MXPA00009685A - Acoustic device relying on bending wave action - Google Patents

Abstract

Acoustic device comprising a member relying on bending wave action with beneficial distribution of resonant modes thereof, wherein the member has its acoustically active area at least partly bounded by means having a substantially restraining nature in relation to bending wave vibration. Operation can be below coincidence, or above if desired for active acoustic device further having beneficial location of bending wave transducer means determined with reference to and taking account of such bounding means.

Description

ACOUSTIC DEVICE Field of the invention The invention relates to an acoustic device of the type comprising a sound radiating element dependent on the action of a bending wave and which results in a surface vibration to produce an acoustic output.

BACKGROUND OF THE INVENTION It is known from US Pat. No. 3,247,925 issued to WARNAKA, which suggests a low frequency loudspeaker consisting of an extremely rigid panel, whose peripheral edges are screwed or glued to a rigid structure, whose structure supports a transducer conventional voice coil that distributes bending wave energy to the center of the panel. This low frequency loudspeaker device is designed to operate entirely above the wave coincidence frequency. It is also known from US Pat. No. 3,596,733 issued to BERTAGN I, where a loudspeaker having a diaphragm formed by a flat-type expanded polystyrene element having a front face tensioned and a rear face of, or includes an irregular shape. The disclosed knowledge relating to acoustic bending wave action devices, are generally conveniently considered as, of the resonant panel type, are given in International Patent Application WO97 / 09842, which includes how to improve or optimize the acoustic performance, according to to panel parameters, including bending geometry and stiffness; including, in particular, an advantageous way of operating at, and below, a coincidence frequency. Geometric parameters of interest include proportions or aspect ratios of panels as such, which are included for use as passive acoustic devices. The flexural stiffness parameters can advantageously interact with the geometrical parameters, including the anisotropy thereof, or in other words, to differentiate the bending stiffness from, or resolvable to, substantial constancy along the axes of the geometric shapes involved for the possible variation of proportions of such forms. Preferably, the locations in the frame of the transducers of the active acoustic devices advantageously have the proportional definition coordinates. Another surface distribution of flexural rigidity can usefully contribute to providing other advantageous locations for the transducers, for example in geometric centers and / or mainly in mass centers, see International Patent Application WO98 / 00621, including for the above combination the action of bending wave with a further piston action on purpose acoustically. The acoustic operation is described and claimed in at least the patent WO97 / 09842, both for panels as a whole, as only being acoustically active forming part of them.

