Cone Speaker Construction

Radio Record, Volume III, Issue 21, 6 December 1929, Page 26

Cone Speaker Construction

Useful Hints for t rospective Builders

(By

MEGOHM

GOOD dynamie cone (or moving coil) is undoubtedly the best type of loudspeaker at the present time, especially where great volume is required. But there are many listeners who will have to be con- === tent with a reprodu"er less expensive both in initial outlay and | in running cost. Those who study economy will still find in the magnetic type of speaker a faithful friend that will give good service, and quality according to its particular design and capabilities. There was a time when cone speakers were less sensitive than the horn type, but with the rapid improvement in the design of units during the last two years, the cone has come to the front, leaving the horn speaker well in the shade. These remarks are not intended to apply to the exponential horn of lengthy dimensions, but to the ordinary small metal horn. Large ex: ponential horns give good reproduce.. tion, but their size makes them unsuitable for general use. To reproduce the rowest note of the piano, 26 cycles, an exponential horn must be 31 feet long, with a mouth over 9 feet square! But 26 cycles per second is a yery low rate a

of vibration, and very good reproduction is obtained in actual practice with a speaker that will respond to 50 cycles as its lowest note. — Deficiencies of Loudspeakers. OOD reproduction of broadcast musical items ghould cdntain all the notes, high, middle, and low, just as they are played at the studio; yet very many listeners never hear many of the lower notes as they are played. This is because their loudspeaker will not respond sufficiently well to notes below a certain audio-frequency to allow of their being audible. In the smallest horn speakers this deficiency is at its maximum, and the "cut-off" may be at 200 eycles, or even higher. The melody or "air" of the tune is present clear enough, but the accompanying bass notes are either much weakened or entirely lost, according to the slowness of their rate of vibration. -~

The worst feature of such speakers is that some of the notes that are only partially reproduced serve to form a background of "mush" which greatly detracts from the quality of the highpitched notes that are strongly brought out, The foregoing briefiy illustrates the. kind of musical struggle that is going on in any speaker doing its best to handle notes that are not within its range. This is of course. apart from any deficiencies that may be present in the audio amplifier, where poor

transformers and other conditions may ulso tend towards the loss of low notes. Range of Reproduction. War is most desired is a speaker that will reproduce at least every note or audio-frequency that the amplifier, especially its last stage, is capible of handling. On account of its limitations the ordinary horn -speaker is not capable of giving reproduction throughout the musical range in generul use, but a cone speaker with a wellconstructed bDbalanced-urmature unit vill give a faithful rendition, provided that the size and texture of the cone Ss suited to the requirements of bassnote reproduction. There is usually little trouble with the high notes un---E

less there is a piercing shrillness, which may possibly be developed in the amplifier, or by a particular crystal detector emphasising the high frequencies so that they are obstrusive when amplified. A fixed condenser of. small value across the speaker leads will often subdue any tendency to "sératchihess," and some units are ‘provided with such a condénser built in, as in the well-known "66" units. In order that bass. notes may be reproduced with reasonable va\me, the diameter of the cone should 144% be too small. From 24 to 27 inches diameter is good and will give all that is required. The double cone as described on page 8) of the "Listeners’ Guide" is a very satisfactory form and looks well when hanging on the wull, as there is no cabinetwork showing if the back of the cone is glued.to a shiul square frame as’ described in this column on June 28, 1929. This frame holds the unit securely and also allows its adjustment in any direction, so that tension on the spindle is reduced to the desired minimum. The diagrams show the 66A unit in position, but if the 66K is used, a hole will be made in’ the wooden support, through which the adjusting knob may pass, The same applies also to. the 66P unit, which also has an adjusting knob, and will attach to the support in a slightly different way, owing to the altered shape of the frame: It would be clamped by two strips of wood, one running through the magnet and the other behind the support. The two strips would then be bolted together ut each end with a suitable thin bolt.

The "Linen-Diaphragm." HE linen-diaphragm" speaker was described on July 12 last,-and on October 11 a different method of -construction was described, using silk for the diaphragnis, and suspending the unit instead of fixing it rigidly, This method suits any unit, and simplifies construction. This is the "top.dog" of home-built magnetic speakers, though the two-foot ‘cone is a near approach. The absence of the paper diaphragm seems to impart to the tone just that little extra touch of pleasing quality that we all desire. Volume obtained is limited chiefly by the amount the unit will carry without rattling. The 66P is a new unit now obtainable, working with a fully neutralised balanced armature, and capable of giving’ good volume and quality. reproduc-

