Patent Publication Number: US-2012024624-A1

Title: Acoustic panel for receiving, emitting or absorbing sounds

Description:
This invention relates to an acoustic panel, designed to emit and to receive sound or to absorb sound. 
     In general, acoustic panel is defined as either an acoustic chamber that comprises a loudspeaker and that provides sound from an electric signal, or a facing, such as a wall facing, designed to passively reduce ambient noise. 
     Such an acoustic chamber can come in various forms and at various costs, according to the quality of sound that is emitted by the loudspeaker. In particular, for obtaining a top-quality acoustic chamber, systems of vibratory membranes have been developed that consist of composite materials, which have a high production cost, both because of their arrangement and the type of the materials used, and their technology, as based on the use of electrostatic chambers. 
     On the other hand, a sound absorption facing is generally used in the reduction of noise in a building or a vehicle. 
     There is therefore need for an acoustic panel that makes possible several operating modes, i.e., able to emit and to receive sound, or to absorb sound. 
     It is the purpose of this invention to eliminate the above-mentioned drawbacks by proposing an acoustic panel structure that makes possible a “triad” operation—either sound absorber, or sound receiver, or sound generator—and, in this sound emission mode, constitutes a loudspeaker of high acoustic quality while being of simple and economical design. 
     For this purpose, the invention relates to an acoustic panel, designed to emit, receive, or absorb sound, characterized in that it comprises two walls that are arranged opposite one another and delimiting an inner space, whereby said inner space comprises at least 50% air and is equipped with at least one cavity, at least one element that can become deformed, means for exerting a force on said at least one deformation element, and means for connecting said at least one element to a surface of one of said walls, with said at least one deformation element, connecting means and surface delimiting a chamber. 
     The invention also relates to a device for generating sound waves, characterized in that it integrates an acoustic panel as described above and comprises at least one piezoelectric element, and in that it comprises means for supplying at least one piezoelectric element with voltage. 
    
    
     
       The invention will now be described in more detail with reference to the accompanying figures provided only by way of example and in which: 
         FIG. 1  is a diagrammatic cutaway view of an acoustic panel in accordance with the invention, 
         FIG. 2  is a representation of the acoustic panel according to several shape variants, and 
         FIG. 3  is a representation of the evolution of the frequency response of the acoustic panel (expressed in terms of dB) of  FIG. 1  in loudspeaker mode, based on the frequency (expressed in terms of Hz). 
     
    
    
