WO1991017636A1

WO1991017636A1 - Improvements in or relating to loudspeakers

Info

Publication number: WO1991017636A1
Application number: PCT/AU1991/000184
Authority: WO
Inventors: Robert Michael Grunberg
Original assignee: Robert Michael Grunberg
Priority date: 1990-04-30
Filing date: 1991-04-30
Publication date: 1991-11-14

Abstract

A direct radiating electroacoustic transducer which exhibits extended axial displacement of the diaphragm (6) by virtue of the moving-roll compliance member (12) extending between the frame (4) of the loudspeaker and the outer perimeter (20) of the diaphragm (6). This provides a basis for an increase in the area of the diaphragm (6) relative to a given diameter for the mounting frame (4), and also provides a basis for a loudspeaker in which the width of the compliance member (12) is independent of the extreme axial displacements of the diaphragm.

Description

IMPROVEMENTS IN OR RELATING TO LOUDSPEAKERS TECHNICAL FIELD

This invention relates to electroacouεtic transducers and more particularly to direct radiating loudspeakers which allow extended axial displacement of the diaphragm or cone in the direction of the force exerted by the voice-coil. Many such loudspeakers are capable of reproducing low frequencies and are commonly referred to as "woofers". BACKGROUND ART

The most practically utilised electroacoustical method of sound production requires the motion of a diaphragm

10 to generate an acoustical wave which propogates through the air; the diaphragm, being driven by an ac modulated electrical signal flowing through a moving-coil which is immersed in the air gap of a permanent magnet motor structure. The motion of this diaphragm-coil assembly must be linear if the resulting

15 sound output is to be linear, that is, distortion free. As the reproduction of sound at low frequencies and high levels requires the generation of considerable volume velocities of air, large axial displacement of the diaphragm is advantageous. The extent of this displacement is contrained by the finite

20 dimensions of the compliance and motor components.

The maximum excursion capability of a loudspeaker (Xin & x ) is defined as the displacement or excursion of the diaphragm for some chosen acceptable or allowable level of nonlinearity. Essentially, X_max is a large-signal Thiele/Small

25 parameter which is indicative of how far a speaker diaphragm can move before significant distortion is encountered; it is therefore indicative of the maximum usable low frequency acoustic power output of a loudspeaker of a given diaphragm-radiating area. Longer excursion loudspeakers

30. currently manufactured for various high-power low-frequency p *rofessional ap*^■p•*^■lications exhibit an X_m a. _ value of less than 8 millimetres.

The loudspeaker motor comprises a coil having a large number of turns of insulated high conductivity wire wound on a cylindrical former of suitable material. The force exerted on the piston by the coil is Bli where B is the magnetic field strength, 1 is the effective length of coil wire in the field, and i is the current flowing in the coil. The acoustic output at a given frequency is essentially proportional to the volume displacement of the cone at that frequency. Lowering the frequency by one octave necessitates doubling the volume displacement of the diaphragm to obtain flat acoustic power response. The maximum low frequency output capability of a woofer is a function of the maximum volume displacement of the diaphragm. This is equal to the diaphragm area (S_D) x maximum linear displacement, X . In order to achieve a high value of ^X _max' ^woσ-_^e--^{s are} usually designed to utilise a coil which is longer than the region of magnetic flux concentration in the air gap. At excursion beyond the linear range of the motor, the Bl product is reduced and the motor-system is inherently unstable due to diminished back EMF. The coil tends to overshoot, however stability is provided by the restoring force of the compliance members, primarily the spider which increases in stiffness at excursions greater than the linear range of the driving ³ motor. This results in a value of Xmax for the loudspeaker which is greater than the motor's linear excursion capability. DISCLOSURE OF INVENTION

It is an object of this invention to provide:

1) a moving-roll compliance of which the increasing stiffness is a function of the position of the roll and its thickness; 2) a moving-roll compliance of constant thickness;

3) a narrow compliance between the diaphragm and the frame of a loudspeaker to result in an increase in the diaphragm area relative to a given diameter for the mounting frame;

4) a means to minimise the overall diameter of a woofer for a given maximum LF output capability; 5) a loudspeaker having a diaphragm compliance member of which the width is indep ^■_endent of Xm a x ;

6) novel compliance members for the suspension and centring of a diaphragm and voice-coil assembly, such that the compliance members, surrounds and/or spiders, inherently posses narrow width and axial mass distribution, whilst being able to provide restoring force and accomodate a very large axial displacement;

6) a highly damped compliance with axial mass distribution to render effective control and suppression of resonances in the diaphragm.

According to one aspect of the invention there is provided an audio transducer comprising a frame; an ax..,ally displaceable diaphragm normally bounded by said frame; a moving-coil permanent magnet motor; and a flexible compliance member extending between the outer periphery of said axially displaceable diaphragm and the inner periphery of said frame; said flexible compliance member being in the form of a moving-roll, whereby said inner periphery is enabled to approach said outer periphery to thereby provide an annular gap between the said two peripheries which is narrower than that in conventional audio transducers; and whereby a larger axial displacement of said diaphragm is made possible.