On an intuitive basis, our specific analysis and design methodology to date, for such resonant mode, the acoustic device of wave action of bending, has been mainly concentrated on all the panels where the edges are totally or substantially free, to vibrate when in acoustic relevance the action of a bending wave is desired, which includes where they are subject to a certain marginal damping. This invention arises from surprising results of subsequent contradictory considerations, in addition to considerations, research and experimentation. Summary of the Invention Follow certain underlying requirements to be applied and be of deep technological / inventive meaning, specifically for an acoustic device element extended transversely in its thickness and capable of providing bending waves through its acoustically active area in a consequent manner, ie , the basic requirements for what is here called an acoustic resonance element or panel; and for parameters, such as geometrical parameters or bending stiffness to be values conforming to the resulting distribution of the natural bending wave vibration of said element that is effective in, or beneficial to achieve an acceptable or desirable acoustic operation, the devices over a frequency range of interest, that is, further requirements for a resonant acoustic element or panel of this type. Specific embodiments of the present invention additionally provide means that give substantial bending wave vibration fixations typically at the edge, periphery or other boundary of such an acoustically active element or panel or area thereof, and furthermore, typically to be at least able to operate at a certain point under matching frequencies. The term "substantial limitation", which has been used intentionally, entails a high requirement of at least part (s) of the edge (s) of the element that is specifically developed in Patent WO97 / 09842, as to what refers to the extension of the edge, as in what refers to the load of subject and effective mass effect. There are two points of view, effects or inventive aspects that are understood as useful to consider them, relative to such edge / limiting / peripheral area fixation. One is that the limitation / reduction of edge / peripheral / border wave vibration movement that can be obtained from the element (compared to what is specifically discerned in WO97 / 09842), can produce a useful composition of acoustic outputs obtained from the vibrating flex wave energy of refreshment in the acoustically active area. The other is that the relevant and acoustically effective natural modes of the action of the resonant bending wave will be different (compared to that discussed in WO97 / 09842) by reason of the limitation / suppression of bending wave vibration movement in the edge (s) periphery / boundary of the element and in this way, reducing / eliminating the contribution (s) of the lowest resonance mode (s) that could be active if the edge (s) ) periphery / boundary of the acoustically effective area of the element is free to have a bending wave distribution as specifically developed in WO97 / 09842; and the substantial reduction / suppression of the resonant modes in which the torsion comes into play. Being nominally less frequent or a less rich content of acoustically active / relevant resonant bending wave modes, it can be exemplified by simple analogy and analysis based on equivalent simple beams taking into account the interactions, in terms that include modes of resonant plates, that relative to each beam starts at a frequency of resonant mode f1 instead of fO, and even more "losing" modes of combinations that include frequencies fO, but with useful and interesting effects available with respect to space uniformity of relative natural resonant modes directly and in combinations that include frequencies f 1. The ramifications are extensive and can be advantageous, including the possibility of obtaining an improved acoustic efficiency of energy conversion and / or often, extensions very advantageously increased of candidate sub-areas for the viable / optimal location (s) of the transducer, at least as identified for the analysis of mechanical impedance as shown in International Patent Application PCT / GB99 / 00404 co-dependent; and / or typically of much greater range of viability of proportions / surface shapes of said elements as exemplified by the isotropic bending stiffness, even in 1: 1 to 1: 3 and more for aspect ratios (s); and / or viability of acoustic performance for materials of the panel element with lower intrinsic bending stiffness, at least as effectively stiffened as a whole by contribution of edge (s) / periphery / boundary fixings thereof and / or capacities in relation to high power input transducer means for speaker embodiments, including everything where such fixations can provide a substantial load either in inertial ground bases, or as is more usual practice, by current fixing in a more rigid / higher mass support or another way of heavy loading. It is a significant advantage of the present invention that useful and novel resonant panel acoustic devices are provided, including active acoustic devices such as loudspeakers, with important ease of fabrication as easily mounted and robust panel type devices, particularly improved in relation to specifically acoustic devices illustrated and described in International Patent Application WO97 / 09842. According to a practical aspect of the present invention there is provided an acoustic device dependent on the action of a bending wave and capable of operating under coincidence, comprising an element that allows said acoustic operation for reasons of advantageous distribution of resonant modes of flexion wave action, wherein the element has its acoustically active area, at least partially limited, by means having a substantially fixed nature in relation to the vibration of the bending wave. According to another aspect of the present invention, there is provided an active acoustic device comprising an element dependent on the action of a bending wave with an advantageous distribution of the resonant modes thereof and the advantageous location of bending wave transducing means, where the element has its acoustically active area at least partially limited by means having a substantially restrictive nature in relation to the vibration of the bending wave, and the location of its transducer means determined with reference to, and taking into account said means of limitation. The entire periphery of an acoustic element such as the one presented here must be substantially fixed, or fastened or only some parts and not the entire edge, for example a rectangular panel, can be fixed or fastened on one or more to all its lateral edges. This can be useful as a flag-like assembly allowing the aforementioned substantial fixation on one side with the active area acoustically protruding from it, or as a mounting on two sides that can be parallel and which can allow such substantial fixation with the acoustically active area between those assemblies and fixed faces and can facilitate the manufacture of fully sealed or very selectively ventilated loudspeakers, for example, medium frequency devices /high. A total or almost total sealing of the diaphragm allows the design of a so-called infinite baffle speaker to contain / control the rear acoustic radiation which could, if not, be detrimental at low to medium frequencies. Totally or substantially fixed or clamped frames also allow for the design of the loudspeaker assembly to be more predictable in mechanical terms, as well as to facilitate a design of a loudspeaker assembly that is relatively robust in construction (compared to a resonant panel loudspeaker in which edges of the panel are substantially free or are suspended in a single elastic manner lightly fastened). Substantial fixings or fastenings of peripheral parts or edges of an acoustic element can be obtained in any desired manner, for example, by intimately securing the edge (s) to a strong structure or similarly by means of an adhesive, or by mechanical means designed to encompass the edges between the elements of the frame. The desired fixing / clamping edge mentioned can be obtained by the structure, also by molding techniques (such as molding by injection of plastic materials) to form the edges of the elements with integral or integrated thickened rounded portions of sufficient stiffness to eliminate movement of edge of the acoustic element. It may be advantageous to mold together an acoustic element and the provision of the thickened edges. Such molding techniques should be particularly appropriate where the acoustic element is formed as a monolith and can be easily finished in an economical manner. Substantial fasteners / restraints can be used to define an acoustic element included in another larger element. Thus a large acoustic panel made to operate at medium / low frequency can be molded to include an acoustic panel developed for high frequency operations and defined by reinforcement ribs. A substantial limitation or clamping action can be designed to obtain a mechanical termination impedance designed to control the reverberation time in the acoustic element, as an aid for the control of the frequency response of the element, perhaps, specifically at lower frequencies . Proportions of appropriate resonant panel elements may be substantially different from the specific knowledge of WO97 / 09842 relating to variations in particular forms. For example, substantially rectangular resonant panel elements of substantially isotropic bending stiffness may be of aspect ratio below 1: 1.5, ie, they generally include prior knowledge for free edge panel elements, but not limited to it will be specifically described later here, that is, greater than 1: 1 .5 as will also be specifically described here. Variations of anisotropy / complex distribution of flexural stiffness are considered above.

