereby the residual sound pressure p res can be measured in several places. If the environmental conditions and sound source remain constant, it is not necessary to alter the approximation of the impedance after the desired level of attenuation has been achieved.
  • the measuring devices 7 for residual pressure can be used continuously, whereby the approximation of the impedance H 2 can be adjusted continuously by means of the devices, and thus the attenuation sound produced by the dipole actuator 4 can also be adjusted very accurately.
  • FIG. 2 illustrates a second sound attenuation element 1'.
  • the same numbers have the same significance as in Figure 1.
  • a monopole actuator 4' is controlled via a filter 5' on the basis of the particle velocity. The intensity of the sound produced by the monopole actuator 4' is described with the volume velocity q s .
  • the monopole actuator 4' is controlled by means of the sound pressure p in such a manner that the measurement result of the sound pressure p is adjusted with the approximation of the admittance H 1/z of the sound field.
  • the admittance of the sound field is the inverse of the impedance of the sound field.
  • the admittance function H 1/z of the sound field is adjusted in the same way as the impedance function H z of the sound field.
  • Figure 3 illustrates a third sound attenuation element 1" according to the invention.
  • the same numbers have the same significance as in Figures 1 and 2.
  • the dipole actuator 4 is controlled directly on the basis of the sound pressure p.
  • the particle velocity u x is not measured at all.
  • the monopole actuator 4' is controlled with the measuring result of the sound pressure p, adjusted in the device producing a correction signal by means of the approximation of the admittance H 1/z of the sound field as in the case shown in Figure 2.
  • Figure 4 illustrates a fourth sound attenuation element 1"' according to the invention.
  • the monopole actuator 4' is controlled on the basis of the value of the particle velocity u x .
  • the sound pressure p is not measured at all, but the dipole actuator 4 is controlled by means of the measuring result of the particle velocity u x , adjusted in the device 6 producing a correction signal by means of the approximation of the impedance H z as was explained in connection with Figure 1.
  • FIG 5 illustrates an overall system of sound attenuation.
  • the overall system comprises several sound attenuation elements 1, 1', 1" or 1"' of the invention. All measuring results achieved with the measuring device 7 for residual sound pressure are collected in the system in order to influence the adjustment of each attenuation element. The adjustment of different elements may depend differently on different residual sound pressures.
  • the measuring devices 7 for residual sound pressure may be mobile and even portable, whereby the minimization of the sound pressure focuses most effectively on the persons carrying the device.
  • the arrangement is suitable for mediums in which sound causes a longitudinal wave motion, i.e. besides air, the medium may be e.g. another gas or a liquid.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
  • Soundproofing, Sound Blocking, And Sound Damping (AREA)
  • Devices For Supply Of Signal Current (AREA)
  • Diaphragms For Electromechanical Transducers (AREA)

Abstract

The invention relates to a method and an apparatus for attenuating sound, comprising measuring at least one component (p, ux) of a sound field and producing attenuation sound on the basis of the measurement result by means of at least one actuator (4). The measurement result is adjusted by means of the approximation of the impedance or admittance of the sound field, and the approximation is adjusted on the measured residual sound pressure (pres). This allows the attenuation of sound to be implemented in a reasonably simple and effective way.

