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Acoustic deviceRelated Patent Categories: Electrical Audio Signal Processing Systems And Devices, Electro-acoustic Audio Transducer, Driven Diverse Static Structure (e.g., Wall, Sounding Board)Acoustic device description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060159293, Acoustic device. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a continuation of U.S. application Ser. No. 11/072,618, filed Mar. 7, 2005, which is a continuation of U.S. application Ser. No. 09/978,663, filed Oct. 18, 2001 (now U.S. Pat. No. 6,904,154), which is a continuation of U.S. application Ser. No. 08/707,012, filed Sep. 3, 1996 (now U.S. Pat. No. 6,332,029). The prior applications are incorporated herein by reference. FIELD OF THE INVENTION [0002] The invention relates to acoustic devices for use in or as loudspeakers when driven or excited, usually by electrical signals via electrodynamic means; or in or as microphones when driven by incident acoustic energy, usually to produce a corresponding electrical signal; or in or for other acoustic devices or purposes. [0003] United Kingdom Patent applications as follows:--No. 95/17918 filed 2 Sep. 1995 for "Acoustic Device" No. 95/22281 filed 31 Oct. 1995 for "Acoustic Device" No. 96/06836 filed 30 Mar. 1996 for "Acoustic Device" from which priority is claimed are hereby incorporated by reference, in their entirety, into this application. GENERAL BACKGROUND [0004] Conventional loudspeakers in widespread use employ acoustic air-driving elements of so-called "cone" type. The or each cone element is mechanically driven at its smaller end in a pistonic manner, normally by a moving coil of electromagnetic means having an operatively associated fixed magnet assembly mounted to a frame or chassis of the loudspeaker in accurate registration with the moving coil and cone assembly. Anti-phase air-excitation to rear of this assembly needs careful baffle/enclosure design to avoid cancellation effects in desired acoustic output from the front of the cone element. Naturally stiff lightweight sheet materials have been used for such cones, as well as very stiff composite sandwich structures that do not bend at all over the working frequency range; even cone elements with tailored reduction of stiffness outwardly with the aim of reducing the effective radiating area with increasing frequency to improve acoustic pistonic effects, including combating increasingly narrow directivity at high frequency. Excellent results are obtainable, e.g. using different sizes/types of cone elements and associated drive units for different frequency ranges, with appropriate electronic "cross-over" circuitry, often all in one loudspeaker housing. However, mass and bulk tend to be substantial. Moreover, sound produced is constrained by its origin with one or more cone elements whose axiality imposes unavoidably high directionality, particularly the higher the frequency; and loudness very noticeably follows the inverse square law of radiation relative to distance, as though from a point source. [0005] Not surprisingly, much interest and effort has long been directed to use of flatter acoustic elements or diaphragms to occupy less space, hopefully be less directional, and preferably be less weighty. Many proposals have resulted. Some use stretched webs or films of flexible material clamped at their edges in frames, e.g. along with bonded-on current-carrying strips or wires for electromagnetic drive using large and heavy arrays of perforated magnets, or with applied surface conduction for electrostatic drive from fixed perforated polarised electrode plates requiring large high-voltage transformers and subject to loudness being limited by voltage breakdown. Drive operation of these stretched film loudspeakers is inherently pistonic, and there tend to be unwanted modal "drum" and related resonances at discrete frequencies requiring specific damping provisions for satisfactory performance. [0006] Other prior proposals have been based on using panels of expanded or foamed polystyrene edge-mounted in housings and also rely primarily on pistonic action. One example, known under the trade name `Polyplanar`, has conventional moving coil drive. Another, known as "Orthophase" has an array of magnets and coils disposed over its surface to try to achieve uniphase drive. Yet others, as available from Sound Advance Systems of California, have variously shaped flat surface polystyrene panels with complex rear ribbing and thinned edge profiling with a conventional moving coil driver mechanisms mounted to a chassis. Bertagni, from Argentina, has patented such proposals made from bonded expanded polystyrene beads, ostensibly based on how musical instruments produce sound, and requiring complex edge-clamped structures of variable thickness/flexibility, but also understood to rely basically on pistonic action. Yamaha of Japan made a large loudspeaker using a thick polystyrene diaphragm of "ear-shape" suspended at its perimeter, with moving coil drive requiring a large chassis for registering powerful magnet provision, effectively as a very large solid-cone, pistonic-action loudspeaker with a degree of self-baffling. [0007] In a sense, these other proposals can be seen as being variations on the simple theme of almost any panel having potential for sound amplification, as long-known relative to musical boxes placed on a table top. In the 1970's, this theme was the basis for a self-contained electrodynamic unit known as "Sonance" (see U.S. Pat. No. 3,728,497) and intended for screwing or gluing to virtually any surface, including under a table top. Not surprisingly, absence of any design control over attachment surfaces/panels, together with no better than moderate efficiency, led only to unpredictable results not satisfactory for high quality sound reproduction. PARTICULAR BACKGROUND TO INVENTIVE CONCEPTS [0008] It is one object of this invention to provide an acoustic element with non-pistonic action in an improved manner leading, inter alia, to ready implementation as more viable flat panel loudspeakers. [0009] Our approach involved relies on exploiting resonance, i.e. departing radically from long