The limiting means may at least partially define said acoustically active area and / or the peripheral edges of a panel element to be fully and acoustically active, typically up to 25%, or more of the extent of the entire border / peripheral edge area. often even the whole.

The elements of the resonant panels are generally self-supporting and would not require pre-tension to achieve mechanical stability, particularly for typical types of free edge or their simple edge supported use.

For a clamped panel element there is an increase of more or less 10 times at a first bending frequency due to the natural stiffening of the panel element when clamped. It is logical and sensible to substantially reduce the property of flexural stiffness to reduce the primary modal frequency and before the lower frequency range. It is envisaged that the rigidity of the panel element in such cases may be as low as 0.001 Nm and the area density as small as 25 g / m2.

From one point of view, those limits of the range of values describe a panel element that for mechanical stability and the function for excitation means can benefit from the application of tensioning forces. These can be applied uniformly or differentially, that is in different directions and / or with different tensions, with respect to the effective geometry of the element.

In the limit, the tensioned panel exhibits a high proportion of the properties of a tensioned film supporting bending waves and with predominantly or non-dispersive second-order wave actions (constant velocity with frequency). For such a panel element the distribution of the resonance can be optimized for a desired acoustic behavior by voltage control and border geometry in large part according to the knowledge of the distributed mode, see WO97 / 09842. Also a preferred modal distribution can be further increased in action to a transducer by preferential / optimized location of the exciter / sensor. Depending on the degree of tension and with increasing density and more particularly with flexural rigidity, there will be a range where the action of a second order bending wave is superimposed and increased in fourth order, dispersive bending action due to rigidity. The optimizations of the two must be derived by calculation and / or experimentation to provide the best result in the given applications. The smaller bandwidth acoustic panels with clamped edges are the object's field of design.

