WO1999059377A1

WO1999059377A1 - Active acoustic impedance control device

Info

Publication number: WO1999059377A1
Application number: PCT/FR1999/001121
Authority: WO
Inventors: Xavier Meynial
Original assignee: Centre Scientifique Et Technique Du Batiment (Cstb)
Priority date: 1998-05-12
Filing date: 1999-05-11
Publication date: 1999-11-18
Also published as: FR2778741B1; FR2778741A1

Abstract

The invention concerns a device for actively controlling acoustic impedance in an audio transducer (1) to a medium wherein it radiates, comprising at least one impedance bridge whereof one branch includes an electrical impedance between the two transducer terminals, means for measuring the transducer membrane velocity, and means for measuring the acoustic pressure exerted on the transducer membrane. The pressure measuring means operates by sampling a measuring voltage (Vd) on the impedance bridge. The excitation voltage (VR) applied on the impedance bridge (2) is a combination of the two measured values.

Description

ACTIVE ACOUSTIC IMPEDANCE CONTROL DEVICE

The present invention relates to the field of active acoustic impedance control.

It finds various applications in the field of acoustics, in particular in acoustics of closed volumes (active control of reverberation in rooms, reduction of noise at low frequencies in transport, improvement of insulation at low frequencies between premises) , in aeronautics (absorption of reactor noise), etc.

The acoustic impedance that a material presents to an incident pressure wave determines its absorption, reflection and transmission properties. Consequently, having a material whose acoustic impedance can be varied makes it more or less absorbent, reflective, transparent. This is, for example, what we strive to do with passive materials: rotating panels with a reflecting side and an absorbing side, rolling curtains, etc. Active impedance control aims to adjust the acoustic impedance of a material by active means using transducers, filters and amplifiers. It differs from other active control techniques by the following points: 1. Operation is local reaction: the sensor and the actuator must be separated by a small distance with regard to the relevant wavelengths. The reaction of the material at a point then does not depend on what happens at another point (located at a distance not small compared to the wavelength);

2. This technique does not require a reference signal from the noise source or an error signal to be minimized. It therefore applies to complex fields such as those emitted in a room by sources of which we know almost nothing (position, spectrum ...). - 2 -

When a sensor is placed near an actuator to which it is connected, stability problems arise. Most authors attempt to solve these problems by signal processing techniques aimed at eliminating the direct actuator → sensor path with LMS or adaptive algorithms. These techniques are quite heavy and costly to implement.

A certain number of works have been undertaken on the active control of the acoustic impedance that a transducer, or a network of transducers assembled in panel, presents in the medium in which it is placed. Examples are reported in the following publications: T. Howarth et al., "Piezocomposite coating for active underwater sound reduction", J. Acoust. Soc. Am. 91 (2), February 1992, pages 823-831; L. D ynn et al., "Acoustically active surfaces using piezorubber", J. Acoust. Soc. Am. 90 (3), September 1991, pages 1230-1237; C. Fuller et al., "Control of sound radiation / reflection ith adaptive foams", Proc. Noise-Con 94, Florida, May 1994, pages 429-436; H. Oison et al., "Electronic sound absorber", J. Acoust. Soc. Am. 25 (6), November 1953, pages 1130-1136; D. Guic ing et al., "An active sound absorber with porous plate", Journal of Vibration, Acoustics, Stress, and Reliability in Design, Vol. 106, July 1984, pages 389-392; D. Thenail, "Active impedance control and optimization of the performance of a porous material", Thesis of the Ecole Centrale de Lyon, defended on April 4, 1995, Order No. 95-11; and D. Guicking et al., "Active impedance control for one-dimensional sound", Journal of Vibration, Acoustics, Stress, and Reliability in Design, Vol. 106, July 1984, pages 393-396). In most of these works, the authors use electroacoustic transducers playing either the role of sensor or that of actuator, but not both simultaneously. The transfer functions between actuators and sensors are - 3 - then quite complicated, which imposes an electronic control circuit which is also complex.

Patents have been published on the reduction of noise emitted by reactors (US 5,702,230 and 5,498,127). These patents do not specify the nature of the feedback, but mention the use of microphones located near the speakers.

