US1990066A - Diaphragm for acoustic devices - Google Patents

Description

Feb. 5, 1935. DUTTON 1,990,066

, DIAPHRAGM FOR ACOUSTIC DEVICES Filed Nov. 15, 1953 2 Sheets-Sheet I I I I l I I I I I I 1 I l I I I A TI'OHNE) Feb. 5, 1935. G. F. DUTTON 1,990,066

DIAPHRAGM FOR ACOUSTIC DEVICES Filed Nov. 15, 1933 2 Sheets-Sheet 2 Patented Feb. 5 1935 UNITED ST S PA EN ,O -I 'E 1,990,066' DIAPHRAGM FOR ACOUSTIC DEVICES Gilbert Faraday Dutton, South Kensington, London, England, assignor to Electric and Musical Industries Limited, Hayes, England, acompany of Great Britain Application November 15, 1933, No. 098,052 4 In Great Britain November 15, 1932 1 20 Claims (01. 181-42 The present invention relates to acoustic de- Kyices and more particularly to large diaphragms,

that is to say diaphragms ofa size adapted toradiate sound effectively without .the aid of a 5 horn, and to arrangements for supporting such diaphragms.

Hitherto large conical diaphragms-have usually been made of paper, and in some cases these have been corrugated. It has usually beenfound that such a paper diaphragm can be made thick fective mass of the diaphragm, and 'hence thefinal response characteristic'curve of the diaphragm willshow a depression at the frequency at which-a radial mode occurs. v

Variation in radial modes produces bending.

stresses in the diaphragm; hence increasing the thickness of the diaphragm raises the frequency at which the radial mode occurs; and. the intro-. duction of internal friction damps the vibrations. A diaphragm of paper having a density of 0.7 gram per cubic centimetre can be made 0.02 inch thick without being unduly heavy. Such a diaphragm is very stiff as regards bending, and this property, together with internal friction, is sufficient to suppressthe radial modes in the lower frequency range, that is below 2,000 cycles per second. If the paper is thinner, for example 0.01 in. thick, radialmodes in the lower frequency range tendto become troublesome.

Paper, whether plain or impregnated with hardeningresins, does not have a very high wave transmission velocity. The value of this velocity is given by the square root of the ratio. of the Youngs modulus to the density. In the best kraft paper the velocity is 1.7 x 10 cm. persecond, and impregnatingthis paper with a hard resin may increase this value to 2.5x 10 cm, per second. The high-frequency response of a conical diaphragm depends on this value of wave velocity, a low value giving a low cut-off frequency and vice versa. At frequencies exceeding 2,000

cycles per second, wave transmission begins to become apparent in a large conical diaphragm. It is necessary that this type of transmission should occur, since it reduces the effective mass of the diaphragm. In an ideal diaphragm-wave transmission would take place without wave'reflection,

allthe energy being'ra'diated before the wave reached the-boundary of the diaphragm.

Hitherto attempts have been made to use aluminium, which has a higher ratio of Youngs modulus to density than paper (its wave trans- -mis'sion velocity, being about 5.1 X 10 cm.- per second), for conical diaphragms, but such 'diaphragms have proved unsatisfactory on account of radial modes and strong reflections. These defects are due to the high density of aluminium, compared with paper, which necessitates the use of thin material about 0.0025 in. thick, and to the small internal friction of such a diaphragm. The

radial modes therefore appear at low frequencies and tend to-cause the diaphragm to rattle.

