GB2278515A - Curved loudspeaker. - Google Patents

Abstract

An electromagnetic loudspeaker comprises a semi-rigid drive surface 4, bearing an imprinted conductor pattern (6, fig. 3) suspended close to an array of rigid, permanent magnets 3; both the array and drive surface having the approximate form of a convex cylindrical segment. The spacing of the magnet array is small, being defined as less than one half wavelength of the maximum frequency to be produced. <IMAGE>

Description

CURVED LOUDSPEAKER This invention relates to the design of a loudspeaker whose drive element is a cylindrical section, whose whole surface is electro-magnetically driven.

The majority of loudspeakers in use for wide frequency response fall into two categories electromagnetic cone speakers and electrostatic sheet speakers. A third category, that of electromagnetic sheet loudspeakers is known, patents placed etc, but are not in common use. The reasons for this are the result of several minor but significant flaws in the theoretical designs used.

Claims have been made previously for wide frequency response speakers, whose surfaces are driven by arrays or groups of relatively widely spaced magnets, with corresponding conductor patterns embedded on the moving element. These result in resonant problems on the diaphragm, poor polar response, and sometimes resonance problems in the air spaces between the magnets. Also, claims have been made for complete cylindrical loudspeakers.

While these work at high frequencies, the stiffness of the complete cylindrical driver inherently prevents the generation of low frequencies, where relatively large displacements must be generated.

Another problem found in a number of previous designs is the use of thin, flexible films for the drive element; often suspended by tensioned materials. While in principle these can be made to work, and indeed do where an electrostatic drive force is used, they generally do not in electromagnetic transducers. The problem arises from practical variations in the field strengths of the magnets used, and of varying spacings between magnets and the conductor patterns. This results in widely varying forces being applied to different areas of the driven surface. As a consequence, at low frequencies, different parts of the driven surface start moving out of phase; with consequent destructive interference in the radiated sound - hence very poor bass response. (In the electro-static case, a bias voltage is applied, causing the membrane to move to an 'equal force' resting position, so the problem does not occur.) This invention overcomes these difficulties, as is discussed below.

The speaker comprises a curved sheet of a semi-rigid material, on which is embossed or printed a pattern of conductors. This sheet is mounted in the close proximity of an array of rigid permanent magnets, whose fields are alternatively North and South poles towards the sheet. The distance between magnets and sheet is of the order of lmmo The sheet is suspended by a 'roll surroud', which permits the sheet to move freely, while returning it to a fixed undriven position. The far side of the roll surround is attached to a rigid frame, which uses curved support members to hold the closely-spaced array of magnets.

The curved driver shape offers a number of acoustic advantages. Being made of a semi-rigid material, for example Bextrene, Cobex, Glass-reinforced plastics or similar, the variations in the force applied to the sheet mentioned earlier become averaged out at low frequencies, and the whole surface moves as a piston. This results in a somewhat disadvantageous polar radiation response, but humnns cannot locate the source of a sound well at low frequencies.

In practice, an emitted low frequency sound reverberates around the room in which the speaker is used, and is interpreted by the listener as coming from the speaker.

At higher frequencies, the equation of motion of the curved sheet is no longer dominated by the stiffness terms, and the semi-rigid surface can expand and contract locally. The sheet drives nearby air as part of an expanding and contracting cylinder, thus providing part of the wave-front which would be generated by an ideal point source. The effect is to make the polar radiation response much closer to omnidirectional, at least in front of the speakers, where it matters. At moderate to high frequencies, where humans can best locate sound sources, the speaker is thus behaving in a close to ideal fashion, and the 'stereo imagery' when a pair of such speakers is used is very good.

The rigidity of the sheet is thus very important. Finns do not meet the low frequency requirements, and completely rigid sheets detract from the high frequency radiation characteristics.

It has been found to be critical to the operation of the speakers that the spacing of adjacent conductors on the membrane, the spacing of the magnets and the depth of the magnets be small. The repeating nature of the array of magnets, conductors etc result in very strong resonances being created in the drive sheet and air spaces. It is thus essential that all of these dimensions are significantly less than one half wavelength at the maximum frequency to be reproduced. In practice, this translates to keeping all of these dimensions less than 8mm, and electrically filtering the input current to remove very high frequencies.

This design is illustrated with the aid of drawings of an example speaker.

Figure 1 shows one form of a rigid mounting, face on.

Figure 2 shows the rigid mounting with magnets fitted.

Figure 3 shows an example of the moving sheet, with conductor pattern.

Figure 4 shows the assembly in profile.

This description is one form of the design discussed. Arrays of rigid magnets 3, of alternating polarity, spaced a few millimeters apart, are mounted on damped supports onto a rigid frame 1. A semi-rigid curved sheet 4, bearing embossed conductors 6, are mounted via a roll-surround 5, onto the rigid frame 1, and clamped 2 or glued The roll-surround is composed of a material with a 'memory', formed into a small curve (Figure 4), and damped.

This retains the driven sheet, whilst allowing free movement. One form of this is cloth strips impregnated with rubber.

Claims (1)

1. An electromagnetic loudspeaker comprising a semi-rigid drive surface, bearing an imprinted conductor pattern, suspended close to an array of rigid, permanent magnets; both the array and drive surface having the approximate form of a convex cylindrical segment. The spacing of the magnet array is small, being defined as less than one half wavelength of the maximum frequency to be produced. This claim specifically excludes complete cylindrical forms intended for full frequency range response, the use of flexible diaphragms, wide-spaced magnet arrays and flexible magnetic sheets.

   A curved loudspeaker as in Claim 1 in which the cross-section is related to a cylindrical section; that is hyperbolic sections, parabolic sections, cosine square sections and other curved surfaces.

   A curved loudspeaker as in Claim 1 and Claim 2 where a multitude of imprinted conductor patterns are used.

   A curved loudspeaker as in Claims 1,2 and 3 where curves are applied in both dimensions of the drive surface.