According to another aspect of the invention there is provided a flexible compliance member for an audio transducer adapted to extend between the outer periphery of an axially displaceable diaphragm of the audio transducer and a frame thereof radially spaced from said outer periphery, said compliance member comprising opposed skirt portions, the upper ends of which are attached to said frame and to the outer periphery of the diaphragm, said opposed skirt portions continuing so as to join at their respective lower extremities forming the moving-roll portion of said compliance member.

Ideally, the axially displaceable diaphragm is a circular radiator diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the reader may gain a better understanding of the present invention, hereinafter will be described a preferred embodiment thereof, by way of example only and with reference to the accompanying drawings in which:- 5 Figure 1 is a schematic cross-section of an audio transducer of the conic radiator kind;

Figure 2 is a fragmentary view, to an enlarged scale, showing a first prior art compliance member;

Figure 3 is a fragmentary view, to the same

10 enlarged scale, showing another prior art compliance member;

Figure 4 is a fragmentary view, again to the same enlarged scale, of a compliance member according to the present invention; and

Figure 5 shows an inventive compliance member, much

-¹-⁵ enlarged, including a transverse cross-section of it. BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 shows, simplistically, a typical audio transducer of the conic radiator kind, in schematic cross-section. Such a conventional direct radiating conic

20 loudspeaker includes a magnet assembly and a dynamic loudspeaker coil. The magnet assembly comprises a magnet 1, a centre pole 2 and an outer pole 3 mounted onto a frame 4 which, in turn, is adapted to be mounted in a baffle or panel (not shown), against a perimeter gasket 5. A diaphragm or cone 6 is

25 joined at its smaller inner periphery to a voice-coil 7 which is positioned concentrically within the magnetic gap by a first compliance member or 'spider' 8. The outer periphery of diaphragm 6 is concentrically located within, but spaced from frame 4 so as to form an annular gap 9 between the outer

³⁰ periphery of the diaphragm and the inner edge of frame 4. Extending across this annular gap 9 is a second compliance member 10, usually referred to as a 'diaphragm surround'. The fixed edge of each compliance member is attached to the loudspeaker frame/magnet assembly and the two members together

35 impart axial stability and radial centring to the diaphragm 6 and voice coil assembly. The members, as such, must be compliant to an extent which permits the normal excursions of the diaphragm when a signal is passed through the coil, such that the restoring force provided by two compliances, acting together, is linear over the peak-to-peak linear displacement range of the motor. Beyond this range, the resultant displacement is linearised by progressively increasing the combined stiffness of the compliance members as a function of displacement. This minimises distortion at higher displacements of the diaphragm. Ideally, a compliance member should possess the following properties and functions; it should:- a) provide linear axial movement below X of the motor; b) possess increasing stiffness beyond Xmax of the motor; c) restrict radial movement of the moving elements, diaphragm and coil, to provide centring of them; d) be highly damped and terminate the diaphragm against diaphragm resonance; e) exhibit no circumferential or radial vibration modes of its own; f) have a narrow radial width so as to maximise diaphragm area (S_D); g) be of low mass; and h) not age or sag.

Two different forms of compliance member are to be found in the prior art and these are shown in Figures 2 and 3. The compliance member 11 of Figure 2 comprises a corrugated, or circumferentially convoluted annulus, usually made of cone paper or impregnated textile material and is employed in low linear displacement speakers - that is, low X_ma3.. This kind of compliance is also almost universally used for spiders.

Compliance member 10 of Figure 3 is termed a "half-roll" compliance and is employed in higher displacement loudspeakers, usually manufactured from soft foam plastic material. The half-roll compliance member has low resistance to radial movement. Low radial stiffness can cause axial tilting, causing eccentricity of the voice-coil turns in the magnetic gap, resulting in improper operation or failure of the loudspeaker. Due to their radial spatial distribution of mass, both types exhibit highly resonant modes. For this reason, a damping compound, or dope, may well be applied. Both of the above forms of compliance member make it necessary for the diameter of outer frame 4 to be substantially larger than that of the diaphragm or cone 6, that is to say the width of the annulus 9 is large.

Since it is an object of this invention to provide an improved compliance member between the outer frame and the diaphragm of a loudspeaker to achieve an increase in diaphragm area S_D relative to a given diameter of outer frame, the use of either of the types of compliance, as shown in Figures 2 and 3, is - whilst acceptable - not ideal as the required overall diameter of the loudspeaker is increased. The radial width of these comp ^cliances is a function of Xmax