A knownvconical aluminium diaphragm has asmall number'of widelyfspaced concentric depressions, or half corrugations with uncorrug ated portions between them. This arrangement-is not satisfactory, since reflection occurs at the corrugations and the metal between the corrugations buckles under the stresses imposed by normal operation and emits rattlingsounds similar to the noise made by shaking a thin sheet of metal. s It-has been proposed to use a conical diaphragm of aluminium or' aluminium alloy having. continuous corrugations in its surface. The

corrugations were, however, of such a shape and depth that portions thereof lay substantially in planes normal to the direction of vibration of the diaphragm. An object of the present invention isto provide a diaphragm which has a good response to the higher as well as to the lower acoustic frequencies. A further object is to provide a metal diaphragm in which the defects hereinbefore re-' ferred to are obviated. According to the presentinven'tion. a large acoustic diaphragm comprises (that is to sayis lformedsu'bstantially wholly or partly of) a material havinga higher ratio of Youngs modulus to density than paper and is provided with corrugations merging into one another over at least a part .of the surface of thismaterial, the

ratio of the pitch of the corrugations to the depth thereof (measured from trough to crest) being greater than 5 to 1.

According to the invention in a further a s-, pect, a large acoustic diaphragm is formedsubstantially wholly of a material having a higher ratio of Youngs modulus to density than paper and provided with corrugations merging into .radially inner portion formed of a material having a higher ratio of Youngs modulus to density than paper, this portion being provided with corrugations merging into one another over at least a part of its surface, the ratio of the pitch of the corrugations to the depth thereof being greater than 5 to 1. The radially outer portion of this diaphragm is preferably of paper and of frusto-conical form. l

The functions of the corrugations are to re-' move irregularities from the surface of the diaphragm, to increase the lowest frequency at which radial modes of vibration take place and to give the diaphragm the stiffness necessary for safe handling. It is therefore important that, at least in the outer region of the material having a high ratio of Youngs modulus to density, the corrugations should merge into one another and not be spaced widely apart by uncorrugated portions.

It is preferred to make the improved diaphragms of dished form, such as of conical form (which term in the appendant claims is -to be read as including a frusto-conical form), and where a dishedform is employed, it is advantageous to make the shape of the corrugations such that all parts thereof lie at a considerable angle to planes normal to the direction of vibration of the diaphragm.

The region round the driving point, of a conical diaphragm or driving zone of a frusto-conical diaphragm may be uncorrugated or provided with corrugations of smaller depth, or longer pitch, or both, than those on a region farther from the driving point or zone. Where the region round the driving point or zone is-uncorrugated the material may be thicker there than elsewhere. Alteratively in a frusto-conical diaphragm the smaller diameter end-may have the form of a cylindrical metal spigot flared out of the frustum so as to provide the necessary rigidity.

When vibrated at high frequencies, say from about 1,500 cycles per second upwards, a cone of, for example, '7 in. diameter acts in the-manner of a mechanical transmission line, the impedance of which increases from the centre outwards. In order to avoid the formation of pronounced standing waves in the diaphragm, the latter must represent a smooth mechanical transmission line. It is therefore necessary to avoid a sudden change of mechanical impedance along the cone radius, such as would occur if the corrugations were deep. The corrugations should consequently be made as shallow as pos-.

sible, consistent with freedom from rattle. It is also necessary to terminate the mechanical line with a suitable resistive element of approximately the same impedance as the cone. In this way the formation of pronounced standing waves due to reflection from the periphery of the diaphragm is prevented.

A preferred arrangement to prevent appreciable refiection from the periphery is to mount the edge of the diaphragm within a circular aperture in a box or baflle with the aid of an annulus of velvet or the like, there being interposed between the annulus and the diaphragm a layer of a substance which offers high internal friction to distortion due to vibrations and which is capable of withstanding considerable distortion within the elastic limit. A suitable substance is a highly-plasticized vinyl acetate polymer, a suitable plasticizer being tricresyl phosphate.

In order to damp vibrations within the diaphragm, it may be formed with one or more joints incorporating a damping substance such as the above mentioned polymer.