Description

The invention as it is defined in the appended claims relates to a method of attenuating sound, comprising measuring at least one component of a sound field and producing attenuation sound by means of at least one actuator on the basis of the measurement result, and adjusting the measurement result by a coefficient.
The invention further relates to an apparatus for attenuating sound, comprising at least one device measuring a component of a sound field and at least one actuator producing attenuation sound on the basis of the measurement result.
It is previously known to attenuate sound by using elements known as JMC elements. The idea of the method is to measure the sound pressure and particle velocity of a sound field and on the basis of the measurements to control actuators, i.e. means for producing attenuation sound, so as to allow a sound field to be produced which is exactly similar to the original sound field but of the opposite sign. In that case, the sound field can be completely eliminated by means of the actuators. In said method a dipole actuator is driven by means of the measured sound pressure, a monopole actuator by means of the normal component of particle velocity, and a quadripole actuator by means of the tangential component of particle velocity. This method is, however, complicated and difficult to implement. Therefore, an approximation disregarding the tangential component of particle velocity has been introduced. This yields relatively good results, but the arrangement and apparatus are still complicated and difficult to implement. Furthermore, the results achieved with such an arrangement are easily spoiled e.g. by background noise. In a further arrangement that has been introduced, only the sound pressure is measured, and attenuation sound is generated by means of only one actuator, assuming that the proportion of sound pressure to particle velocity is constant. This approximation is based on plane wave approximation of the original sound field, whereby both the detector and the actuator have cardioid detection/radiation patterns. Such an approximation, however, often leads to too inaccurate results especially in confined space, and is thus not satisfactory.
An object of the present invention is to provide a method and apparatus by means of which sound attenuation can be implemented in a reasonably effective and simple way.
The method of the invention is characterized in that a measurement result is adjusted with the approximation of the impedance or admittance of a sound field, and the approximation is adjusted on the basis of the measured residual sound pressure.
The apparatus of the invention is characterized in that the apparatus comprises at least one device measuring residual sound pressure and a device producing an error signal, said apparatus being arranged to adjust the attenuation sound to be produced on the basis of the approximation of the impedance or admittance of a sound field, which has been adjusted on the basis of the measured residual sound pressure.
The basic idea of the invention is that at least one component of the sound field is measured, and at least one actuator producing a sound field of the opposite sign is controlled on the basis of the measurement, and the result is adjusted by utilizing the approximation of the impedance or admittance of the sound field, and the approximation can be estimated and adjusted by measuring the residual sound pressure. Any detector or actuator can be left out of the apparatus consisting of two detectors and two actuators and replaced with the approximation of the impedance or admittance of the sound field in question. The idea of one preferred embodiment is that the value of the impedance or admittance is adjusted substantially continuously by measuring how high the level of the residual sound pressure is.
An advantage of the invention is that with respect to sound attenuation very good and reliable results are achieved with a relatively simple method and apparatus. By adjusting the impedance or admittance function by means of feedback substantially continuously, very accurate results can be achieved with respect to sound attenuation.
The invention will be described in greater detail with reference to the accompanying drawings, in which
  • Figure 1 illustrates a general diagram of an apparatus according to the invention,
  • Figure 2 illustrates a general diagram of a second apparatus according to the invention,
  • Figure 3 illustrates a general diagram of a third apparatus according to the invention,
  • Figure 4 illustrates a general diagram of a fourth apparatus according to the invention,
  • Figure 5 illustrates an overall system of sound attenuation, utilizing the method and apparatus of the invention.
    • Figure 1 illustrates a sound attenuation element 1. The sound attenuation element 1 comprises a detector 2 for the sound pressure p. The sound attenuation element 1 further comprises a detector 3 for the particle velocity. The device is used for indicating the normal component ux of particle velocity. The sound pressure detector 2 controls a dipole actuator 4 directly via a filter 5 on the basis of the measurement result of the sound pressure p. The dipole actuator 4 is used for producing a sound opposite to the original sound field. The intensity of the sound produced by the dipole actuator 4 is described with the strength fs of the dipole. The normal component ux of particle velocity measured with the particle velocity detector 3 is also utilized in controlling the dipole actuator 4. The effect of the particle velocity is taken into account by the device 6 producing a correction signal. In said device, which may be e.g. a microprocessor, the velocity component is multiplied by the approximation of the impedance Hz of the sound field. The real value of the impedance of the sound field is not known in advance, and thus Hz = ρ x c can be considered the initial value, where ρ is the density of the medium and c is the sound velocity in the medium. The approximation of the impedance Hz is adjusted by measuring the remaining sound pressure, i.e. the residual sound pressure pres, which is sent to the device 6 producing a correction signal. The algorithms needed are fully known per se to one skilled in the art, and therefore, this application will not deal with adjustment criteria in any greater detail. By controlling the dipole actuator 4 directly with the sound pressure detector 2, control can be implemented with substantially no delay, and furthermore, by utilizing the velocity signal ux adjusted with the approximation of the impedance Hz, an accurate result can be achieved. There may be several measuring devices for the residual sound pressure pres, whereby the residual sound pressure pres can be measured in several places. If the environmental conditions and sound source remain constant, it is not necessary to alter the approximation of the impedance after the desired level of attenuation has been achieved. However, if the conditions are dynamic, the measuring devices 7 for residual pressure can be used continuously, whereby the approximation of the impedance H2 can be adjusted continuously by means of the devices, and thus the attenuation sound produced by the dipole actuator 4 can also be adjusted very accurately.
    Figure 2 illustrates a second sound attenuation element 1'. In Figure 2 the same numbers have the same significance as in Figure 1. In a device according to Figure 2, a monopole actuator 4' is controlled via a filter 5' on the basis of the particle velocity. The intensity of the sound produced by the monopole actuator 4' is described with the volume velocity qs. The monopole actuator 4' is controlled by means of the sound pressure p in such a manner that the measurement result of the sound pressure p is adjusted with the approximation of the admittance H1/z of the sound field. The admittance of the sound field is the inverse of the impedance of the sound field. The admittance function H1/z of the sound field is adjusted in the same way as the impedance function Hz of the sound field.
    Figure 3 illustrates a third sound attenuation element 1" according to the invention. In Figure 3 the same numbers have the same significance as in Figures 1 and 2. In the device shown in Figure 3 the dipole actuator 4 is controlled directly on the basis of the sound pressure p. The particle velocity ux is not measured at all. Instead, the monopole actuator 4' is controlled with the measuring result of the sound pressure p, adjusted in the device producing a correction signal by means of the approximation of the admittance H1/z of the sound field as in the case shown in Figure 2.
    Figure 4 illustrates a fourth sound attenuation element 1"' according to the invention. In Figure 4 the same numbers have the same significance as in Figures 1 to 3. The monopole actuator 4' is controlled on the basis of the value of the particle velocity ux. The sound pressure p is not measured at all, but the dipole actuator 4 is controlled by means of the measuring result of the particle velocity ux, adjusted in the device 6 producing a correction signal by means of the approximation of the impedance Hz as was explained in connection with Figure 1.
    Figure 5 illustrates an overall system of sound attenuation. The overall system comprises several sound attenuation elements 1, 1', 1" or 1"' of the invention. All measuring results achieved with the measuring device 7 for residual sound pressure are collected in the system in order to influence the adjustment of each attenuation element. The adjustment of different elements may depend differently on different residual sound pressures.
    The drawings and the related description are only intended to illustrate the idea of the invention. The details of the invention may vary within the scope of the appended claims The measuring devices 7 for residual sound pressure may be mobile and even portable, whereby the minimization of the sound pressure focuses most effectively on the persons carrying the device. The arrangement is suitable for mediums in which sound causes a longitudinal wave motion, i.e. besides air, the medium may be e.g. another gas or a liquid.

