US1699903A - Loud-speaker - Google Patents

Description

Jan. 22, 1929 7 1,699,903

N. w M LACHLAN LOUD SPEAKER Filed Jan. 16, 1928 2 Sheets-Sheet 2 gwvenfoz /l 0/"/770/7 M/m MZM/M dbtom w i Patented den. 22, 1929.

UNITED STATES V eeann PATENT OFFICE.

NORMAN NILLIAM MCLACHLAN, OF LONDON, ENGLAND, ASSIGNOR T WIRELESS MUSIC LIMITED, OF LONDON, ENGLAND.

LOUD-SPEAKER.

Application filed January 16, 1928, Serial No. 247,228, and in Great Britain January 11, 1927.

The present invention relates to loud speakers of the type in which a diaphragm or other surface of relatively large size is in operative connection with a'driving element which is adapted to be set in vibration electrically.

Many attempts have been made to produce a loud speaker of this type, which shall re-,

It is also difficult to design a driving element which shall give an even response over this band of frequencies without excessive damping which renders the system insensitive. The present invention takes advantage of the fact that certain sound producing surfaces break up progressively as the frequency applied to them is increased, that is to say, the effective area in vibration decreases progresively with increasing frequency. By using a driving element having a natural frequency higher than has been customary in the past, and by coupling to it a sound producing surface'which is of such size and of such material and shape that it only commences to vibrate as a whole in the neighbourhood of the lowest frequency to be reproduced, a very uniform overall response characteristic can be obtained and at the same time 40 pronounced resonances are eliminated.

According to the present invention the natural frequency of the driving element of a loud speaker of the type set forth when detached from the sound producing surface is made greater than 2500 cycles per second.

All parts which can be regarded as vibrating efiectively with the driving element are considered to form part thereof: Thus where a cantilever reed is utilized as driving 5 member and is connected to a conical diaphragm by means of a metal rod and metal nuts, the connecting means are regarded as forming part of the driving means.

The sound producing surface is made of e such size and shape and of such material that it does not freely vibrate as a whole above 150 cycles per second. The question as to whether the surface is vibrating as a whole or not may be tested by dusting a fine powder on to the surface. If any nodes are present these will then be rendered visible by a piling up of the powder upon them. The presence of nodes indicates that the surface is not vibrating as a whole.

The natural frequency of the vibratory parts of the loud speaker, which comprise the driving element, the connecting means and the diaphragm, as calculated from the stiffness of the driving element and the mass of the vibratory parts, regarded as lump loaded upon the driving element, lies preferably between 700 and 1100 cycles per second.

The diaphragm is preferably conical and supported at its periphery with only suiiicient restraint to provide the desired terminal impedance.

Suitable means are preferably provided for minimizing the interaction of pressure waves generated by the front and rear surfaces of the diaphragm respectively, in order to preserve the lower tones. Such means may comprise a box, the front end of which is sealed by the diaphragm and its flexible annular member, whilst apertures of suitable size may be formed in the rear of the box to reduce undesirable box resonance, and possible reduction of the lower tones by interference.

The invention is illustrated in the accompanying drawings, in which Fig. 1 shows diagrammatically a method of construction of a diaphragm;

Fig. 2 a schematic sectional elevation of a loud speaker in accordance with the inventiOIl; i

Figure 3 is a sideview of a movement arranged to be mounted inthe loud speaker casing showing a portion thereof taken along the line 33 of Figure 4, and

Figure 4 is a plan" view of the movement.

Referring to the'drawings, the diaphragm '1, which in its'final form is conical, is made of hot press paper about 0.2 millimetres in thickness and weighing about 0.015 grammes per square centimetre. Acircle A, 8% inches 5 radius, is first cut from the paper and two radii B and C, 60 apart are drawn thereupon. One cut is then made along one of these radii (say B) and another cut D along a line parallel to and about inch from the other radius between the two radii and ex.- tending from the former radius (B) to the periphery of the circle. The inch wideare made around the edge of the cone extending'inwards about inch, and the tongues so formed arebent outwards --to lie in the plane of thecone base. These tongues are then glued on to a sheet of thin paper and when dry the circular portion of this paper ,inside the-tongues is removed. Alternatively, instead of making the radial incisions the outer rim may-be pressed into shape by means of a male and female jig, the outer edge of paper being previously moistened. There is now, therefore, a cone of. about 18% inches base .diameter,. having therefore a projected area slightly less than, 1006 sq. cms. and with an apex angle of about 113, bounded by a fiat annular member of thin paper, such as that commonly employed for charcoal sketches.