BRIEF DESCRIPTION OF THE DRAWINGS The practical embodiment of the present invention is illustrated by diagrams, by way of example, in the accompanying drawings in which: Figures 1 and 1 A are a schematic perspective and a cross-sectional view of a generally rectangular resonant panel acoustic element 10 fastened at its edges between rectangular perimeter frame elements facing 1 1 A, B using nuts and bolts 12A, B which may also be useful for mounting to a chassis or other parent structure; Figure 2 is a cross-sectional view showing a clamping / termination edge of acoustic element of resonant panel 20 with edge fixed to a structure 21 by means of an adhesive 22; Figure 3 and 3A are a partial perspective and a cross-sectional view of an injected plastic mold 35 formed by a wall element having stiffening ribs 36 which intersect with a rectangular edge 31 as the fixing boundary of an area of acoustically active panel 30, the border 31 can also be formed by ridges protruding to give rigidity to the edges of the operating area 30; Figure 4 is a partial perspective view of an acoustic resonant panel element 40 stretched on a frame 41 and held on its edge by a surrounding frame fastening element 42; Figure 4A is a partial section of the embodiment of Figure 4; Figure 4B is a partial cross section similar to Figure 4A for an alternative embodiment of a stretched panel acoustic element; Figures 5A, B are graphs showing the frequency response of a respective resonant panel element of size A4 and A5, respectively, and in which the thick line strokes represent the subject edge panel and the thin line strokes represent a suspended panel of elastic or free border; Figures 6A, B and 7A, B and 8A, B are graphic representations of mechanical impedance versus frequency for relationships of selected aspects of panel elements with subject edges; Figures 9A, B, C are graphical representations of the relative uniform inverse mean square deviation for the location of the transducer means; Figure 10 is a drawing of the mechanical impedance of a quarter panel calculated for a subject edge panel element; Figure 1 1 is a graphic representation for several aspect ratios of a panel with subject edge; Figure 12A-H are mechanical impedance drawings of a panel room measured for various aspect ratios; Figures 13A-H are drawings of the acoustic outputs referred to a reference value; Figure 14 draws the inverse of the maximum mean quadratic power deviation for different aspect ratios; Figures 15 AJ are polar diagrams of acoustic outputs for low resonance modes of 1: 3 aspect ratio in panel elements with subject edges, and Figures 16 AD are drawings of acoustic output power comparisons for specific panel structures differing in size and / or stiffness.