The principle of motional feedback (see E. De Boer, "Theory of Motional Feedback", IRE Trans. On Audio, January-February 1961) has been used for many years to control the operation of bass speakers frequencies: slaving of the displacement of the membrane to extend the response at low frequencies, and reduction of harmonic distortion (see GB-A-1 438 724). The main advantage of this principle is that it overcomes the need for a speed (or acceleration) sensor, which poses certain problems. It also makes it possible to obtain an excellent signature of the movement of the membrane, in particular with a zero propagation delay.

US Patent 5,588,065 describes a bass speaker using the motional feedback, with speed information coming from the impedance bridge controlling the speaker and pressure information measured in front of a secondary diaphragm of the speaker. .

Some authors have more recently attempted to adapt the motor feedback to noise reduction in ventilation ducts (see J. Okda et al., "Active noise control System using motional feedback loudspeaker", J. Acoust. Soc. of Japan (E), 12 (6), 1991, pages 291-296). But the published results are not very encouraging because these authors use only one quantity for the feedback. They are therefore very limited in terms of device stability. US Patent 4,712,247 describes a feedback device in which the transducer is both a sensor and an actuator. The target application is control - 4 - active reverberation in rooms. This circuit is however too simple to allow obtaining interesting results.

Patent application O97 / 03536 describes the use of a pressure and speed feedback to actively control the acoustic impedance of a loudspeaker system. Taking into account the two parameters, pressure and speed, improves the feedback. Other uses of motional feedback are described in articles by X. Meynial: "Active acoustic materials for room acoustics", CSTB notebook, delivery 367, March 1996; "Active materials for applications in room acoustics", 3 ^rd ICIM / ECSSM '96, Lyon, 1996, pages 968-973. In these articles, the feedback was based on only one electrical quantity, which limited the results.

An object of the present invention is to provide an effective and relatively simple active impedance control device.

The invention thus proposes a device for active control of the acoustic impedance that an electroacoustic transducer presents to a medium in which it radiates, comprising at least one bridge of impedances of which one branch comprises an electrical impedance taken between two terminals of the transducer control means for applying an excitation voltage to the impedance bridge, a first measurement means for obtaining a first quantity representative of the speed of movement of the transducer membrane, and a second measurement means for obtaining a second quantity representative of the acoustic pressure exerted on the diaphragm of the transducer. The second measurement means operates by taking at least one measurement voltage from the impedance bridge. The excitation voltage applied by the control means is a combination of said first and second quantities. The device thus uses a double reaction, one resulting from a measurement of the pressure in the immediate vicinity of the membrane of the actuator, the other resulting from a measurement of the speed of this membrane. By adjusting the degree of these two reactions, one can adjust the acoustic impedance that the device has in the environment in which it is placed.

The pressure measurement is carried out in a simple and economical way by means of the impedance bridge, which makes it possible to dispense with a special pressure sensor. Furthermore, interference in the frequency response corresponding to the membrane-sensor propagation time is avoided.

In a particularly advantageous embodiment of the device, each of the two measurement means operates by taking respective measurement voltages from the impedance bridge, or from two circuits connected to two separate electrical inputs / outputs of the transducer. This eliminates any separate measurement sensor from the transducer control circuit. The structure of the device is then particularly simple.

Other features and advantages of the present invention will appear in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

- Figures 1 and 2 are diagrams of active impedance control devices using a speed and pressure feedback;

- Figures 3 to 5 are diagrams of variant devices.

Since the (complex) acoustic impedance of a group of one or more transducers is Z _a , the reflection coefficient in plane waves of this group is:

_{R =} Z _a . cos (θ) - Z _c Z _a . cos (θ) + Z _c where θ is the angle of incidence of the wave (θ = 0 for normal incidence), - 6 - and Z _c is the characteristic impedance of plane waves: Z _c = p. c with p: density of air (in kg / m ³ ), c: speed of waves in air (in m / s).