The diaphragm so formed is warmed to remove moisture and then treated all over both sides with a suitable waterproofing yarnish, such as a solution of cellulose. After this treatment the diaphragm is preferably left for a week in a dry warm atmosphere. The diaphragm is then glued to the edge of a wooden frame 2, 16 A and 17% inches wide, internally and externally respectively, and

" about 2 inches deep, with the conical portion projecting through the frame. If desired, the edge of the thin paper of the diaphragm which is to be'glued may be left unvarnished to facilitate gluing, and when dry, varnish may be applied in the neighbourhood of the oin.

' wooden box 3, for example of inch thick a total area of about 60 square inches. The

wood, whose dimensions are 17 inches square internally and 7 inches deep, The back of the box is provided with apertures 4 having frame carr ing the diaphragm is inserted within the fi'ont of the box and is held firmly against fillets secured around the inside of the box by suitable blocks secured to the inside of the casing in front of the frame. The diaphragm and frame may therefore'be readily replaced if necessary. The front of the dia phragm in its final position is preferably about inch behind the front edge of the box and the front of the box may be covered with thin silk (not shown) secured in position by means of a beading.

- V In order to strengthen the cone in the neighbourhood of its apex a small cone 5 of the same material as the larger cone is preferably The loud speaker casing consists of a stiff overlapped portion of the smaller cone should be arranged at the opposite side to the overlap of the large cone. I

The annulus 1' of light-paper around the conical portion of the diaphragm, which annulus is of minor or negligible import as a sound radiating surface, serves to support the diaphragm to prevent the passage of pressure waves around the periphery of the cone, to preserve. the lower tones, and to impose a terminal impedance upon the diaphragm such as will substantially prevent reflection and consequent resonance. Instead of paper, other materials which fulfil these conditions may be used.

The diaphragni hereinbefore described is adapted to be impulsed by a reed loud speaker movement 6, through a threaded rod 7, which is attached thereto by screwed conical cups 8 and 9, as shown in Fig. 2. The movement is mounted in the rear of the box 3 and is preferably acoustically insulated therefrom by means of a rubber ring or the like 10.

As stated above, any suitable type of movement complying with the requirements of the present invention may be employed, but in order that the relative dimensions of the apparatus as a whole may be appreciated, one suitable movement as shown in Figures 3 and 4: will now be described in detail.

The said movement comprises a base piece 12 of flat brass which is adapted to be attached to the-back of the box 4. A relatively large boss 13, havinga central hole 1a, is provided on the rear side of the base. A U-shape'd magnet 15 is secured at its base to a plate 16 and a spindle 17 secured to the plate projects therefrom away from the magnet. This spindle is made a good sliding fit in the hole 14 through the boss and a featherway 18 is provided in the spindle and is engaged by a small block 19 which is pressed into position by means of a screw 20 passing from the outside to the inside of the boss and raven-ting rotation of the spindle relative y to the boss but permitting free sliding thereof. The end of the spindle remote from the plate is threaded to receive an adjusting knob 11, whilst a helical spring 21 surrounds a small locating spigot 22, formed upon the base piece upon the opposite side to the boss 13 andco-axial therewith, and tends to force the plate 16 carrying the magnet in a direction away from the base. The knob is provided with a groove which is engaged by a plate secured to the rear face of the boss, and this permits rotation of the knob but prevents translation thereof although this arrangement is not per surfaces, in order to concentrate the flux in the air gap. A coil 25, adapted to carryaudio frequency currents,is arranged around eachpole piece, and these coils may be connected in series. The resistance of the coils in series may be 2000 ohms. A pillar 26 is secured to and projects from the base on the opposite side to the boss, and parallel with the spindle and upon its ends, projecting at right angles to the pillar, is secured a reed 27 of mild steel or suitable silicon steel alloy which passes just above the gap between the pole pieces. the adjusting knob permits of the movement of the magnet pole pieces relatively to the reed. The thin mild steel rod 7 is riveted into the reed at a point over the middle o'f It will be readily understood that the air gap and projects at right angles to the reed. The mild steel rod 7 is provided with a thread upon which is screwed a conical cup 9 into which the apex of the cone diahragm 1 seats when the movement is placed 1n position uponthe box (see Fig. 2). Alternatively,the conical cup may be formed integral with the rod or may be soldered thereto. The end of the threaded rod projects through tothe front of the cone and a coni-' cal nut or cup 8 is screwed on to secure the centre of the diaphragm firmly to the rod. The angles of the conical cup and of the coni cal nut should be approximately equal to that of the cone diaphragm.