Detailed Description of the Invention Relative to Figures 1 and 2, the acoustic elements can be and are shown as substantially rectangular and can have aspect ratios as is preferably considered in WO97 / 09842, however, broader ranges of aspect ratios they will be shown to have a useful potential with the general objective of obtaining a high modal density and a uniformity of modal dispersion in the element. Figures 4 and 4A show an embodiment of a resonant acoustic element 40 stretched on a rectangular perimeter frame 41 and attached to the rectangular peripheral frame by a clamping frame 42 to hold the acoustic element in place. Tension forces are applied to the element 40 in the direction of the arrow F. Alternatively, as shown in Figure 4B, the clamping frame 42 can be replaced by tension means 43, for example with the inclusion of springs tension 44 in the frame 45, the tension means will be fixed to the edges of the acoustic element to stretch the element on the rectangular perimetric frame. Vibration exciters, for example of the kind described in WO97 / 09842, should be located in the acoustic elements in the embodiments of Figures 4, 4A and 4B to excite in resonance the acoustic elements produce an acoustic output so that the acoustic elements can act as loudspeakers or loudspeaker excitation units. These vibration exciters are not shown in Figures 4, 4A and 4B in the interest of clarity. A strong fixation or fastening of the edges of the panels allows the use of relatively low stiffness materials (compared to the general practice of substantially free edge panels), which can be aided by reducing the frequencies of the fundamental folding of the panels , including even below levels in practice for typically more rigid free edge panels typically (and in spite of effectively losing the free edge mode at the lowest frequency in a fully clamped panel). For example, where the stiffness range for a practical example of free edge panel of the kind described in WO97 / 09842 should be of the order of 0.1 to 50 Nm, the stiffness of a subject edge panel of the same general class should be minor by at least an order of magnitude, even as low as 0.001 Nm. Also, where the range of the surface density of the aforementioned practical example of free edge panels should be 100 to 1000 g / m2, the surface density of the subject panels should be only a fraction, even as low as 25 g / m2. However, it will be appreciated that significantly stiffer and / or denser materials may be employed for acoustic panels described herein with substantial edge fixing or clamping at least where performance, at a lower frequency, is not a requirement. Such applications include speakers for high frequencies, sirens, ultrasonic sound players. The use of relatively low stiffness panel materials can result in a greater coincident frequency, for example over the normal audio band, which can improve the uniformity of the directivity of the sound from a resonant loudspeaker panel. Also the panels of lower rigidity, can realize effective increases of modal density in the lower registers, consequently improving sound quality. Advantageous variants to the total / peripheral fastening / fastening of the edge / border, as illustrated, include any device of smaller extension / fastening extension that, for active areas / elements of substantially rectangular panels, could be a default side of what it is shown by three sides, or two typically parallel sides by default of what is shown by the other two sides. The acoustic radiant elements described herein can be excited in any of the ways suggested in WO97 / 09842, for example through at least one inertial electromechanical exciter device. The or each exciter device can be arranged to excite the radiating element in any geometrical position appropriate to the surface of an acoustic element.; in accordance with the principles mentioned in WO97 / 09842 or in accordance with the mechanical impedance analysis in PCT / GB99 / 00404 or as determined experimentally. Such vibration exciters have been omitted from Figure 1 in the interest of clarity. The references are made to WO97 / 09842 in reference to the types of exciters applicable, and the positioning of such exciters can be determined in accordance with the same principles shown in the WO97 / 09842 and / or PCT / GB99 / 00404, normally with difference allowed for the current locations compared to Patent WO97 / 09842. Some useful investigations of fully subject edge resonant panel elements such as or in active acoustic devices, specifically loudspeakers, are first described in and relating to Figures 1 1 to 16 of the PCT Patent Application codept PCT / GB / 00404 filed on 9 February of 1,999; and the Figures have been repeated in this as Figures 6 to 11, respectively. These investigations are naturally based on analyzes that involve power transfer parameters, particularly uniformity of the input power, specifically related to the mechanical impedance; and in particular particularly firmly fixing the viable / optimal locations of transducers and shapes of the panel