FR2661540A1 - METHOD FOR ATTENUATING ACOUSTIC WAVES IN A FLUID CIRCUIT. - Google Patents

Abstract

The process involves the insertion of at least one transducer into the wall of a pipe belonging to this circuit. The transducer causes the wall to vibrate, thus creating an active moving wall whose radial displacements are in phase opposition to the local acoustic pressure. This produces a condition in which the acoustic waves are not propagated so that waves coming from the pipe towards the transducer are reflected.

Description

The present invention relates to a new method for the attenuation of

  acoustic waves in a fluid circulation circuit.

  We know that in a fluid circulation circuit, whether pneumatic or hydraulic, noises are created by different elements, namely motor elements such as pumps or fans, structural elements such as diaphragms, elbows or valves , and the like The acoustic waves thus generated propagate in said circuits, up to the terminations where they create nuisances harmful to the personnel. Thus, for example, in a ventilation circuit, the transmission of these acoustic waves to the outside results in

  an increase in the noise level in the room to be ventilated or air conditioned.

  We therefore sought means to reduce or control the waves S e propagating in this type of circuit, without disturbing the flow of the fluid which circulates there. A first type of means uses passive systems, based for example on the use of absorbent materials or resonators of the HELMHOLTZ type. However, these solutions based on the dissipative qualities of the

  materials can only relate to the area of high frequencies.

  A second type of means uses active systems, that is to say determining an attenuation of the waves. For this, it is known to associate on the one hand a member for measuring the waves propagating in the circuit, and on the other hand an acoustic transmitter controlled from the signals collected by said organ, for example a microphone, in such a way that, by this transmitter, a train of waves is emitted in opposition to the previous one, and that their result is in principle zero . However, such a system, attractive in principle, in reality only provides significant attenuation for sources of harmonic noise; for noises

  broadband, attenuation remains weak.

  The present invention also relates to an active system, but based on a different principle, namely that according to which, if the circuit wall is subjected over a certain portion to local deformations corresponding to a vibratory state depending on the frequency of the train incident acoustic waves, the latter are reflected and can no longer propagate in this portion, which

  is thus acoustically isolated.

  Due to the fact that there is no longer any composition between two wave trains as in the prior system mentioned above, the efficiency limitations of the latter are eliminated and the new system according to the invention allows significant attenuation. incident waves in the whole frequency range This system

  therefore represents very significant progress.

  We will now describe the invention in more detail with reference to the

  following description, illustrated by the appended drawing, in which:

  FIG. 1 represents a diagram illustrating the theoretical presentation of the invention; FIG. 2 illustrates a practical embodiment of a circular active movable wall according to the invention; FIG. 3 illustrates another practical embodiment of a circular active mobile wall; FIG. 4 illustrates a practical embodiment of a square active movable wall; FIG. 5 illustrates an example of assembly of the entire system according to the invention and;

  FIG. 6 illustrates a variant construction of the elementary active filter.

  Referring first of all to the diagram in FIG. 1, there is symbolically represented a portion of the circuit consisting of a cylindrical tube with radius R,

  assumed to be infinitely rigid, and in which there are three portions 1, 2 and 3.

  The intermediate portion 2 comprises an element supposed to create on the walls of the tube a condition of the impedancial type: (2.1) P = Z Vr PV In this relation, as in all those which will follow in the present

  description, the different variables and the units in which they are

  expressed are as follows: P: acoustic pressure (in Pascals) Vr: radial speed of the wall vibrations (in m / s) Z: acoustic impedance, homogeneous to the product of the density of the fluid, P, by the speed of the sound , c, in the fluid (in Kg Im 2 Is) pulsation (in rd / s) k = c: number of acoustic waves (in m-1) CI>: velocity potential (in mri 2 / s): admittance wall acoustics and separation constants

(am Ai) are dimensionless.

  In this portion of the circuit, it is possible to construct an analytical solution by product of functions with separate variables. We only consider here the components of zero order according to the angular variable, but the reasoning could be extended to non-pressure fields. uniforms along the angle around the axis of the conduit, which is expressed by the relation: (2.2 l C) (Wo) = 5 l An exgpikum 2) + Bm exp & -ikom l Jo (Am i) m = OR in which:

(2.3)

cam = f (1 -4 to 2)

(2.4)

Arn = kam R

(2.5)

  Am = Jo (A; = i Ak R Jc Au) In the case where the wall is rigid, the first separation constant Am is

  null, which leads to the existence of a plane wave.

  By writing the continuity of the pressure and the axial speed at the interfaces between the portions 1, 2 and 3 of the circuit, it is possible to express the amplitude of the wave transmitted in the tube 3 as a function of the amplitude of the incident wave in the tube 1 It immediately appears that this amplitude can be canceled if the number a becomes zero This consists in fact of imposing a condition of non-propagation of the acoustic waves in the direction z We then have as immediate consequence that the separation constant am is unitary, which translates for admittance by: (2 6) i OI (k R) The relation (2 6) must be supplemented by the fact that admittance D must be an imaginary number pure whose value must remain strictly negative in order to prohibit the existence of quasi-plane wave This type of wave is represented by the solutions An of equation (2 6) with zero real part and can only exist the condition that the

  imaginary part of admittance either positive or zero.