and strongly established presumptions regarding quality of sound reproduction being critically reliant upon avoiding resonance effects. [0010] Our approach involves use of materials capable of sustaining bending waves and generating sound from action of those bending waves. General theory for analysis and calculations concerning bending wave action and related resonances in two-dimensional panel structures is long known and understood, for various purposes. For purposes hereof, we find that finite element analysis is particularly suitable and useful in analysing bending wave action in panel-like structures; and arriving at remarkably effective and compact loudspeakers, including with capability for wide-band performance of great clarity/intelligibility, and well-suited to good quality sound reproduction. Moreover, other valuable passive as well as active acoustic devices and applications arise. At least one prima facie attractive and likely mathematical technique, namely statistical energy analysis, is actually ineffective. [0011] We know of a few prior proposals for sound reproduction based on bending wave action, though none appreciate or foreshadow the analysis, understanding and practical teaching hereof. Two of these proposals emphasise importance of "the coincidence frequency", where the speed of sound in panels subject to bending wave action matches the speed of sound in air. One, see U.S. Pat. No. 3,347,335 (Watters), proposes a light stiff strip element of composite structure that is excited whilst clamped so that controlled intendedly substantially single-axis bending waves are generated over a specified frequency range for which the panel is deliberately designed for a constant velocity of sound. The particular intention is to produce a highly directional sound output, and operate the strip only above the coincidence frequency, stated as typically in the range 700 Hz to 2 KHz. A weakened shear property is said to help meet the constant sound velocity desideratum. [0012] Another, see WO92/03024, specifically illustrates and describes a one-metre square loudspeaker panel wholly of aluminium alloy having a honeycomb cellular core between facing sheets giving an extremely high stiffness in all orientations. This panel is required to be mounted to a support in a free undamped manner, and is shown mechanically excited at a corner by a vibrator device acting reactively from secure mounting to the support. Only limited working acoustic range is indicated, said to be suited to applications such as public address systems; and operation is again limited to being wholly above the coincidence frequency. Whilst very high mechanical efficiency is indicated for sound energy conversion, the described panel is so stiff that it is difficult to drive, requiring a very large and cumbersome moving-coil driver. In fact, overall efficiency from the viewpoint of electrical input is even less than for conventional loudspeakers. It is also very expensive to make; and rather heavy, particularly with its support frame. Limitations in operating frequency range of this proposal were confirmed, and at first appeared to preclude achieving practical loudspeakers, even for public address applications where there is acceptance of limited range/quality of sound reproduction but expectation/requirement for quite low-cost units. [0013] However, our own theoretical and practical research and development reveals, as just outlined above, ways of designing and operating a panel as a well-behaved acoustic element that can have a surprisingly wide frequency coverage and remarkable sound distribution and loudness capability, including with low or virtually no perceived directionality and reduced proximity effects as to loudness. Indeed, when appropriately driven by electromechanical transducers, a wide range of lightweight flat or curved loudspeakers can be produced, for a wide range of applications, to some of which such loudspeakers hereof seem uniquely suitable, whether or not with wide frequency coverage. It is noted again that other acoustic devices arise of profoundly novel and useful characteristics, as will become apparent. It is also noted that, in contrast to other mathematical techniques, specifically statistical energy analysis, use of finite element analysis could, at least as applied by us, have revealed structural problems as the reasons for frequency limitation of WO92/03024 and its poor placement of its drive means being an inappropriate choice. [0014] Essentially, our research reverted to seeking basic understanding of acoustic implications of the phenomenon by which any member with extent mainly transversely of its thickness and capable of bending wave action will have a characteristic complex natural bending wave vibration. Of particular acoustic relevance are contributions of resonant modes at frequencies related to inherent fundamental frequencies for the member concerned to such characteristic complex natural bending wave vibration. Each such resonant mode contributes a particular component of bending wave vibration ranging over said extent of the member between vibrationally most active subareas and vibrationally inactive subareas, corresponding to "anti-nodes" and "nodes" (or "dead spots"), respectively. We found that combination of the bending wave vibration components from the totality of the natural resonant modes results in grouping of nodes and anti-nodes by superposition and clustering at subareas forming regions of substantially more and less vibrational bending wave activity that can be considered as "combined anti-nodes" and "combined dead spots", respectively. It was confirmed that most such members have poor acoustical performance, especially at frequencies effectively excluded by WO92/03024, i.e. at lower frequencies going below the coincidence frequency and down towards the largest or fundamental wavelengths of possible bending wave vibration for the member concerned. [0015] However, further as foreshadowed above, we have specifically established how, by careful analysis and orderly design, some such members can very greatly outperform the teaching of WO92/03024 by reproducing a surprisingly wide audio frequency range with