Yo combined with sensitivity. With a\ given input, the linen-diaphragm will give rather more volume than will a paper cone. The units will, of. course, work well from a larger amplifier than just mentioned, but whilst not giving the full ‘possible volume, it will be more than ample for any ordinary house, When it becomes necessary to supply dance music to fill a hall, a dynamic cone speaker is the most suitable, as heavy volume is required for dancing, for which an exceptionally heavy unit would be required.in a linendiaphragm speaker. The latter speaker with an ordinary good unit will be found suitable for dancing in the home. Nothing is gained by increasing the dimensions of the linen-diaphragm sygarer over those givén. Increased 8 only means more tension on the diaphragms to get the right effect, and the extra tension may be risky. The speaker as described will reproduce notes well below 50 cycles-as deep as

any pedal organ notes that the writer has encountered on a gramophone record. It is just as well to mention here that the application of. celluloid ‘solution to the linen or silk diaphragm is not intended to result in a celluloid diaphragm in which the fabric is embedded. The dope" is only intended to stiffen the fabric sufficiently to take away the "deadness" which it has when used without stiffening. Three coats of thin solution are ample, and thus ta@ated, the diaphragms will give pleasingly mellow reproduction. Baffle-Board Speakers. ANOTHER successful cone speaker is that in which a cone of small diameter is mounted with a flexible edging or "surround" in a circular hole in the centre of a sheet of heavy threeply or building-board not less than 2ft. square. A cone 9 inches in diameter is suitable, and the hole in the baffle may be 4 to: #in. larger in.diameter. The eone is then connected to the baffle by a one-inch circle of thin chamois leather, or the nearest suitable materia! procurable-thin india-subber sheet is good, but perishes in time; a strip of ‘ thin serge cut "on the bias," that is er diagonally; waterproofed silk is also ‘used. The material must be thin, pliable, and free from "grain." The construction of this type of speaker was described on May 8 last. A one-inch batten is attached to two opposite ends of the baffle; and the unit is supported upon a stout cross-piece joining the two, but standing away from the back of

the baffle to the thickness of the battens. This method of mounting preserves the deep notes and gives them necessary prominence. Free-edge Cone. HIS, the simplest form of conespeaker, is quickly made, and gives good results. A 12-inch circle of heavy paper is cut out, a small segment removed, and the two edges. glued together. A hole is then made in the centre, through which the spindle is passed, one of the felt-covered washers on either side of the cone. When the nuts are tightened up,.the speakef is ready for use if the unit is laid flat on the table. The spindle will hold the cone in place. Suitable means may be devised for hanging the unit on the wall or upon a stand. "It may be attached to. the ceiling with the cone downwards in a horizontal position. This is a handy method of trying out the unit before the construction of a more complete speaker, but does not bring out the full capabilities of a unit. A segment of 25 degrees is sufficient to eut from any cone to get the "dished" effect. This is about 30 degrees when overlapped and glued. One eut runs to the centre, but the second cut runs slightly to one side, so that an overlap for glueing continues to the | centre. See diagram. Material for Cones. QHEETS of paper suitable for cones may be purchased, but very few dealers keep it in stock, and it requires careful packing for postage. Suitable paper, known as "cover paper," may often be obtained from a printer, and if not sufficiently heavy, two sheets may be pasted together. The correct thickness for a large cone would be equal to two thicknesses of the "Listeners’ Guide" cover. Dark browns or similar colours are the most suitable. An important point is to choose a "soft" paper rather than one with a hard fin-ish-judge it by shaking a sheet-it should not have much "rattle." Heavy brown paper is fairly good for the purpose. When sheets are pasteil together they will remain flat if allowed to dry for 24 hours under flat pressure. Drying at a fire or without pressure will result in buckling. A small table turned upsidedown on uw larger one makes "a good press, but a few sheets of newspaper must be used as a pad to take up unevenness. Haye clean paper next to the cone, as the dampness may cause newspaper print to transfer and spoil the appearance. Two or three sheets of newspaper faced up with thin brown or a fancy paper will make good cone material. The main thing to avoid in cone construction is unsound joints, as these are liable to rattle and give annoyance. If the edges are glued together with secotine as directed in the "Guide," no trouble will be experienced. . General. {t might be said that the manufacture of a loudspeaker as outlined above is the simplest of all radio construction work, especially when the great amount of satisfaction to be derived is taken into consideration. With a good unit ready for use, it is only a matter of a little time and careful hand-work to produce an article that will please the eye as well as the ear. (Coneluded. on page 28.)

Cone Speakers (Continued from page 27.)

The units now available are capable é of giving high quality reproduction by any of the methods mentioned, but the volume and exact range of frequencies depends to a certain extent upon the method sele‘t«" and the manner in which constrivi.on is camried out. A little patience wil! conquer many difliculties and ensure success.