  At the low frequencies of the spectrum, this only condition relating to the fact that the admittance is a pure imaginary number whose sign is negative makes it possible to ensure

a non-propagation condition.

  In summary, it is therefore found that by imposing on the wall of the tube 2 the condition (2.6) it is possible to completely cancel the wave transmitted from the tube 1 to the tube 3 and vice versa This condition actually translates the absorption by the wall of the acoustic energy in the tube 2 The wave vector is indeed oriented in the radial direction. This principle has various advantages which mainly consist in the fact that one is reduced to a local condition linking the pressure and the speed of deformation of the wall induced by the vibrations and that this condition remains true whatever the external disturbances, l incoherence of the pressure field, the reflections on the termination, etc. We can also note that the principle is not based on the existence of plane waves as are the 'classic' systems before, but that it concerns all wave types This system will therefore be less limited in frequency or at least will have different restrictions For this purpose, we can notice that in formula (2 6), the denominator is canceled out for the zeros of the Bessel function this

  which corresponds to the frequencies where the wall is placed on a pressure node.

  The admittance is then infinite, which would lead to imposing wall displacements of infinitely large amplitude In fact, this problem could be circumvented by using several stages of 'filters' of different diameters, but of very short length, which would function in bands of different frequencies These filters will in practice consist of small transducers placed on the circumference of the tube, as will now be described in connection with several embodiments

  practical, with reference to Figures 2 to 5.

  If the term “active movable wall” designates the portion 2 of the pipe wall playing the role of “filter” for an incident wave train, it is intended, according to the invention, to use, to carry out the above-mentioned impedance condition on this wall, electro-mechanical systems of the transducer type, which can for example use piezoelectric effects, which are placed in such a way that they can ensure a

  rate of deformation of the wall over the entire periphery thereof.

  One means of achieving this result may consist, as illustrated in FIG. 2, of inserting a transducer 10 into a ring 11, the vibrations of which cause

uniform variations of the radius.

  Another means may consist in regularly distributing transducer elements 12 of the piston type all around the duct C, either circular as illustrated in FIG. 3, or square as illustrated in FIG. 4, in which case the

  transducers are placed respectively equiangularly or on each side.

  In general, any vibrating system 12 which can be controlled by an electric control (electromagnet, piezoelectric or the like) can be used

  to impose on the wall C vibrations inducing appropriate deformations.

  If we refer more particularly to FIG. 5, there is illustrated diagrammatically

  an assembly allowing the control of the elements 12.

  In order to allow the elements 12 of the movable wall to be properly controlled, it is necessary to know the uniform component of the acoustic pressure. To do this, the forces exerted on each element are measured.

  constituting the movable wall, by means of strain gauges, piezo-quartz

  13 These measurement signals provide an evaluation of the circumferentially uniform sound pressure which allow

  deduce the command to be imposed on the movable elements 12.

  The pressure signals coming from the measuring elements 13 are then processed by a control member 14 which will deduce therefrom the displacements to be imposed on the mobile wall elements 12 The control law is such that the local impedance (ratio between the pressure and the radial velocity of deformation) checks the law: (3.1) Z (k R) P = Vipcjo (k R)) (3.1) vr 10 (k R) For reasons of feasibility, we can use a similar Zc law at best the exact law given by the formula (3 1) In this case, the displacement speed to be imposed on the elements will be given by: p (3.2) r Z (k R) This control system can use electronic components or else

  use an analogical principle allowing to realize the law (3 2).

  Referring now to Figure 6, there is shown a variant of the system according to the invention intended to obtain even more effective attenuation results Indeed, the system described above, for reasons of feasibility of the law can have performance limitations in certain frequency bands, particularly for those which cancel the BESSEL JO function In this case, to obtain attenuation on the complete frequency band, it is possible to combine several active filters having operating characteristics different By increasing the radius of the filter, it is indeed possible to shift in the spectrum, its operating regime By placing filters of different radii 12 and 12 'in line C, we will obtain

  thus a more efficient mitigation system.

Claims (5)

  1 Method for attenuating acoustic waves in a fluid circulation circuit, characterized in that it consists in inserting into the wall of a pipe belonging to this circuit at least one transducer imposing on this wall a vibration, so to create an active movable wall canceling by absorption of the acoustic energy the wave transmitted by the portion of the conduit located upstream   from this transducer to the downstream portion, and vice versa.   2 Method according to claim 1, characterized in that the transducers   are distributed uniformly around the entire periphery of a section of the pipe.   3 Method according to claim 2, characterized in that the pipe being of circular section, the transducers are distributed equiangularly around its periphery. 4 Method according to claim 2, characterized in that, the pipe being at   square section, the transducers are placed in each side of the square.   Method according to any one of Claims 1 to 4, characterized in that   that, with a view to piloting said transducers, means are also provided for measuring the forces exerted on each element of the movable wall, these measures providing an assessment of the uniform sound pressure around the periphery of the pipe and then being processed by a controller   determining at all times the displacements to be imposed on the transducers.   6 Method according to claim 5, characterized in that said means are chosen between strain gauges, piezoelectric quartz and microphones.   7 Method according to any one of the preceding claims,   characterized in that, in order to make the attenuation system more efficient, one   place side by side in the line of transducers of different radii.