Panel-form loudspeaker

Details Panel-form loudspeaker Panel-form loudspeaker

1998-07-13

2001-06-19

Dang, Khanh (Department: 2837)

Acoustics

Diaphragm

Particular shape

C181S170000, C381S431000, C381S425000

Reexamination Certificate

active

06247551

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a panel-form loudspeaker utilising a resonant multi-mode radiator, which is suitable for applications requiring thin speaker sections such as in public address loudspeakers. The speaker exhibits a conversion efficiency approaching unity so it is suitable for applications requiring high acoustic power output from the loudspeaker.
2. Discussion of Prior Art
Current loudspeakers utilise a diaphragm or similar element which is caused to move in a gross fashion in an essentially pistonic manner to create the acoustic output. The motion of the diaphragm should be in-phase across its surface so that the diaphragm moves backwards and forwards in response to the driver actuation and this is achieved, inter alia, by the nature and size of the diaphragm in relation to the frequency band over which the loudspeaker is required to operate. In these loudspeakers the diaphragm operates largely at frequencies below those at which it exhibits resonant modes (though typically they can operate above the first resonant frequency of the diaphragm by suitably damping-out this mode) and this imposes spatial and/or frequency limitations upon the loudspeaker which are undesirable. In order to raise the threshold of resonant frequencies small diaphragms are used but these are not efficient radiators at low frequencies.
There are two main kinds of loudspeaker in current use and both of these utilise a diaphragm driven in pistonic manner. The first of these is the electrostatic loudspeaker in which the diaphragm is driven by the charge difference experienced between the diaphragm and a rigid backplate closely spaced behind the diaphragm. Electrostatic loudspeakers are capable of yielding a high fidelity output across a wide frequency band and they are of relatively planar configuration suitable for public address applications. However they are expensive and have very low conversion efficiency which detracts from their advantages. The other established form of pistonic-diaphragm loudspeaker is the conventional dynamic loudspeaker which incorporates an edge mounted diaphragm driven by an electro-mechanical driver. These loudspeakers have relatively narrow bandwidth and although they are more efficient radiators than the electrostatic loudspeakers they still have low conversion efficiency. In loudspeakers of this form is necessary to prevent destructive interference between the forward and rearward outputs of the diaphragm. This usually requires that the diaphragm be mounted in the front face of a substantial box housing and consequently precludes flat panel formats.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide a high conversion efficiency flat panel-form loudspeaker having a frequency band at least adequate for public address purposes. This is achieved by making use of the possibilities offered by certain modern composite panels to produce a loudspeaker which operates in a novel way. Composite panels comprising thin structural skins between which is sandwiched a light spacing core are commonly used for aerospace structures for example and certain of these may be used in the speaker as claimed herein. Certain sandwich panel materials have been used previously in the construction of diaphragms in conventional dynamic loudspeakers, eg as disclosed in patent specifications GB 2010637A; GB 2031 691A; and GB 2023375A, but have not been used, to our knowledge, in the manner of this invention as resonant multi-mode radiators.
The invention claimed herein is a panel-form loudspeaker comprising:
a resonant multi-mode radiator element being a unitary sandwich panel formed of two skins of material with a spacing core of transverse cellular construction. wherein the panel is such as to have ratio T of bending stiffness (B) in Nm to the cube power of panel mass per unit surface area (&mgr;) in kg/m
2
in all orientations of at least 10 Nm
7
/kg
2
, i.e., T=B/&mgr;
3
≧10;
a mounting means which supports the panel or attaches it to a supporting body, in a free undamped manner;
and an electro-mechanical drive means coupled to the panel which serves to excite a multi-modal resonance in the radiator panel in response to an electrical input within a working frequency band for the loudspeaker.
The term “transverse cellular construction” as used in the above definition and elsewhere in the specification refers to honeycomb core forms and other cellular based core constructions having non-hexagonal core sections with core cells extending through the thickness of the panel material.
In the above definition of the invention and throughout the specification and claims all units used are MKS units, specifically Nm and kg/m
2
in the above paragraph. We term the value of the above-given ratio “T” and a T value as specified above is necessary in order that the radiator panel might function properly in the manner required. Preferably the value of T should be at least 100. This T value is a measure of the acoustic conversion efficiency of the radiator panel when the loudspeaker is operating as intended at frequencies above its coincidence frequency (see below). A high T value is best achieved by use of honeycomb cored panels having thin metal skins. Our presently preferred panel type is those panels having honeycomb core construction and thin skins with both skins and core being of aluminium or aluminium alloy. With such panels T values of 200 or more can be achieved. It is most unlikely that any solid plate material could provide the required minimum value of T. A solid steel panel of any thickness would have a T value of about 0.5, well below that required. Solid carbon fibre reinforced plastics sheets with equi-axed reinforcement would have a T value around 0.85, still well short of the minimum requirement. The mode of operation of the speaker as claimed is fundamentally different from prior art diaphragm loudspeakers which have an essentially “pistonic” diaphragm motion. As mentioned previously such loudspeakers are intended to produce a reciprocating and in-phase motion of the diaphragm and seek to avoid modal resonant motions in the diaphragm by design of the diaphragm to exclude them from the loudspeaker frequency band and/or by incorporating suitable damping to suppress them. In contrast the present invention does not incorporate any conventional diaphragm but rather uses a panel, meeting the criteria described, as a multi-mode radiator which functions through the excitation of resonant modes in the panel not by forcing it to move in a pistonic, non-resonant manner. This difference in mode of operation follows from the panel stiffness to mass criterion, from the avoidance of edge damping and the absence of internal damping layers etc within the radiator panel, and also from operation of the radiator at frequencies above both the coincidence frequency and the fundamental frequency of the composite panel.
The “coincidence frequency” is the frequency at which the bending wave speed in the radiator panel matches the speed of sound in air. This frequency is of the manner of a threshold for efficient operation of the loudspeaker for at frequencies above their coincidence frequency many modern composite sandwich panels radiate efficiently. It is possible using the information provided herein to produce a radiator panel suitable for given frequency bands in which the concidence frequency of the radiator panel will fall at or below the required bandwidth so that the loudspeaker will convert almost all mechanical input from the electro-mechanical drive means into acoustic output. This is more than a mere desideratum for it is this characteristic of high conversion efficiency which overcomes potential problems in a resonant multi-mode radiator based system. A high conversion efficiency (which can be achieved by suitable selection of materials in accordance with the design rules given herein) is achieved when panel motions are constrained by acoustic damping rather than internal structural damping within the panel material

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