The absorption coefficient is a = 1- | R | ² , and the

2. z „transmission coefficient is T = 1 - R =

Z _a . cos (θ) + Z,

If the real part of Z _a is positive (| R | <1 and a> 0), the device is absorbent since it reflects less energy than it receives. If the real part of Z _a is negative (| R |> 1 and a <0), the device reflects more energy than it receives, which is only possible with active materials. | τ | tends to 0 when | Z _a | tends to infinity, and can exceed 1 if Z _a approaches -Z _c , i.e. if the real part of Z _a approaches -Z and is much greater in absolute value than its imaginary part .

The present invention makes it possible to adjust the value of the acoustic impedance Z _a of a transducer. Such a transducer can be used, alone or in a network, in various applications where it is desired to adjust the reflection coefficient R.

The device according to the invention is described below in the particular case of an electrodynamic transducer. It will be understood that a similar device would make it possible to adjust the impedance of another type of transducer (piezoelectric, electrostatic -...).

The device represented in FIG. 1 comprises an impedance bridge 2 to which an amplifier 3 applies an excitation or feedback voltage V _R , obtained by amplifying the output of a mixer 5. The impedrffîœg bridge 2 comprises four branches : the first includes the electrical impedance presented by the transducer 1 between the two excitation terminals of the - 7 - coil setting in motion its membrane; the second branch, mounted in series with the first branch, has an electrical impedance Z '-_; the third branch has an electrical impedance Z _Q ; and the fourth branch, mounted in series with the third, has an electrical impedance Z'r _j . ^The excitation voltage V _R is applied between a common end of the first and third branches and a common end of the second and fourth branches. In the example of FIG. 1, the bridge 2 is arranged so that the differential measurement voltage V _d is representative of the speed V of displacement of the membrane of the transducer 1. For this, the impedances are chosen so that Z ' ₀ x Z _e = Z ₀ x Z ^, ₁ , where Z _e represents the blocked electrical impedance of the transducer

1 (electrical impedance obtained when the membrane of the

Z Z ' ₀ transducer is kept stationary). We note γ = - =

Z _e Z ₀ the ratio between the impedances. The differential voltage V _d is taken between the node located between the first and second branches of the bridge and that located between the third and fourth branches. This voltage V _d = V _v is proportional to the speed V of displacement of the diaphragm of the transducer 1 when the input impedance of the mixer 5 is much greater than each of the impedances of the bridge.

It should be noted that, if the éléσtrodynamique transducer was replaced by an electrostatic or piezoelectric transducer, the differential voltage V _d taken from the bridge would be proportional to the displacement of the membrane (independently of the frequency if the impedance Z ' _χ is capacitive), a differentiating circuit then making it possible to obtain the signal V _v representative of the speed V.

In the device of figure 1, that does not enter - 8 - directly in the context of the invention, a pressure sensor 4, placed in the immediate vicinity of the membrane of the transducer 1, provides a voltage V _p representative of the sensed acoustic pressure. The mixer 5 receives the measurement voltages V _v and

V _p and constructs, with amplifier 3, the feedback voltage V _R by combining these two voltages. The applied combination is linear: V _R =. P + β .V _v (it is assumed here that the coefficient α integrates the proportionality between the measurement voltage V _p and the acoustic pressure P, and that α and β integrate the voltage gain of amplifier 3). Frequency filtering can be applied to the input of the mixer 5 to one of the signals V _p , V _v (or both) to limit the bandwidth processed by the device. The frequency filtering can be carried out after the mixer 5 if the two combined signals are to be filtered in the same way.

When the output impedance Z _s of amplifier 3 is negligible (that is to say | Z _S | «β.Z

and | z _s | "| (Β + γ) .Z _e .γ / (l + γ)), with β" γ and β "1 / γ, the acoustic impedance Z _a presented by the electrodynamic transducer 1 of force factor Bl (B being the induction in teslas in the air gap of the magnet and 1 the length in meters of the conductor located in this field), in a frequency band framing the resonant frequency of the transducer, is:

Z- ≈ - Bl. -. α 1 + γ

An appropriate choice of the coefficients α, β and γ then makes it possible to adjust the value of the acoustic impedance Z _a .