elements, specifically aspect ratios for at least substantially rectangular panel elements and transducer locations based on a provided coordinate. Thus, the graphical representation of Figure 6A, B and 7A, B and 8A, B for mechanical impedance with frequency for panel elements of aspect ratios and isotropy such as flexural stiffness are accompanied by graphic representations of Figures 9A, B , C for uniform mechanical impedance measured by the inverse square root mean standard deviation for the location of particularly promising transducer sites. Obtained favorable aspect ratios accurately calculated 1 .160, 1 .341 and 1 .643, also together with the location coordinates of preferential transducers calculated accurately (0.437, 0.414), (0.385, 0.387) and (0.409, 0.439), respectively. Figure 10 is a drawing of the mechanical impedance of a panel room calculated for an aspect ratio of 1.16 and shows a substantial extension of promising areas for locations of the transducers, including two separate (scratched) areas. Figure 11 gives a comparison of such preferential aspect ratios at subject edges and locations of transducers, also included for aspect ratios 1.138. Subsequent investigations are based on current measures of mechanical input power that includes elements of substantially rectangular resonant panels that have increasing aspect ratios; and in each case adapting the frequency response to a reference value or flat line per decade over the frequency of the lowest effective resonant mode. Figures 12A-H give contour drawings of a quarter panel of the inverse of the mean square deviation of such adaptation including for the same or close the aspect ratios above (Figures 12 A, B, D) corresponding to to Figures 13A-H the frequency adaptation with the planar line, respectively, so that the clearest coloration / shape represents the most viable location (s) of the transducer and bursting discrete areas of viability indicated in ratios of look older The extension of those subsequent investigations to aspect ratios as high as 1: 4 is remarkable, perhaps especially the establishment of clear viability from, in, or near the square. This was not expected, it is the least we can say of our previous revelation work and knowledge concerning resonant panel elements with substantially free edges for bending wave vibration. The increase, also up to now, unexpected operational power as established here of Figures 5A, B for 1.41 aspect ratios is established even more consistently towards other aspect ratios now investigated. The subsequent unexpected marked reduction of the criticism of the aspect ratios to give even frequency spacing of the resonance mode so advantageous for acoustic actions have been the cause for subsequent considerations of detail and analysis. The following consequence is presented in terms of a simplified beam theory for substantially rectangular resonant panel elements having substantially isotropic flexural rigidity. Usually, there is confirmation thanks to prior knowledge / work, that is, for panel elements with substantially free edges, the frequency of the lowest resonance mode as determined in the dimension of the longest side and is the best in conjunction with the dimension of the shortest side corresponding to a frequency of a higher resonance mode giving respective relative series of frequency of a higher resonance mode which are substantially interleaved in values. Indeed, a higher aspect ratio for such a substantially free edge panel would result in the second (perhaps even more) of the resonance mode frequencies of a panel member directly attributable to the larger edge dimension also being smaller than the first attributable to the smaller edge dimension, resulting in a frequency jump (s) too large for truly satisfactory acoustic realizations, which are based on the bending wave action at the lowest frequencies concerned. In contrast, the first effective resonance mode frequency for a fully clamped edge resonant panel element requires the contribution of the first resonant mode attributable to the shortest edge length, that is, the first combination mode for the vibration action of plate for the two series (fxi, fx2, ..., fxn) and (fyi, fy2,. - -, f and m) for the x axes, and parallel to the edge represented for the resonant mode spectrum equation: feyrm The effects of this quadratic realization are that a high aspect ratio can produce a succession of fairly close resonance mode frequencies especially attributable to the contributions of the next higher in the related series of the larger edge before the next contribution of the greater to the series related to the minor edge. Figure 14 draws the deviation of the inverse average quadratic power versus the aspect ratio and shows the increase in power stability (above the lowest effective resonant mode) with peaks in the increased ratio of 1: 3. Indeed, higher aspect ratios for limited elements on the sides described herein have frequencies in closer resonant modes, while the opposite applies to the relative free edge panels of Patent WO97 / 09842.

