GB2246684A

GB2246684A - Panel form loudspeaker

Info

Publication number: GB2246684A
Application number: GB9017133A
Authority: GB
Inventors: Kenneth Harry Heron
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1990-08-04
Filing date: 1990-08-04
Publication date: 1992-02-05
Also published as: GB9103969D0; GB9017133D0

Abstract

A panel form loudspeaker comprises a composite panel 1 having a honeycomb or similar core structure and thin stiff skins, and means 4-6 for mechanically exciting the panel in response to an electrical input. At frequencies above the panels coincidence frequency the panel radiates sound efficiently and suitable choice of panel materials avoids significant output distortion from panel modal motions. The skins may comprise aluminium sheets and the core be of honeycomb formed aluminium or resin impregnated paper. <IMAGE>

Description

PANEL-FORM LOUDSPEAKER This invention relates to a novel form of loudspeaker suitable for applications requiring thin speaker section such as in flat panel-form speakers. This form of loudspeaker exhibits a conversion efficiency of close to unity so it is also suitable ior applications requiring a high acoustic power output from the loudspeaker Current panel-form speakers are of the electrostatic variety capable of yielding a high fidelity output. These are expensive and have low conversion efficiency so they are not suitable for many applications.

The other conventional form 0+ speaker incorporates a mechanical driver coupled to a thin diaphragm which is caused to move in gross manner by the action o-f the driver., This form of speaker is more efficient than the electrostatic variety, although it is still an inefficient converter of electrical power into acoustic power.In this form of speaker it is arranged that the first resonance frequency of the diaphragm is s above the required output frequencies but this design criterion, which dictates small diaphragm size, conflicts with the neecl for a large diaphragm size for adequate radiation at low frequencies.In consequence speakers of this form or often incorporate more than one channel esch allocated to a particular frequency range within the overall bandwidth of the speaker and having its own speaker unit, but this imposes a concomitant requirement +or a dividing network to provide appropriate al location of the electrical input between the various channels.Speakers of: this form need to be mounted in such a manner that destructive interference between the forward and rearward outputs is avoided and this dictates that they are normally mounted on the front face of a substantial box housing which precludes flat panel formats.

The present invention provides a spsaker form capable of achieving high conversion efficiency, suitable for flat panel forms and requiring no box housing.

The invention is a loudspeaker comprising a composite panel formed of two , skins having a high stiffness to mass ratio between which is sandwiched a light core of transverse cellular construction ,a mounting means for supporting the panel or attaching it to a supporting body in a manner permitting free vibration of the panel , and a means to mechanically excite the composite panel in response to an electrical input signal.

The term "transverse cellular construction" refers to honeycomb cores and other cel 1 based core patterns having Si mi 1 ar properties of high shear stiffness combined with light weight.

This form of speaker works in a diffenent way to the established speakers discussed above. It is intended to operate largely at frequencies above the coincidence frequency of the panel. The "coincidence frequency" is the frequency at which the bending wave speed in the panel matches the speed of sound in air. At frequencies above the coincidence frequency many modern composite sandwich panels radiate efficiently and in consequence by utilising a panel having a coincidence frequency below the frequency range required of the speaker and using a panel form with the required transverse cellular construction it is possible to produce a speaker which converts very nearly all mechanical input from the excitation device into acoustic output.This is more than a mere desideratum for it is this characteristic which overcomes potential problems from the modal motions in the panel.

high conversion efficiency implies that the panel motions are constrained by acoustic damping rather than internal structural damping and this in turn implies that acoustic distortions will be small.

The material of the sandwich panel core should be such as to avoid excessive shear at all frequencies required in the speaker output or otherwise internal shear motions will cause the panel to have unacceptably low bending stiffness at the higher frequencies.This criterion may be satisfied b use of a panel form whose shear modulus exceeds the value: c /d ; where "p" is the panel mass per unit area,"c" is the speed of sound in air, and "d" is the depth of the panel core. Note that all units used in this specification are MIS units.

There is a second criterion used to assess the suitability of a particular composite panel for use in the speaker as claimed and this is related to the acoustic conversion efficiency on the assumption that the panel is working at frequencies above the coincidence frequency. The parameter which we term "T" which is the product of the expression Bp- should be very much greater than 2. In the above expression "B" is the bending stiffness of the panel and is the mass per unit area of the panel. For an isotropic steel panel of any thickness T would be less than C). 5.