The invention will be further described with reference to the examples shown in the accompan'ying drawings, in which Fig. 1 is a section of part of a metal frustoconical diaphragm,

Figs. 2, 3 and 4 are diagrammatic sections of alternative forms of radial joint in the diaphragm shown in Fig. 1,

Fig. 5 is a section of part of a' moving-coil loud speaker comprising an alternative'form of metal frusto-conical diaphragm,

Fig. 6 is a front elevation of a modification of the diaphragins shown in Figs. 1 and 5,

Fig. 7 is a section of part of a. metal and paper frusto-conical diaphragm,

Figs. 8 and 9 are diagrammatic sections of aldiaphragm shown' in Fig. 7.

Referring to Fig. l, the diaphragm from a circular sheet of aluminium about 8 in. in diameter. After a central disc and a sector have been cut from it, the sheet is formedin known manner into a conical frustum by joining the two radial edges 2 and 3, the apex angle'of the frustum being about 105 deg. The joint is made by applying the vinyl acetate polymer softened by heat and lapping the edges. The frustum is then placed in a press and corrugated. While the diaphragm is in the press, the smaller diameter end is drawn into a cylindrical spigot 4 adapted to receive a driving coil. The corrugations are concentric with the cone and extend from the periphery inwards for about two-thirds of the distance from the periphery to the spigot, the radially inner portion being uncorrugated.

The corrugations are formed by merging arcs of which the radius r is 0.4 in. and the depth d is 0.015 in. The pitch p is thus about 0.3 in., and the ratio of pitch to depth is about 20 to 1. Further, the corrugations are all of such shape and depth that no parts thereof approach parallelism to planes normal to the direction of vibration of the diaphragm as a whole, that is normal to the axis 5 of the frustum.

1 is formed ternative forms of circumferential joint in the Owing to the reaction following the stretching process of drawing the spigot, the metal of the spigot is put into circumferential compression and the metal in the zone immediately adjoining the spigot is'put into circumferential tension. This prevents the occurrence of slackness over any small area of the radially inner part of the diaphragm and obviates crackling noises during operation. 1

The aluminium has the following properties; Breaking stress, 9 to 10- tons per square inch; Youngs modulus, 7.3 x 10 dynes per sq. cm. For this aluminium coneof about 7 in. in diameter the preferred thickness is 0.0025 in. Radial modes have been found to become prominent in diaphragms of this kind if the thickness-ericeeds 0.004 in. and the diaphragm becomes too fragile if the thickness is below 0.0015 in.

- The value of the square root of the ratio of 1,990,066 Youngs modulus to density for this quality of The driving'coil former 14 is attached to the aluminium is about 5.1 x10 cm/sec. whereas the corresponding value for paper is between about 1.7 x 1-0 'and 2.5. x cm./sec., as hereiribefore mentioned. w

The aluminium used in constructing the above diaphra'gmis -known as .semi-hard. If harder material is used it will be found to have become too hard (and therefore liable to fracture in use) after the corrugations have been formed if these 'are formed in the cold. A softer material has not sufiicientstrength to withstand sudden low fre-' quen'cy impulses such as maybe met with in prac 'tice. If thematerial is annealed during the corrugating process it may be possible to use a harder material. In any case endeavour should be made to arrange that when the corrugating process has been finished the'material' of the diaphragmhas.

thehighest. possible degree of hardness consistent with capacity to resist alternating stress without fracture. l v

In order to damp circumferential vibrations,,the

diaphragm shown in-Fig. Imay be modified by the provision of a plurality of radial 'joints, for example four, spaced at equal intervals, and incorporating a damping substance. These joints may be plain lapped joints, as previously described I and as shown in Fig. 6. eAlternativeforms of joints are shown in Figs. 2, 3. and 4. In Fig. 2., the lapped edges 2 and 3 are united by a layer 6 of .fplasticizedvinyl acetate polymer reinforced by -thread stitches 7. Fig. 3 shows a folded joint,