In an embodiment of the device according to the invention (FIG. 2), the impedance bridge 2 is - 9 - dimensioned so that the differential voltage V _d is proportional not to the displacement or the speed of the diaphragm of the transducer 1, but to the force exerted on this membrane (or, equivalently, to the pressure acoustic: V _d = V). The voltage V _v for measuring the speed of the membrane, used in the development of the feedback voltage V _R , can then result from an acceleration measurement by an accelerometer 14 installed on the membrane, integrated by a pass-through circuit. low 15. An example of choice of the impedances Z _QI Z and Z ¹ suitable for such an embodiment with an electrodynamic transducer checks Z ' ₀ x (Z _e + (Bl) ² / Z _m ) = Z _Q x Z' _lr where Z _m represents the mechanical impedance of the transducer 1, for example Z ' ₀ = δ.Z ₀ and Z = δ. (Z _e + (Bl) ² / Z _m ), or even Z' ₀ = δ.Z ^ and Z _Q = δ. (Z _e + (Bl) ² / Z _m ). This pressure measurement V _d = V is independent of the frequency if the impedance Z '-_ is active and such that Z ¹ ₁ = -Z _e . Yes

Z ' ₁ is of another form, the differential voltage V _d , always independent of the speed V, depends on the frequency, which can be compensated by filtering.

If the transducer 1 is of the electrostatic type with a blocked capacity C _e , a transduction coefficient N is a mechanical impedance Z _m , the differential voltage V _d measures the acoustic pressure exerted on the membrane if Z ' ₀ = Z ₀ x Z x [jC _e ω + l / (N ² Z _m )], independently of the frequency if Z ' _χ = -l / (jC _e ω).

Some transducers have multiple pairs of electrical terminals. One can in particular have two coils 7.8 wound around the same core 9 actuating the membrane of an electrodynamic transducer 1 (Figure 3), with exςy-tation of one of these coils or both from 1 'airvp_Li.ficateur. These two windings 7.8 are associated with respective circuits 2.10. In another realization - 10 - of the invention, illustrated in FIG. 3, one of these circuits is the impedance bridge 2 to which the feedback voltage V _R is applied. This bridge 2 is for example dimensioned so that its differential voltage V _d = V _v is proportional to the speed V of the membrane of the transducer 1. The other circuit 10 can be an active circuit arranged to present a negative resistance -R opposite to the resistance R _e of the winding 8 with which it is connected in series. The voltage V _p proportional to the force exerted on the membrane can then be taken from the terminals of the winding 8. The two voltages V _v , V _p are combined as before to obtain the excitation voltage V _R of the transducer.

If the circuit 10 connected to the winding 8 also consisted of an impedance bridge receiving the excitation voltage V _R , the pressure information V _p would be taken as the differential voltage of this bridge, dimensioned to obtain the desired proportionality with the force exerted on the membrane, as explained with reference to Figure 2.

In another embodiment of the invention, the same impedance bridge 2 makes it possible to obtain the two pieces of information necessary for the feedback. The main interest of this technique, besides the fact that it allows to do without an additional sensor (microphone, accelerometer ...), is that there is no delay in the feedback loop , and therefore that the open loop transfer function of the system has a phase varying less than that obtained with an additional sensor. The stability of the system is therefore improved.

A particular case of a circuit allowing the capture of two pieces of information is shown in FIG. 4. The arrangement of the bridge is comparable to that previously described with reference to FIG. 1, a first measurement resistor R "-_ being connected in series. - 11 - with the impedance Z '-_ in the second branch. A first measurement voltage V "- _^ is taken from the terminals of this resistor R '.

Between the node situated between the first and second branches and that situated between the third and fourth branches, the bridge comprises a measuring branch composed of a measurement impedance Z _d and of a second measurement resistance R _d connected in series and such as | Z _d ”R _d . A second measurement voltage e _d is sampled across the resistor R _d , and a third measurement voltage V _d is sampled across the impedance Z _d .

The impedances Z _Q , Z ' ₀ and Z' -_ are, as

previously, chosen such that - =: -. We choose Z _d

very much higher than the impedances Z _Q , Z ' ₀ and Z' _χ in all the frequency band to be treated, so that the voltage V _d is very little affected by the presence of the measuring branch. The measurement resistances are such that R _d «| Z _d |

and R " ₁ " | Z ' ₁ | in the whole frequency band to be processed.