    Claims (14)

    1. A method of attenuating sound, comprising measuring at least one component of a sound field, and producing attenuation sound by means of at least one actuator, and adjusting the measurement result with a coefficient, characterized in that the measurement result is adjusted with the approximation of the impedance or admittance of the sound field, and the approximation is adjusted on the basis of the measured residual sound pressure (pres).
    2. A method as claimed in claim 1, characterized in that the pressure (p) and particle velocity (ux) of the sound field are measured, and a dipole actuator (4) is controlled substantially directly by means of the sound pressure (p), and the influence of the particle velocity (ux) on the dipole actuator (4) is taken into account, adjusted with the approximation of the impedance of the sound field.
    3. A method as claimed in claim 1, characterized in that the particle velocity (ux) and sound pressure (p) of the sound field are measured, and a monopole actuator (4') is controlled substantially directly by means of the particle velocity (ux), and the influence of the sound pressure (p) on the monopole actuator (4') is taken into account, adjusted with the approximation of the admittance of the sound field.
    4. A method as claimed in claim 1, characterized in that the sound pressure (p) of the sound field is measured, and a dipole actuator (4) is controlled substantially directly by means of the sound pressure (p), and a monopole actuator (4') is controlled by means of the sound pressure (p) adjusted with the approximation of the admittance of the sound field.
    5. A method as claimed in claim 1, characterized in that the particle velocity (ux) of the sound field is measured, and a monopole actuator (4') is controlled substantially directly by means of the particle velocity (ux), and a dipole actuator (4) is controlled with the value of the particle velocity (ux), which has been adjusted by means of the approximation of the impedance of the sound field.
    6. A method as claimed in any one of the preceding claims, characterized in that the residual sound pressure (pres) is measured substantially continuously.
    7. A method as claimed in any one of the preceding claims, characterized in that the residual sound pressure (pres) is measured in several different places.
    8. An apparatus for attenuating sound, comprising at least one device measuring a component of a sound field and at least one actuator producing attenuation sound on the basis of the measurement result, characterized in that the apparatus comprises at least one device (7) measuring residual sound pressure and a device (6) producing a correction signal, and the device (6) is arranged to adjust the attenuation sound to be produced by means of the approximation of the impedance or admittance of the sound field, which is adjusted on the basis of the measured residual sound pressure (pres).
    9. An apparatus as claimed in claim 8, characterized in that the apparatus comprises a sound pressure detector (2), particle velocity detector (3), and dipole actuator (4) producing attenuation sound, whereby the sound pressure detector (2) is arranged to control the dipole actuator (4) substantially directly, and the particle velocity detector (3) is arranged to control the dipole actuator (4) via the device (6) producing a correction signal, and the device (6) is arranged to produce the correction signal by means of the approximation of the impedance of the sound field.
    10. An apparatus as claimed in claim 8, characterized in that the apparatus comprises a sound pressure detector (2), particle velocity detector (3), and monopole actuator (4') arranged to produce attenuation sound, whereby the particle velocity detector (3) is arranged to control the monopole actuator (4') substantially directly, and the sound pressure detector (2) is arranged to control the monopole actuator (4') via the device (6) producing a correction signal, and the device (6) is arranged to produce the correction signal by means of the approximation of the admittance of the sound field.
    11. An apparatus as claimed in claim 8, characterized in that the apparatus comprises a sound pressure detector (2), and a dipole actuator (4) and monopole actuator (4') producing attenuation sound, whereby the sound pressure detector (2) is arranged to control the dipole actuator (4) substantially directly and the monopole actuator (4') via the device (6) producing a correction signal, and the device (6) is arranged to produce the correction signal by means of the approximation of the admittance of the sound field.
    12. An apparatus as claimed in claim 8, characterized in that the apparatus comprises a particle velocity detector (3), and a dipole actuator (4) and monopole actuator (4') producing attenuation sound, whereby the particle velocity detector (3) is arranged to control the monopole actuator (4') substantially directly and the dipole actuator (4) via the device (6) producing a correction signal, and the device (6) is arranged to produce the correction signal by means of the approximation of the impedance of the sound field.
    13. An apparatus as claimed in any one of claims 8 to 12, characterized in that the apparatus comprises several devices (7) for measuring residual sound pressure (pres).
    14. An apparatus as claimed in any one of claims 8 to 13, characterized in that at least one device (7) for measuring residual sound pressure (pres) is arranged to be carried by a person in the sound field.
    EP97913207A 1996-11-20 1997-11-19 Method and apparatus for attenuating sound Expired - Lifetime EP1002311B1 (en)