with the vinyl acetate polymer 6 lying between the folded edges 2 and 3. In Fig. 4 the edges 2 and 3 are substantially butted and joinedby a butt strap 6' of resistance material;,'which"may be the vinyl acetate polymer. The diaphragm provided I withsuch joints consists, nevertheless; substanthe spigot .may be shorter,- the necessary stiffness being provided by making the metal-of the rad i-- ally inner zone thicker than elsewhere. v

conical blank may be spun from a sheet of a thick-' ness' equal to the-maximum thickness of the fin- Thus the isheddiaphragm, the radially outer portion being drawn thinner'by the spinning operation. Alter- .natively the conical blank may beformed'from a i 1 xfiat sheet by a series of-drawi ng operations inter-- 55.

'spersed with annealing operations, by which the;

thickness can be graduated as desired.

"In the modified diaphragm shownin Fig. 5,- which however has not beenfound quite so satisfactory as that shown-in-Fig, 1, such a jointless -this case thepitch p is about 0.36 in.-,'and the construction is adopted. The size and material are substantially; the same, but the corrugations over the outertwo-thirds of the diaphragm-sun:

face are formed'by 'merging arcs of which the radius r is 0.21 in.;.and the depth' d 0.04 in. In

pitch-to-depth ratio is therefore about .9 to -.1.

The inner one-third of the cone-surface is pro vided withla little more than one shallow coriu-igation of which the dpth D is 0034131.; formed by mergingarcs of 1.25m. radius R. The pitch is then about 0.95 in.- and the pitch-to-depth ratio i is 32 to 11' The'thickness of the diaphragmis preferably graded from 0.004'in." at the spigot to 0.002 in. at the periphery.

radially outer portion 11 of paper.

spigot 4 by cellulose cement, and the leads to the driving coil (one of which is shown and denoted by. 15) are secured to the former l4 and not to the cone, to avoid rattling. The diaphragm.

1" is supported at its centrein known manner by meansof a corrugated buckrar'n spider l6 (see Fig. .1) secured byan adhesive such as cellulose cement to the cone; audit is mounted within an apertureB in a boxor battle 9' with the aid of a ring 10 of velvet attached by means of plasti- "cized' vinyl acetate polymer -6 to the periphery The velvet-ring 10 may Fig. 7 shows a further form of diaphragm having a radially inner portion 1' of aluminium and a The maximum diamter is about '7 in. and the frustum' of paper'll extends from the periphery to the mid 'point between the periphery and the spigot 4, be-

the same-section as those shown in'Fig; l and extending from the joint 12 to about the mid point- The joint 12" between the joint and the-spigot. shown in Fig.. 8 is lapped, a resistance material, for example plasticized vinyl acetate polymer, being used as'the adhesive.-'v The joint may, if desired, be reinforced, for example by sewing with thread, similarly to the radial joint shownin Fig. 2. Alternative forms'of resistance joints .between the aluminium andthe paper are shown in Figs. 8 and 9. In Fig. 8 the paperand the aluminium cones are flanged, the-resistance material: 6 being employed to couple the flanges to-. I

gether. In Fig. 9 the two portions are lap jointed and secured. by a relatively rigid adhesive 13, such as cellulose cement, the joint being coated with. a damping layer of the resistance, material 6.

I claim:

- 1. A large'ac oustic diaphragm comprising a materialhaving a higher ratio of Youngs modulus to density than paper, andprovided with corrug'ations" merging into one another over at least.

a part of the, surface of said material, ther'atio thereofbe-ing greaterthan 5 to 1.

of the pitch. of 'said corrugations to the depth 2. A large acoustic diaphragm formed substantially wholly of a material having a. higher ratio of Youngsmodulus to density than paper and provided with corrugations merging into one another over at least-the radially outer portion of its surface, the ratio of the pitch of said corrugations to the depth thereof being not less than 3. A large aeoustic'diaphragm the radially inner portion of which is formed of a. material having a higher ratioof Youngs modulus to density than paper, saidportion being provided with corrugations merging into one another over at least'a part of its surface, the ratio of the pitch .other over at' least apart of the" surface of said .material, the ratio of the pitch of said corrugations tothe depth thereof being greaterth '5 to'l.