For example, for an electrodynamic transducer with mechanical impedance Z _m and force factor Bl, with

Z _d = δ.Bl ² / Z _m , the feedback voltage V _R produced by the mixer 5 can be V _R = G ₁ .e _d + G ₂ ^{, v} " _l ⁺ β- ^v _d with

Bl.δ γ + l] (Bl

^G ι = α and G ₂ = α. In this case we have Rd-s γ SR "

^G _l - ^e _d ^{+ G} ₂ - ^V " _l ^≈ α.P, and V _d ≈ -V.B1 / 2. The feedback voltage V _R is therefore, as before, a linear combination of P and V. En d In other words, the measurement voltage representative of the pressure P is in this case a linear combination of two voltages V " ₁ , e _d respectively taken from the terminals of the first and - 12 - second measurement resistors R " ₁ , R _d .

In the embodiment illustrated in Figure 5, the transducer 1 and the impedance Z ' _χ belong to two impedance bridges 2,2' forming a double bridge. The bridge 2 is for example similar to that of FIG. 1, its differential voltage V _v measuring the speed. The bridge 2 'has a similar arrangement, the impedances Z _Q and Z' ₀ being replaced by other values Z ₂ and Z ' ₂ so that the differential voltage V and of this bridge 2' measures the pressure.

Claims

- 13 - CLAIMS 1. Active control device of the acoustic impedance that an electroacoustic transducer (1) has in a medium in which it radiates, comprising at least one bridge of impedances (2; 2 ') of which a branch comprises a electrical impedance taken between two terminals of the transducer, control means (3,5) for applying an excitation voltage (V _R ) to the impedance bridge, a first measurement means to obtain a first representative quantity (V _v ) the speed of movement of the transducer membrane and a second measuring means to obtain a second quantity (V _p ) representative of the acoustic pressure exerted on the transducer membrane, in which the excitation voltage applied by the control means is a combination of said first and second quantities, characterized in that the second measurement means operates by taking at least one measurement voltage (V _p ; e _d , V "χ) from the impedance bridge s (2).

2. Device according to claim 1, wherein the first and second measurement means operate by taking respectively first and second measurement voltages (V _v , V _p ; ea, V "ι) on the impedance bridge (2) .

3. Device according to claim 2, wherein the impedance bridge (2) comprises: a first branch comprising one electrical impedance between the two terminals of the transducer; a second branch connected in series to the first branch and comprising a first measurement resistor (R "ι) connected in series to an impedance (Z'χ) proportional, in a ratio γ, to that presented by the first branch when the moving element of - 14 - transducer is blocked and modulus significantly greater than the value of the first measurement resistance;

- a third branch; a fourth branch connected in series to the third branch and of impedance (Z'o) proportional to

(Zo) that of the third branch in the ratio γ; and

- a measurement branch comprising a second measurement resistor (R _d ) and a measurement impedance (Z _d ), of modulus significantly greater than the value of the second measurement resistance, connected in series between the node located between the first and second branches and the node located between the third and fourth branches, in which the excitation voltage (V _R ) is applied between a common end of the first and third branches and a common end of the second and fourth branches, in which the first measurement voltage (V _d ) is taken from the terminals of the measurement impedance (Z), and in which the second measurement voltage is a linear combination of two voltages (V "ι, e _d ) respectively taken from the terminals of the first and second measurement resistors (R "ι, Rd) •

4. Device according to claim 1, in which the transducer comprises several pairs of electrical terminals, in which one of the first and second measurement means operates by taking a measurement voltage

(V _d ) on the impedance bridge (2) comprising the electrical impedance taken between the two terminals of one of the pairs, while the other measuring means comprises a circuit (10) connected to the two terminals of a other pair.

5. Device according to claim 1, comprising two impedance bridges (2,2 ') having at least one common branch comprising the electrical impedance taken between the - 15 - terminals of the transducer, the first and second measurement means operating by respectively taking measurement voltages (V _v , V _p ) from the two impedance bridges.