The free length of the reel is 2 centimetres, the distance from the support to the threaded rod 1.25 centimetres, the width of the reed 0.64 centimetres and the thickness of'the reed 0.18 centimetres. The mass of the connecting means (threaded rod, cup and nut) is 2.5 grammes and the mass of the conical portion of the diaphragmis 23 grammes. The stiffness of the reed, which may be of turbostalloy or steel, is found by calculation from its dimensions to be 9 x 10 dynes per centimetre, and its natural frequency about 3500- 4000 cycles per second.

lVhere a simple cantilever reed of constant rectangular cross section is used its natural frequency may be determined by Rayleighs approximate formula:

where is the natural frequency, '15 the thickness in centimetres and Zthe length in centimetres.

With the connecting means the natural frequency is approximately 2800 cycles per second.

If the mass of the conical portion of the diaphragm, together with that of the connecting means, be assumed to be lump-loaded on the reed at the point of attachment of the connecting means thereto, the calculated natural frequency of the vibratory system will be found to be about 1000 cycles per sec- 0nd. An allowance for the mass of the reed lowers the calculated frequency to 950 cycles per second. The natural frequency of the diaphragm considered as a mass vibrating upon its paper annular member, is below good audibility about 25 cycles per second.

Although the values given above have been found to give the best results, certain small 4 being between 2500 cycles and 3000 cycles per second, said sound producing member being of such nature and dimensions that it does not freely vibrate as a whole above 150 cycles per second.

2. A sound reproducing device comprising a sound producing member of relatively large size, a vibrating reed operatively connected with said member, the natural frequency of said reed being greater than 2500 cycles per second, said sound producing member being of such size, shape and material that it does not freely vibrate as a whole above 150 cycles per second.

3. A sound reproducing device comprising a sound producing member of relatively large size, a vibrating driving element, means operatively connecting said producing member with said driving element whereby said element is so dimensioned that its natural frequency when loaded at its point of attachment with said member, with a mass equal to the mass of said member and the mass of said connecting means, is between 700 and 1100 cycles per second, said sound producing member being of such nature and dimensions that it does not freely vibrate as a whole above 150 cycles per second.

4;. A sound reproducing device comprising a conical sou'nd producing member of relatively large size, and a'vibrating driving element operatively connected with said member, the natural frequency of said driving element being between 2500 cycles and 8000 cycles per second, said sound producing member being of such nature and dimensions that it does not freely vibrate as a whole above 150 cycles per second.

5. A sound reproducing device comprising a conical sound producing member of relatively large size, a vibrating driving element operatively connected with said member, the natural frequency of said driving element bemosses which is adapted tobe set in vibration, and means for connecting said driving element With said sound producing member, wherein 5 the natural frequency of said driving element is greater than 2500 cycles per second and wherein the natural frequency of said sound producing member, when detached from said connecting means, is less than 30 cycles per 20 second.

In testimony whereof I have signed my name to this specification.

N ORMAN WILLIAM MGLACHLAN.

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