remarkable clarity when used in loudspeakers. Indeed, the acoustic characteristics of such members can be so well-ordered and/or prescribed that uses are seen for the members, in themselves, as passive acoustic devices for such purposes as providing reverberation or acoustic filtering or setting/altering, or generally improving, room etc acoustics. [0016] Sounding boards are, of course, very well-known for stringed musical instruments such as pianos and the violin family. The making of successful such sounding boards is very old, and they are invariably held effectively rigidly at edges and/or clamped medially. To date, getting good/acceptable results has tended to be very much of a "black art" nature, involving quite complex shapes and generally achieved essentially pragmatically, typically by using quite complex proven templates. Indeed, even now, best results are generally accepted as involving highly skilled crafting by hand. This contrasts strongly with what is now put forward herein as being readily achievable, including for essentially free-standing or self-contained devices, by straightforward application of highly practical teachings, including what can be, or can include, very simple panel-like structures. Indeed, affording simpler alternatives to traditional sounding boards, particularly of calculated and orderly design as herein, is seen as an aspect of this invention. [0017] The calculated and orderly design criteria we are now able to put forward, from analysis and experiment in the light of insights gained from profoundly improved explanation and understanding of acoustical bending wave action, are based on achieving beneficial distributions of bending wave vibrations associated with resonant modes in a said member. These improvements are achievable by the invention even for simple shapes of said members that satisfy proportioning criteria taking appropriate account of actual physical parameters relevant to bending wave action, see further below. Also, for any associated transducer means, highly effective location and other criteria are developed herein, see also further below. In relation to this invention herein, the term "transducer means" is intended to encompass, as appropriate, single and plural transducers, as well as any type and structure of transducer that will serve to excite bending waves, whether of electromagnetic or piezoelectric types specifically described later, or of other types, such as magnetostrictive. [0018] Moreover, desired or required effective control of edge vibration effects is readily available by edge framing simply carried by the member itself, even, in some cases, relying only on holding edges or resting same on a surface; and selective damping can be applied at localised medial positions in operative areas of said members. Edge framing may be selective at positions affecting resonant modes at frequencies of interest, but more usually complete, though perhaps with selective enhancement of intermediate damping material. Generally, however, such intermediate damping material should not clamp the edges against desired vibrational action but should be at least in light contact, particular requirements necessarily being determined case-by-case in finalising any product design, but generally not being highly critical. Requirement(s) of or for edge-damping will depend on such factors as testing performance to assure avoidance of "ringing" in desired operation, materials of members hereof and size, including vibrational energy reaching edges. Localised medial damping will also be at areal positions appropriate to frequencies of interest and/or by way of affixed damping and/or stiffening material dimensioned to correspond with wave-length(s) of frequencies of interest. GENERAL ASPECTS OF INVENTION [0019] Reverting to our research and development work, poor acoustic performance in said members, considered generally, appears to be influenced by presence and distribution of the above-mentioned dead spots and combined dead spots in the transverse extent of the member; and/or the converse, i.e. by spacing and distribution, or spread, of said anti-nodes and combined anti-nodes and/or complexity of combined anti-nodes. We find, as aspects of invention, that inherently better acoustical performance or action arises from care taken to reduce, preferably as near as practicably eliminate, occurrence of combined dead spots; and/or the converse of more evenly distributing the anti-nodes and combined anti-nodes over said transverse extent of the member, preferably at or approaching as evenly as practicable. At least in/for embodiments of the invention for application in audio frequency ranges, particular attention is directed to resonant modes at lower frequencies than considered to be of useful interest in WO92/03024. It is found to be highly effective for attention to be directed to lower frequency resonant modes in a much broader frequency range of actual operational interest, even to resonant modes that may be below a high-starting such frequency range of interest, i.e. closer or closest to the lowest possible, or conceptual fundamentals, for natural bending wave vibration in the member. The normal variation of bending wave speed with frequency in practical materials and structures is fully accepted in members hereof without finding problems. [0020] We further find, as an aspect of invention that best location of transducer means is at one or more positions coupling to one or more said combined anti-nodes whereby many, preferably a number considered to be a practical maximum or optimum, further preferably all, of said lower frequency resonant modes in an operational acoustic range of interest have vibrationally active anti-nodes--which can be by way of plural transducers on a combination basis using two or more said positions, advantageously in as complementary a manner as available relative to the resonant mode vibrations at the positions concerned. Such positions are different from all known prior art, and are even found to be advantageous in use beyond said members with preferentially orderly distribution of said anti-nodes and combined anti-nodes. 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