Figure 4 shows, in fragmentation, a compliance member 12 according to the present invention; inventive compliance member 12 may aptly be termed a "moving-roll" compliance and is comprised of a small radius half-roll 13, the position of which alternates up and down the height of the skirts of the compliance member in the axial direction, by half the value of the displacement of diaphragm 6. Half-roll 13 is shown in contact with two surfaces, one formed by the inner-wall 14 of frame 4 and the other by an in-turned rim 15 of cone 6, of appropriate length. As can be seen, the width of the annulus 9 between the two peripheries is much narrower than with the prior art loudspeakers, necessitating the inner-wall portion 14 of frame 4 to be set almost perpendicularly to the mounting flange 16 of frame 4. The lips 17 and 18 of compliance member 12 are bonded, or otherwise affixed, to frame 4 and diaphragm 6 at 19 and 20 respectively. In the process of manufacture, the inventive compliance member 12 is moulded in unrolled form as shown in Figure 5 such that the diameters at the corners 21 and 22 formed by two lips 17 and 18 and the compliant portion of the surround correspond to the outer diameter of the radiating diaphragm and the diameter of the inner wall of the frame respectively. Flexible materials possessing high internal damping, such as silicon rubber, neoprene, or elastic polyurethane foam are particularly suitable. Figure 5 also shows a cross-section 23 of the compliance member. If the thickness of the compliance member were constant, then the restoring force would be zero. If required, this force may be provided by making the thickness of the compliance member vary, such that it is progressively thicker away from its middle linear range, which is determined by the linear range of the motor indicated by 24 in Figure 5. The mid-point of this linear range 25 is the line about which the half-roll 11 positions itself when there exists no driving force, and coincides with the symmetrical orientation of the coil in the gap. The magnitude and linearity of the restoring force are in the way of being a design factor dependent on material properties, thickness variance and roll radius. In this fashion, the compliance can be adapted for use as a spider. INDUSTRIAL APPLICABILITY

The invention provides for loudspeaker construction in which the radial width of the compliance member is independent of X_max/ as opposed to the prior art compliances in which the radial width of the compliance member is a function

In comparsion with prior art loudspeakers, the diaphragm diameter can approach the inner diameter of the frame more closely. Thus, for a loudspeaker of a given outer frame diameter, provision of the inventive compliance member as the diaphragm surround maximises S_D and, accordingly, the displacement volume is greater. The greatest gains will be for loudspeakers of small diameter and large displacement such as the 4" units manufactured by Bose Corporation in the U.S.A. The loss of S_D necessitated by prior art compliances is minimi sed .

The susceptability of the moving-roll compliance to self resonate is also inherently minimised by the narrowness of the half-roll and the large area of loose contact of its skirts with the frame on one side and the in-turned rim of the diaphragm on the other.

As well as exhibiting no significant resonances of its own, further benefits of the moving-roll compliance are superior termination properties for the suppression and control of diaphragm resonances of which there are two types; modal and nodal. Modal resonances are antiphase actions of different sectors of the diaphragm at lower frequencies and nodal resonances are antiphase concentric annular actions at higher frequencies. Both types exhibit their greatest amplitude at the periphery of an unterminated diaphragm.

For a diaphragm of which the periphery is terminated with a moving-roll compliance, modes are suppressed by well resisted stresses induced in the surround. Nodes are controlled by the coupled mass and the rolling resistance of the surround. Both functions are dependent on the properties of the flexible material from which the compliance is formed, and its thickness.

Circumferential stressing in the skirts of the compliance is caused by axial tilting of the diaphragm, and this yields a large restoring force to return the diaphragm to its normal axial concentric orientation.

From the abovegoing, it will be readily appreciated by those skilled in the art that several variations and modifications may be made to the invention without departing from the spirit and scope thereof.

Claims

1. An audio transducer comprising an outer frame; an axially displaceable diaphragm normally bounded by said outer frame; a moving-coil permanent magnet motor; and a flexible compliance member extending between the outer periphery of said axially displaceable diaphragm and the inner periphery of said outer frame; said flexible compliance member being in the form of a moving-roll, whereby said inner periphery is enabled to approach said outer periphery to thereby provide an annular gap between the said two peripheries which is narrower than that in conventional audio transducers; and whereby a larger axial displacement of said diaphragm is made possible.

2. The audio transducer as claimed in Claim 1, having an axially displaceable diaphragm in the form of a conic or curvilinear radiator diaphragm incorporating an inturned rim on the periphery which contacts with a compliant member.

3. The audio transducer as claimed in Claim 1 or Claim 2, wherein said flexible compliance member is thinner towards the middle section thereof.

4. The audio transducer as claimed in any one of the preceding Claims, wherein said flexible compliance member is utilised as a spider.

5. The audio transducer as claimed in any one of the preceding Claims, wherein the material from which said flexible compliance member is made is selected from the group consisting of silicon rubber, neoprene and clastic polyurethane foam.

6. A flexible compliance member for an audio transducer adapted to extend between the outer periphery of an axially displaceable diaphragm of the audio transducer and an outer frame thereof radially spaced from said outer periphery, said compliance member comprising opposed skirt portions, the upper ends of which are attached to said outer frame and to the outer periphery of the diaphragm, said opposed skirt portions continuing so as to join at their respective lower extremities forming the moving-roll portion of said compliance member.

7. A flexible compliance member as claimed in Claim 6, being of variable cross-section across its width whereby the thickness of the compliance member is greater towards its peripheral edges and lesser towards the middle section thereof,

8. A flexible compliance member as claimed in Claim 6 or Claim 7, consisting of silicon rubber, neoprene or elastic polyurethane foam.

9. A diaphragm for an audio transducer of the type as defined in Claim 1, with an in-turned rim on its periphery which contacts with a compliant member.