This result, naturally, in no case cancels the useful and good results for the acoustic devices with fully fixed edges that fulfill minor aspect ratios, elements of resonant panels; which is also very practical with operations of desired acoustic devices of frequency of resonant modes interspersed as advertised by the analysis also developed in Patent PCT / GB99 / 00404. There are, however, significantly better design possibilities. In some particular case, and in the desired application for acoustic devices described herein, the particular frequency spectra of resonance modes will obviously vary with aspect ratios for given bending stiffness or described relationships, and the alternative will normally be made with calculable results, measurable or perceptible as an acceptable or desirable acoustic device performance. Another relevant factor has been established or investigated, that is, the acoustic action and the realization related to the axes and the positioning, so that the differences can be significant; and being useful / effective in designing a particular acoustic device for a particular application, in particular, where such differences may be positively desired or may be undesired, or some particular preferred or acceptable configuration. Figures 15A-J are polar diagrams for a 1: 3 aspect ratio resonant panel element for the lowest resonance mode frequencies, respectively; and with each case shows horizontal (continuous) and vertical (dotted) planes, that is, with a greater horizontal or vertical dimension, respectively. In general, as expected, the patterns of realization are significantly different, so that in the plane of shorter length it is generally more stable and in the plane of greater length it is more diffuse. The design options include the acceptability of higher frequencies of the lower resonance mode as directly dependent by any particular panel element structure or aspect ratio; the acceptability of the addressability where the vibration of the panel element is markedly different in different axial directions; consequently a different power stability in the corresponding radiation planes relative to the orientation or positioning selection of the used panel element; and possible compromise relationships between power stability in different planes and / or total power stability versus similarity or in other cases responses in landscape / vertical planes or azimuth / elevation planes. The panel element of Figure 16A is composed of 0.05 mm. of thickness of 4mm black crystalline layers. of alveolate aluminum thickness resulting in a substantially isotropic bending stiffness of 12.26 Newton meters, mass density of 0.76 Kilograms / square meter, and coincidence frequency of 4.6 kHz. The panel element of Figure 16B is composed of 0.102 mm. of thickness of black glass layers in a core of 1 .8 mm. of thickness in Rohacel core, resulting in a substantially isotropic bending stiffness of 2.47 Newton meter, mass density of 0.60 Kilogram / square meter and coincidence frequency of 9.1 kHz. The panel element of Figure 16C is 0.05 mm. of Melinex ™ layer thickness in 1.5 mm. Rohacel core, resulting in a substantially isotropic flexural rigidity of 0.32 Newton meters, mass density of 0.35 Kilograms / square meter and coincidence frequency of 19.2 kHz. These panel elements all have a similar aspect ratio between 1.13 and 1.14 and are excited with similar exciters of 13mm active diameter. and input impedance of 8 Ohms. In each one an acoustic power output has been measured with all the panel edges free to vibrate by the action of the resonant bending wave of the panels, and with all the edges held against such vibration. Figures 16 AC shows that the clamping allows a substantial increase in the acoustic output power under the coincidence frequency, but not above, where there is a more beneficial effect of clamping at higher frequencies than those of coincidence, that is, the lower bending stiffness of a panel member concerned. The panel elements for Figures 16 D-E of the same stiffness structure as Figure 16A, but of larger sizes, i.e., 360 mm. x 31 5 mm. and 545 mm. x 480 mm , respectively, they are compared with 260 mm x 230 mm. for Figures 16 A to D, and there is confirmation that the subject panels fully produce an improved acoustic output power from the coincidence frequency by lowering to the lowest resonant mode frequency of the smallest panel member (Figures 16 AC) and lower for a larger and larger panel element. It is worth mentioning that the larger the panel elements, the closer the shapes of the modes are to a sine wave. Mechanical input power check measurements were made for all those panel elements when excited with free edges to vibrate and with fully clamped edges, and showed that all panel elements support approximately the same power. It may be interesting to speculate according to the assumption, prior to the contrary teaching of WO97 / 09842, in relation to useful acoustic radiation being unviable under coincidence frequencies based on the theory that perfect sine waves are expected in an infinite plate; and suppositions in consequence to the teaching of Patent WO97 / 09842 clearly establishing that the useful acoustic radiation is that available in a finite plate under coincidence, namely, that such radiation results from parts of a finite plate vibrating deviating from a sinusoidal distribution perfect as it appears as may be the case for the lower frequency modes and both next to an excitation transducer and edges that are free to vibrate, hence, of course, the emphasis, until now, on the latter. However, from what has been taught here, it is clear that setting the edges particularly in terms of capacity for bending wave vibration has beneficial effects for acoustic coupling to the air, particularly by increasing the efficiency below the coincidence frequency. This, of course, occurs in the self-evident context of acoustic output power, necessarily being related in a net manner to losses in a resonant panel element and a nearby acoustic field, at least the latter clearly, being reduced by the setting of said edge effectively eliminating acoustic shorts on such edges subject to such fixation. It seems reasonable to attribute the increased acoustic coupling to the air below the coincidence with the reflection of such energy that it could otherwise be lost in a nearby acoustic field, if only on the basis that such energy is in a wave vibration. bending of resonant mode of a panel element with acoustic range of interest and should leave the panel element as acoustic energy, as improved coupling to the air on limited edges or to the middle of the panel elements. The situation above the coincidence frequency is not affected, of course. This is, of course, all in the subsequent context of permitted resonant modes of fastening panel elements, of edges being necessarily without vibration torsion modes that are effectively reduced or eliminated by the preferably fastening edge fastening. Further research has been done related to advantageous locations for a second transducer based on the average effects using a relocatable / mobile second transducer; and in relation to discrete fixing / holding edges using inertial masses at localized positions. The consequence according to the location of the second transducer mainly emphasized the extent and complexity of interaction between the effects on a 2-transducer resonant panel element. In fact the best location indicated for a secondary transducer to a first transducer advantageously located for a resonant panel element of substantially rectangular shape and substantially isotropic bending stiffness were currently and close to center and in, or close to, the three fourths of positions of length along the limiting axes of the panel room in which the first transducer was located, and the quality of the acoustic output tended to be adversely affected (however viable without hesitation for some applications). The consequence for discrete fasteners / fasteners was particularly interesting in the potentially useful transition indicator of near equivalence to continuous constraints / restraints for the effects of frequency pitch filters relative to a greater spacing and ratio to wavelengths of bending waves in element of panel described.