    Applications Claiming Priority (3)

    Application Number Priority Date Filing Date Title
    FI964636A FI105602B (en) 1996-11-20 1996-11-20 Method and device for sound attenuation
    FI964636 1996-11-20
    PCT/FI1997/000707 WO1998022933A1 (en) 1996-11-20 1997-11-19 Method and apparatus for attenuating sound

    Publications (2)

    Publication Number Publication Date
    EP1002311A1 EP1002311A1 (en) 2000-05-24
    EP1002311B1 true EP1002311B1 (en) 2002-03-06

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    Family Applications (1)

    Application Number Title Priority Date Filing Date
    EP97913207A Expired - Lifetime EP1002311B1 (en) 1996-11-20 1997-11-19 Method and apparatus for attenuating sound

    Country Status (6)

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    EP (1) EP1002311B1 (en)
    AT (1) ATE214191T1 (en)
    AU (1) AU5054498A (en)
    DE (1) DE69710928T2 (en)
    FI (1) FI105602B (en)
    WO (1) WO1998022933A1 (en)

    Families Citing this family (1)

    * Cited by examiner, † Cited by third party
    Publication number Priority date Publication date Assignee Title
    GB2542860B (en) * 2015-10-04 2021-10-20 Labxero Ltd Acoustic processing device

    Family Cites Families (3)

    * Cited by examiner, † Cited by third party
    Publication number Priority date Publication date Assignee Title
    US4473906A (en) * 1980-12-05 1984-09-25 Lord Corporation Active acoustic attenuator
    GB8328997D0 (en) * 1983-10-31 1983-11-30 Secr Defence Active noise reduction
    JPH05223334A (en) * 1992-02-14 1993-08-31 Matsushita Seiko Co Ltd Active silencer

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    Publication number Publication date
    ATE214191T1 (en) 2002-03-15
    DE69710928D1 (en) 2002-04-11
    AU5054498A (en) 1998-06-10
    FI964636L (en) 1998-05-21
    FI964636A0 (en) 1996-11-20
    DE69710928T2 (en) 2002-09-19
    WO1998022933A1 (en) 1998-05-28
    EP1002311A1 (en) 2000-05-24
    FI105602B (en) 2000-09-15

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