.-'5. A large acoustic diaphragm comprising aradially inner portion of conical form and of the shape of said corrugations being such that.

surface, and a radially outer portion of 'frustoconical form and of paper.

7. A large acoustic diaphragm of dished form and provided with corrugations merging into one another over at least a part of its surface,

all pafts'thereof lie at a considerable angle to planes normal to the direction of vibration of the diaphragm.

' 8. A large acoustic diaphragm o frconical'form, comprising ,amaterial havin'g a higher ratio of Youngs'modulus to density than paper, and provided with corrugations merging into one another over at least a part of the surface of said material, the shape of said corrugations being such that all parts thereof lie at a considerable angle to planes normal to the direction of vibration of the diaphragm.

9.- A large acoustic diaphragm of conical form, comprising a material having a higher ratio of Youngs modulus to. density than paper, the part of said material in the region round the driving point or zone being uncorrugated, and the part of said material in a region farther from said point or zone being provided with corrugations merging into one another, the ratio of the pitch of said cgrrugations to the depth thereof being greater than 5 to l.

10. A large acoustic diaphragmof conical form, comprising a material having a higher ratio of Youngs modulus to density than paper, said material being provided with corrugations merging into one another over its surface, and said corrugations being of smaller depth in the region round the driving point or zone than in the region farther from said point or zone.

11. A large acoustic diaphragm of conical form consisting substantially wholly of a material having a higher ratio of Youngs modulus to density than paper, the region round the driving point or zone being uncorrugated, and the radially outer region being provided with corrugations merging into one another and having a pitcht -depth ratio of between 15 to 1 and 25 tol.

12. A large acoustic diaphragm of frusto-conical form of which at least the radially inner portion is of metal uncorrugated in the region round the driving zone and provided with corrugations merging into one another over a region farther from said zone, the smaller diameter end of the diaphragm having the form of a cylindrical spigo t flared out of the frustum in such a manner as to put the spigot into circumferential compressionand the metal in the zone immediately adjoining the spigot into circumferential tension.

13. A large acoustic diaphragm of conical form comprising light metal having corrugations merging into one another over at least a part of said the ratio of the pitch to the depth of the corrugations on said radially outer portion being between 8 to 1 and 30 to 1.

15. A conical acoustic diaphragm of which at least the radially inner portion is of aluminium between 0.0015 and 0.004 in. thick and has an apex angle of between and \deg., the region of said portion more remote from the driving point or zone being provided with corrugations merging into one another and having a pitch-todepth ratio of between 8 to 1 and 20 to 1.

16. A large acoustic diaphragm comprising a material having a higher ratio of Youngs modulus to density than paper, and a joint incorporating a substance which offers high internal friction to distortion and which is capable of withstanding considerable distortion within the elastic limit, said diaphragm material being provided with corrugations merging into one another over at least a part of its surface, the ratio of pitch to depth of said corrugations being greater than 5 to 1.

17. A large acoustic diaphragm formed at least in part of a material having a higher ratio of Youngs modulus to density than paper, and comprising a joint incorporating a highly plasticized vinyl acetate polymer which serves as a damping substance. I

18. A large acoustic diaphragm of conical form comprising a material having a higher ratio of Youngs modulus to density than paper and a joint incorporating a highly plasticized vinyl acetate polymer, said diaphragm material being provided with corrugations merging into one another over at least a part of its surface.

19. A large acoustic diaphragm comprising a material having a higher ratio of Youngs modulus to density than paper, and a plurality of radial joints incorporating a material which offers high internal friction to distortion and which is capable of withstanding considerable distortion within the elastic limit.

20. In combination, a large acoustic diaphragm comprising a material having a higher ratio of Youngs modulus to density than paper, and a

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