     In the figures, an acoustic panel—designed to emit, receive or absorb sound, comprising two walls  12 ,  14 , arranged opposite one another and delimiting an inner space  16 —is referenced as  10 . The inner space  16  comprises at least 50% air and is equipped with at least one cavity  18 . The acoustic panel  10  also comprises at least one element  20  that can become deformed, means  22  for exerting a force on said deformation element, and connecting means  24  of said element to an outside surface  26  of the wall  12 . A chamber  28  is delimited by the at least one deformation element  20 , the connecting means  24 , and the surface  26  of the wall  12 . 
     According to one embodiment, the at least one deformation element  20  is a membrane  30  of an electromechanical transduction element. The electromechanical transduction element is preferably a piezoelectric element, but any converter of mechanical energy into electrical energy and vice versa is also suitable for the implementation of this invention. This piezoelectric element is advantageously tuned to operate in a frequency range of between 20 Hz and 25,000 Hz. 
     According to the embodiment that can be seen in  FIG. 1 , the connecting means  24  of the membrane  30  to the surface  26  come in the form of a peripheral edge  34  of the membrane  30  and are made integral, for example, by bonding to the outer surface  26 . According to one embodiment, the means  22  exert a counter-reaction force F and are arranged in contact with the membrane  30  from an outer side  31   a,  opposite to the chamber  28 , or, in contrast, from an inner side  31   b,  in the chamber  28 . 
     According to a variant, not shown, the chamber  28  can be arranged in the inner space  16 , and the connecting means  24  connect the membrane  30  to the inner surface  33  of the wall  12 . The means  22 , integral with the membrane  30 , are also arranged in the inner space  16 . 
     The means  22  can come in various forms. They can involve, as shown in  FIG. 1 , at least one mass  38 , in which case the force F that is exerted is equal to mγ, where γ is the acceleration of the mass. They can also come as a mass-spring system that is tuned to the low frequencies and that makes it possible to reduce the runaway of the piezoelectric element in response to a strong electric pulse. Actually, at low frequencies, a large portion of the wall  12  radiates mechanical energy and is vibrating. The presence of the mass then makes it possible to amplify and to tune the amplitude of the vibrations of the panel  10 . 
     According to one variant, the mass  38  is a metal ball, connected to the center of the membrane  30  by means of a glue, preferably of the elastomer type such as neoprene or butyl. The presence of the glue between the mass  38  and the membrane  30  means that the connection between the mass  38  and the membrane  30  is elastic and dampened. The support between the mass  38  and the membrane  30  being specific and central, it allows a controlled deformation of the membrane  30 , which contributes to a high sound quality of the acoustic panel. 
     According to another variant, the membrane  30  carries two masses  38  that are arranged in such a way that the deformation effects are symmetrical, ensuring a high quality of the sound emitted by the acoustic panel. 
     The means  22  can also come in the form of a viscoelastic element or a fairly rigid spring linked to a heavier element. 
     The walls  12  and  14  preferably consist of a material that is light and rigid, such as plastic, polycarbonate, cardboard or else expanded polystyrene. The advantage of these types of materials is that they are economical to use in comparison in particular with composite materials that are generally used for a top-quality loudspeaker. The acoustic panel  10  can be monolithic and consist of, for example, a cardboard sheet or a section of expanded polystyrene. It is also possible that the walls  12  and  14  form two layers of a trilayer made by the two walls  12 ,  14  and the inner space  16  filled with an absorbent material, such as foam. 
     The acoustic panel can be filled at least partially by an absorbent material, such as foam, which then provides to the inner space  20  a multitude of cavities  18 , as shown in a diagram in  FIG. 1 . The filling material is advantageously light and comprises cells, such as alveolar cardboard. 
     The acoustic panel  10  can assume various shapes shown in  FIG. 2 . It can be a parallelepipedic panel, as for the panels  10 - 1  and  10 - 2 , or curved, as for the panels  10 - 3  and  10 - 4 , or else cylindrical, as for the panel  10 - 5 . The acoustic panel  10  can also be a sphere. The panel  10  can comprise a recess, even with a dimension that is greater than 20% of the total surface area of the acoustic panel. The acoustic panel can be extremely flat, as shown in  10 - 1 , or, in contrast, very thick; see panel  10 - 5 . 
     It is noted that the invention is not limited to the embodiment of  FIG. 1 , and, in particular, the number of piezoelectric elements and membranes  30  as well as their distribution in the panel  10  depend on numerous parameters such as the type, dimensions, and the number of sheets of material, and the presence or absence of cavities. 
     In particular, when the panel  10  comprises several piezoelectric elements, the excitation of a maximum of suitable modes of the vibratory wall is preferred, over that which is known in the state of the art in the design of a loudspeaker. This excitation of suitable modes makes it possible to obtain low frequencies, reduces the possibility of inter-acoustic wave interference that degrades the sound quality of the acoustic panel, and makes it possible to excite other suitable modes that would not be excited by a single piezoelectric element. 
     It is noted that the various piezoelectric elements that are used are not necessarily in phase. For example, a spatialization effect can be obtained using two phase-shifted piezoelectrics. 
     It is also noted that when several piezoelectric elements are mounted according to a determined network, it is possible to focus the emitted acoustic waves. 
     The acoustic panel  10  can operate according to three different modes, either by emitting, or by receiving sound, or by absorbing sound. 
     The first operating mode is the acoustic absorption mode according to which the absorbent material of the acoustic panel makes it possible to reduce the sound level. This operating mode can be totally passive and permanent, or else active by emitting sound in phase opposition with the one arriving above. 
     The second operating mode is acoustically active, making it possible to generate sound emissions. In this operating mode, the acoustic panel is used as a loudspeaker: a sound signal is generated by a computer, an MP3 key, or a Walkman, for example, and then it is sent digitally by network or in analog form to the piezoelectric element after being pre-amplified. The piezoelectric element converts this electrical energy into mechanical energy, which causes the membrane  30  to vibrate, and the vibrations are transmitted to the wall  12  by the air of the chamber  28  and then to the entire panel  10  by means of the at least one inner cavity  18 , which becomes resonant, and by means of the connecting means  24  that carry the vibrations of the membrane  30  to the wall  12 . The presence of the mass  38  makes it possible to amplify and to tune the amplitude of the vibrations. Acoustic waves are then emitted. 
     The piezoelectric element is supplied with a voltage supply, for example at ±20 V, because of the high impedance of the piezoelectrics that are used. 
     Thus, according to the acoustic emission operating mode, it is possible, in particular, to emit masking noise, ambient sound, or to create a musical atmosphere or to broadcast information. 
     The operating mode corresponds to that of a loudspeaker whose acoustic characteristics are particularly high-performing, as can be seen in  FIG. 3 . The curve in  FIG. 3  is obtained without active or passive filtering. It thus is noted that in the absence of any filtering, the passband at −6 dB extends over a wide frequency band, from 100 Hz to 20,000 Hz, ensuring a very high sound quality of the panel  10 . 
     The third operating mode is that of an acoustically active receiver. Actually, the ambient noise causes the panel  10  to vibrate, and it is possible to collect a signal by the piezoelectric element. This signal is very weak, but it can in turn be amplified by an integrated or offset amplifier. In this case, it is possible to ensure the surveillance of an unoccupied site—the detection of intrusions in closed sites, for example. 
     The invention also relates to a device for generating sound waves, characterized in that it integrates at least one acoustic panel  10  that is equipped with at least one piezoelectric element and in that it comprises means for supplying at least one piezoelectric element with voltage. 
     The acoustic panel according to this invention has a simple and economical design, in particular due to the use of cardboard or expanded polystyrene, while exhibiting noteworthy acoustic properties. In addition, it can have a thickness of only some millimeters and is thereby suitable for numerous applications. It can be, for example, integrated with furniture, in particular design furniture, such as a dresser or a lampshade.