Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction
  • H04R7/04—Plane diaphragms
  • H04R7/045—Plane diaphragms using the distributed mode principle, i.e. whereby the acoustic radiation is emanated from uniformly distributed free bending wave vibration induced in a stiff panel and not from pistonic motion
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K1/00—Devices in which sound is produced by striking a resonating body, e.g. bells, chimes or gongs
  • G10K1/06—Devices in which sound is produced by striking a resonating body, e.g. bells, chimes or gongs the resonating devices having the shape of a bell, plate, rod, or tube
  • G10K1/062—Devices in which sound is produced by striking a resonating body, e.g. bells, chimes or gongs the resonating devices having the shape of a bell, plate, rod, or tube electrically operated
  • G10K1/066—Devices in which sound is produced by striking a resonating body, e.g. bells, chimes or gongs the resonating devices having the shape of a bell, plate, rod, or tube electrically operated the sounding member being a tube, plate or rod
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R1/00—Details of transducers, loudspeakers or microphones
  • H04R1/02—Casings; Cabinets ; Supports therefor; Mountings therein
  • H04R1/025—Arrangements for fixing loudspeaker transducers, e.g. in a box, furniture
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction
  • H04R7/04—Plane diaphragms
  • H04R7/06—Plane diaphragms comprising a plurality of sections or layers

Definitions

• 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.
• 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.
• small diaphragms are used but these are not efficient radiators at low frequencies.
• 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.
• 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 it 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.
• 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.
• 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 of bending stiffness (B) to the cube power of panel mass per unit surface area ( ⁇ ) in all orientations of at least 10;
• a mounting means which supports the panel or attaches it to a supporting body, in a free undamped manner
• 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.
• transverse cellular construction 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.
• T 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.
• T 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.
• 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.
• 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.
• 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 or damping imposed by virtue of the panel mounting. When this is achieved acoustic distortions will be small.
• Figure 1 is an isometric view from the rear of a frame-mounted loudspeaker
• Figure 2 is a lateral view of a ceiling mounted loudspeaker.
• the loudspeaker as illustrated in Figure 1 comprises a resonant multi-mode radiator 1, a simple support frame 2 from which the radiator is suspended by means of suspension loops 3t and an electro-mechanical exciter .
• the radiator 1 comprises a rectangular panel of aluminium alloy-skinned, aluminium alloy honeycomb sandwich construction. Details of the panel and sizing rules etc are given later.
• the electromagnetic exciter *•! has a shaft and is mounted upon the support frame 2 such that this shaft 5 bears against the rear of the radiator panel 1 and excites the latter by a reciprocating movement of the shaft when an electrical signal is supplied to the exciter 4. At the point of contact between the shaft 5 and the panel the latter is reinforced by a patch 6 to resist wear and damage.
• the exciter 4 is positioned such that it excites the radiator panel 1 at a position thereon close to one of its corners not at a position close to its centre point to avoid exciting the panel preferentially in its symmetrical modes.
• the inertial masses of the exciter *•! and the radiator panel 1 are matched to secure an efficient inertial coupling between the two for efficient power transfer.
• This version of loudspeaker is suspended from a ceiling 7 rather than a support frame.
• Four suspension loops 3 are used instead of two in the previous version, so that the radiator panel 1 underlies the ceiling rather than hanging down from it.
• the exciter 4 is positioned above the radiator 1.
• Both versions of the loudspeaker operate in exactly the same way and are subject to the same design rules regarding selection of panel materials and construction and dimensioning of the panel having regard to the required frequency band of the loudspeaker.
• the "T" criterion and the shear modulus criterion both of which have been mentioned previously relate to panel forms and panel materials rather than panel dimensions and loudspeaker frequency ranges.
• To produce a speaker optimised for a particular frequency range it is useful to refer to some design rules which are given below.
• the low end of the desired frequency range of the loudspeaker sets a limit upon the fundamental frequency of the panel for this must be below the lowest frequency of interest. Moreover the coincidence frequency of the panel should also be below the lowest frequencies of interest.
• the coincidence frequency (f c ) is independent of panel area and is given by the expression:
• the desired bandwidth for a particular speaker sets a value of f and hence establishes a relationship between ⁇ and B. If a value of the fundamental frequency (f t ) is also set then this fixes an approximate value for the area of the panel for f ⁇ is given by the approximate expression:
• Design considerations are illustrated by way of example below with reference to one version of the loudspeaker which utilises a radiator panel comprising a lm x lm square of aluminium skinned, aluminium honeycomb cored composite.
• the core depth for the panel is 0.04m and the thickness of each skin is 0.0003m.
• For this panel B is l8850Nm, ⁇ is 3.38kg/m 2 , and T is 488Nm 7 /kg 2 .
• f c is [3-38 x 340* /4 x 3-l4l6 2 x 18850] 1/2 246 Hz.
• the shear stiffness of the panel varies with orientation within the plane of the panel.
• ⁇ .c 2 /G.d has a value of O.O56 and for the axis of of its maximum value the same expression has a value of 0.122. Both these values are much less than the limiting value of 1 and indicate that the loudspeaker will not be limited in performance across the intended frequency band by core shear motions.
• a loudspeaker as claimed utilising a radiator panel in the form of a lm square of the material detailed above would have a frequency bandwidth of 250 Hz to 4 kHz within which it would have a high conversion efficiency and low distortion. It is anticipated that such a bandwidth would be quite satisfactory for a public address loudspeaker.

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)
• Fittings On The Vehicle Exterior For Carrying Loads, And Devices For Holding Or Mounting Articles (AREA)
• Laminated Bodies (AREA)

Priority Applications (4)

Applications Claiming Priority (4)

Related Child Applications (1)

Publications (1)

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Family Applications (1)

Country Status (8)

Cited By (58)

Families Citing this family (31)

Citations (6)

• 1991
  • 1991-07-26 AT AT91914046T patent/ATE117155T1/de not_active IP Right Cessation
  • 1991-07-26 JP JP3512829A patent/JP3034952B2/ja not_active Expired - Lifetime
  • 1991-07-26 EP EP91914046A patent/EP0541646B1/en not_active Expired - Lifetime
  • 1991-07-26 WO PCT/GB1991/001262 patent/WO1992003024A1/en active IP Right Grant
  • 1991-07-26 DE DE69106712T patent/DE69106712T2/de not_active Expired - Lifetime
  • 1991-07-26 DK DK91914046.7T patent/DK0541646T3/da active
• 1993
  • 1993-01-27 GB GB9301628A patent/GB2262861B/en not_active Revoked
• 1997
  • 1997-11-03 HK HK97102078A patent/HK1000544A1/xx not_active IP Right Cessation
• 1999
  • 1999-12-08 JP JP11349073A patent/JP3084281B2/ja not_active Expired - Fee Related

Patent Citations (6)

Non-Patent Citations (2)

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