Claims (2)

1. CLAIMING ES Having described the present invention, it is considered as a novelty and, therefore, the content of the following REVINDICATION IS is claimed as property. 1 . An acoustic device that depends on a bending wave and capable of operating under coincidence, which comprises an element that provides said acoustic operation due to an advantageous distribution of the resonance modes of the bending wave actions therein, characterized in that means that at least partially limit the element, said means having a substantially restrictive nature in relation to the vibration of the bending wave of the element, the limiting means being at a peripheral end of said element and being continuously peripheral on at least 25% of said end.
2. 2. An active acoustic device comprising an element that depends on the action of a bending wave with advantageous distribution of modes of resonance therein and the advantageous location of bending wave transducing means, characterized by means that at least partially limit the element, said means having a substantially restrictive nature in relation to the vibration of the bending wave of the Limiting means extend completely around said area or end. 7. An acoustic device as described in any of the preceding Claims, further characterized in that said limiting means contribute significantly to the overall stiffness of the element, effectively in the desired acoustic operation. 8. An acoustic device as described in Claim 7, further characterized in that the flexural stiffness of the element in said acoustically active area is less than about 5 Newton meters. 9. An acoustic device as described in Claim 8, further characterized in that said bending stiffness is greater than about 0.001 Neutometers. 10. An acoustic device as described in Claims 7, 8 or 9, further characterized in that the density of the surface of the element in said acoustically active area is approximately 25 grams / square meter. eleven . An acoustic device as described in any of the preceding Claims, further characterized in that said area of said element has an isotropic bending stiffness. 12. An acoustic device as described in any of Claims 1 to 10, further characterized in that said area of said elements has bidirectional anisotropy of the bending stiffness. 13. An acoustic device as described in any of Claims 1 to 10, further characterized in that said area of said member has a flexural stiffness distribution that adapts the desired advantageous location for the transducer means. 14. An acoustic device, as described in any of the preceding Claims, further characterized in that said area is substantially rectangular with isotropy of the flexural stiffness thereof, and has an aspect ratio of between 1: 1 and 1: fifteen. 15. An acoustic device as described in any of Claims 1 to 13, further characterized in that said area is substantially rectangular with isotropy of the flexural stiffness thereof, and has an aspect ratio greater than 1: 1. 5. 16. An acoustic device as described in Claim 15, further characterized in that said aspect ratio is about 1.3 or greater. 17. An acoustic device as described in any of the preceding Claims, further characterized in that the limiting means serve to limit at least the resonance modes of the action of the bending wave comprising the roll. 18. An acoustic device as described in any of the preceding Claims, further characterized in that the limiting means serve to reduce at least near the cancellation of the acoustic output field. 19. An acoustic device as described in any of the preceding Claims, further characterized in that said limiting means serve to increase the acoustic output, coming from the energy of the resonant action of the bending wave in said area. 20. An acoustic device as described in any of the preceding Claims, further characterized in that the element is stressed. twenty-one . An acoustic device as described in Claim 20, further characterized in that the element is tensioned in two mutually transverse directions. 22. An acoustic device as described in Claim 21, further characterized in that the tensioning is equal in both directions. 23. An acoustic device as described in any of Claims 20 to 22, further characterized in that the flexural stiffness of the element is low in the acoustically active area. 24. An acoustic device as described in any of Claims 20 to 23, further characterized in that the density of the surface of the element is low in the acoustically active area. 25. An acoustic device as described in any of the preceding Claims, further characterized in that the limiting means are integral with the element. 26. An acoustic device as described in Claim 25, further characterized in that the limiting means are integrally molded with the element. SUMMARY Acoustic device comprising an element dependent on a bending wave action with advantageous distribution of resonance modes, where the element has its acoustically active area and at least partially limited by means taking into account the nature of substantially fixation in relation to vibration of bending wave. The operation may be below the coincidence or above, if desired, by the active acoustic devices further having an advantageous location of bending wave transducing means determined with reference to and taking into account such limiting means.