Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Driven diverse static structure
Reexamination Certificate
1996-09-03
2001-12-18
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Driven diverse static structure
C381S337000, C381S423000, C381S431000, C181S166000
Reexamination Certificate
active
06332029
ABSTRACT:
FIELD OF THE INVENTION
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.
United Kingdom Patent applications as follows: No. 95/17918 filed Sep. 2nd 1995 for “Acoustic Device” No. 95/22281 filed Oct. 31st 1995 for “Acoustic Device” No. 96/06836 filed Mar. 30th 1996 for “Acoustic Device” from which priority is claimed are hereby incorporated by reference, in their entirety, into this application.
GENERAL BACKGROUND
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 light-weight 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 loud-speaker 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.
Not surprisingly, much interest and effort has long is 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 electro-magnetic drive using large and heavy arrays of perforated magnets, or with applied surface conduction for electro-static drive from fixed perforated polarised electrode plates requiring large high-voltage transformers and subject to loudness being limited by voltage break-down. 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.
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.
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 electro-dynamic 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
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.
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.
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.
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.
Another, see WO92/03024, specifically illustrates and describes a one-meter 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 energ
Azima Henry Firouz
Colloms Martin
Harris Neil John
Foley & Lardner
Kuntz Curtis
New Transducers Limited
Ni Suhan
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