For a carbon fibre reinforced panel with fibres laid equally in the 0, 45, 90, and 135 degree directions T would be approximately .85. Therefore it is unlikely that any isotropic panel would have a suitable balance of stiffness and light weight. This T criterion can be satisfied by many modern composite sandwich panels having a core of transverse cellular construction such as those having resin-impregnated paper honeycomb cores or even with constructional panels having a honeycomb core of aluminium alloy material.

The invention is now described by way of example with reference to the drawings of which: Figure 1 is an isometric view from the rear of a frame-mounted loudspeaker; and, Figure 2 is a lateral view of a ceilingrmounted 1 oudspeaker.

The speaker as illustrated in Figure 1 comprises a rectangular panel designated 1 made of a sandwich composite having a haneycomb core. Details of suitable materials and sizing rules are given later. The panel 1 is hung +rom a support frame 2 by means of a pair of simple suspension loops 3 in a manner which permits the panel to vibrate freely when excited without significant damping from the support frame 2.An electromagnetic exciter 4 is also attached to the support frame 2 and this has a shaft 5 which bears against the rear face of the panel 1. The arrangement is such that an electrical input to the exciter 4 causes its shaft to move in a reciprocating motion and by contact to provide a corresponding mechanical excitation to the speaker panel 1. At the contact point the rear face of the panel is reinforced by a patch 6 to prevent damage to the panel skin. The location of the contact point is towards one corner of the panel l so that the panel is excited in all its modes.

An alternative ceiling-mounted version of the loudspeaker depicted in Figure 2. This version is the like of that described above save that the panel 1 is mounted upon a ceiling 7 which takes the place of the support frame 2 mentioned above and save that this version of the loudspeaker uses four not two of the suspension loops 3.

In both the above versions of the loudspeake5r the inentia of the exciter 4 is matched to that of the panel 1 to provide optimise the amount of power transferrad to the panel. The mechanical excitation of the panel 1 causes sound to be radiated from both its front and rear daces but the lateral extent of the panel and the special character of the sources of radiation ensure that there is no signifitant destructive inter-fer-ence from these two sources and hence no need for any bo mounting for the speaker.

The "T" criterion and shear modulus criterion both of which reflect the effectiveness of any speaker in the form claimed have been described above. These control chuice of materials and panel forms rather than dimensions or frequency ranges. To produce a speaker optimised to a particular frequency range it is useful to refer to snme design rules which are given below.As mentioned previously y, the coincidence Frequency of the panel should be below or at least at the lower end of the desired frequency range of the speaker. The panel coincidence frequency, fc, is given b the expression fc = c / 4@ B and setting a volue of fc establishes a relationship between and S. The fundamental frequency, f1 of the panel should be below the lowest frequency of interest and this is a function inter alia of panel area. f1 is given by the following approximate exgression +1 = where A is the panel area.. Lastly, the frequency of the first air resonance within the core of the panel must be above the highest frequency of interest and this frequency, fa is given by the following expression fa = c/2d where d is the depth of the panel core.

One version of speaker panel and its parameters is described below. It comprises a im x im square of aluminium skinned, resin impregnated paper honeycomb cored composite. The depth of the core is o.04m and the thickness of each skin of aluminium is 0.0009m. For this panel the estimated performance parameters, given by the expressions stated previously, are as follows : = 4250Hz ; f1 = 98 Hz ; fc = 190 Hz ; and T = 280.

The operation of the speaker as claimed has heen confirmed using a convenient piece of aluminium skinned aluminium honeycomb core structural composite in a rectangular panel of about 1.5 m square and C). 14m thick. This was used in the suspended configuration shown in Figure l and achieved reproduction of spoken sounds to a standard satis+actory for at least public address systems.

Claims

CLRIMS

1. R loudspeaker comprising a composite panel formed of two skins having a high stiffness to mass ratio between which is sandwiched a light core of transverse cellular construction ,a mounting means for supporting the panel or attaching it to a supporting body in a manner permitting free vibration of the panel, and a means to mechanically excite the composite panel in response to an electrical input signal.

2. A loudspeaker as claimed in claim l in which the composite panel has skins of aluminium or aluminium alloy.

3. n loudspeaker as claimed in claim l or claim 2 in which the composite panel has a core of resin impregnated paper material.