Patent Publication Number: US-6215882-B1

Title: Panel-form loudspeaker

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to resonant multi-mode radiator elements for panel-form loudspeakers. In particular, resonant multi-mode radiator elements which can improve the sound quality of such speakers by reducing the occurrence of air resonance within the functional bandwidth of the speaker are considered. 
     2. Discussion of the Prior Art 
     U.S. Pat. No. 3,247,925 discloses a low frequency panel-form loudspeaker which operates by exciting bending waves in a lightweight, stiff panel which remains essentially stationary but for the bending waves. Such radiating panels may comprise skinned composites with honeycomb or similar shaped cellular cores. 
     Acoustic distortion may occur in panel form loudspeakers where air resonance occurs in the core of the panel. Patent application No. PCT/GB91/01262 discloses how this form of acoustic distortion can be reduced by ensuring that the frequency at which the first air resonance occurs within the core of the panel is above the functional upper frequency limit of the loudspeaker. This frequent ƒ core  is a function of the speed of sound in air, c and the depth of the panel, d as follows; 
     
       
         ƒ core   =c/ 2 ·d    
       
     
     This expression fixes the depth of the panel core according to the desired functional frequency bandwidth of the loudspeaker, the desired functional upper frequency limit, ƒ max , of the loudspeaker being inversely proportional to the panel depth. 
     It can be shown that a second level of acoustic distortion occurs in panel-form loudspeakers at certain frequencies within the panel, when a localised resonance occurs in the skin immediately above individual core cells. The effect of this localised skin resonance is to reduce the effective bending stiffness of the panel speaker and consequently the radiation efficiency of the panel. 
     SUMMARY OF THE INVENTION 
     This invention provides a resonant multi-mode radiator element for a panel form loud speaker having a functional upper frequency limit ƒ max , the element having skins and a cellular based core, the skins having a thickness h, a Young&#39;s modulus Ε a Poisson&#39;s ratio of ν and a material density ρ, and the core having a cell size l cell  characterised in that the l cell  size is less than:              π                 h       4        f     m                 a                 x                  (     E     3        ρ        (     1   -   v2     )           )       1   2                         
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described below by way of example, with reference to the drawings of which: 
     FIG. 1 is an isometric view from the rear of a frame-mounted loudspeaker; and 
     FIG. 2 is a side cross-sectional view of the panel sandwich. 
    
    
     DETAILED DISCUSSION OF PREFERRED EMBODIMENTS 
     The loudspeaker as illustrated in FIG. 1 comprises a resonant multi-mode radiator 1, a simple support frame 2 from which the radiator is suspended by means of suspension loops 3, and an electromechanical exciter 4. The radiator 1 comprises a rectangular panel of aluminum alloy-skinned, aluminum alloy honeycomb sandwich construction. Details of the panel and sizing rules etc. are given later. The electromagnetic exciter 4 has a shaft 5 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. 
     FIG. 2 illustrates the construction of panel 1 having material skins 10 which sandwich a transverse cellular core 12. 
     It can be shown that the first resonance frequency for an individual skin ƒ skin  can be predicted by matching the skin bending wavelength with twice the cell size of the core. Thus for a bending stiffness B skin , a mass per unit area m skin  and cell size l cell  the first resonance frequency of a skin f skin  can be defined by;          f   skin     =       π     2        l   cell   2                  B   skin       m   skin                           
     For an isotropic skin of thickness h with a Young&#39;s modulus of Ε, a Poisson&#39;s ratio of ν and a material density of ρ, this becomes:                f   skin     =         π                 h       4        l   cell   2                E     3        ρ        (     1   -     v   2       )                       Equation                   (   1   )                           
     Thus, by choosing a core cell size to ensure that ƒ skin  is greater than the functional upper frequency limit ƒ max  the loud speaker, the resonance effect is substantially reduced. Rearranging equation (1) and relating to the functional upper frequency limit of the panel form loud speaker, a suitable cell size l cell  is predicted from the equation:                l   cell     ≤           π                 h       4        f     m                 a                 x                  (     E     3        ρ        (     1   -     v   2       )           )       1   2                   Equation                   (   2   )                           
     Reduction of cell size can therefore provide improved sound quality by taking the first resonance frequency of the skin outside the functional bandwidth of the speaker. As this frequency is inversely proportional to the square of the cell size, considerable improvements can be obtained with small variations in the cell size. 
     A further aspect of the invention provides a panel-form loudspeaker containing a multi-mode radiator element as described hereinabove. The loudspeaker further comprising a mounting means which supports the radiator element or attaches it to a supporting body, in a free undamped manner; and an electromechanical drive means coupled to the panel which serves to excite a multi-modal resonance in the radiator element in response to an electrical input within the functional frequency band of the loud speaker. 
     Preferably the loud speaker will also be characterised in that no planform dimension is less than half the bending wavelength at the lowest functional frequency limit of the loud speaker. 
     Although in describing a radiator and speaker in accordance with the invention, the term “loud speaker” is used as a convenient nomenclature, it will be understood this should not be read as suggesting any limitation to, say, hi-fi speakers alone. Rather, the invention is applicable across a range of speaker sizes from the smaller scale to the very large for use, for example, in large scale public address systems. 
     A particular panel core of this invention is illustrated below by way of example with reference to a panel core comprising a 0.5 m×0.5 m square of aluminium skinned, aluminium honeycomb cored composite. The core depth is 0.02 m and the thickness of each skin is 0.003 m. For Aluminium, Ε=7×10 10 , ρ=2700 and ν=0.3 (mks units), thus          E     3        ρ        (     1   -     v   2       )                           
     approximately equals 3081.7 m/s. Assuming the desired upper frequency limit to be 28 kHz, and applying equation (2) above, the maximum desirable size for l cell  becomes approximately 0.016 m.