A Linen-Diaphragm Loud-Speaker

Radio Record, Volume II, Issue 52, 12 July 1929, Page 36

A Linen-Diaphragm Loud-Speaker

Covers an Excellent Range

HIS speaker has proved very popular among amateur constructors — so much so that our stocks of the number carrying the description have long since been exhausted. As still more inquiries have come in, we have complied with request and repeated the description with @ few minor alterations. This is a type of loudspeaker called the balanced-tension type of reproducer. ‘There are several patterns. One pattern has both frames of the fame size with linen diaphragms, another. has the back diaphragm much smaller, and employs skin in place of linen, and so on. The employment of skin or parchment obviates the stiffening of the linen by impregnating" it with varnish, celluloid solution, or else with four or five coats of thin collodion or Duco. All patterns alixe give very fine reproduction, including the lower notes that ars mostly .suppressed by the average small horn speaker. Construction is simple, and if the instructions are carefully followed, a loudspeaker of which the constructor may be proud will result. Construction of Frame. ‘THE first operation is the assembly of the wooden frame upon which

the linen diaphragms are, stretched. The back view of the frame, Fig. °, sitows how the various pieces of rimu or other suitable wood are -cut and mortised together. This construction should be followed, for if the parts _ of the frame are not securely fastened by glueing, a rattle will be introduced in the speaker that cannot be elimin‘ated without a great amount -of trouble. After the pieces A and B have been glued, the corner pieces E gre attached. The pieces marked C and D are next joined and screwed to the sides of B, making sure that the pieces D are centred. Cae a a eceammanatiammnemmtetenhentensulesinnnitiantionnasismnsnsnnslenmimmsensnthetaarantateisians

While the points of the wooden frame are drying, the two diaphragms of high-grade linen are prepared. It is necessary that a hem one half inch wide be sewed along each side of both the large and the small squares; the one being 26 inches equare and the other 8 inches. When the joints are thoroughly dried, the large square of cloth is placed ove: the front of the frame,

tacking.dowr one edge; be careful to place the tacks fairly close to one another, so that there will be little danger of the cloth’s pulling out. When one edge has been fastened stretch the linen as tightly as possible and tack down the opposite side. This process is repeated for the other two sides. The 8-inch-square of linen is fastened to the rear frame in the same manner, ‘ Preparing the Diaphragm. OW locate the exact centres of both diaphragms and carefully, with the point of a compass or a sharp nail,

force a hole in the linen. Be careful not to break any threads, but spread them apart until the hole is }-inch in diameter. ‘Then prevail upon one of the ladies of the family to work a buttonhole stitch around these two holes. The next operation should be. performed either outdoors or in a room with the windows open, as otherwise the. fumes from the collodion are liable to cause an unpleasant sensation. Paint the face of each diaphragm with the thin collodion and allow it to dry. Four or five coats are required; let each coat dry before applying the next one. When the last coat is dried the diaphragms will be stiff and slightly flexible, and when tapped with the finger will sound like a drum. An alternative method of treatment to the above is to impregnate the diaphragms with celluloid dissolved in -amyl-acetate, or with a suitable varnish. The small coupling bolt is next prepared. This is a 8/32 bolt, tin. in length, through which is drilled lengthwise a small hole, just large enough to take the driving-rod of the loudspeaker unit which is to be used. One of the washers is put over the bolt, and the hea¢ with the washer is put through the hole in the large diaphragm, from the front. The two diaphragms are forced together until the bolt can be slipped through the hole in the smaller square, after which the other washer and the nut are put on and tightened down. The edges of all metal washers coming in contact with

the diaphragm should be rounded off so that they will not cuv the fabric. They can also use silk for the diaphragm. The mounting: of the unit itself is left to the ingenuity of the constructor. The method employed with good results by the writer can be seen in the accompanying illustrations. It is important to remember that the unit must be so lined-up that the driving-pin will come exactly in line with the hole in the bolt. ° The finished speaker may be placed in a cabinet or hung from the ceiling. If it is desired to colour the linen diaphragms this must be done before

treating them with the collodion. The tacks should be covered over with an attractive passe-partout binder for appearances sake. The material needed for constructing this speaker is as follows :- 4 pieces of rimu, 24 x 2 x 1} inches CA, B"). : 2 pieces of rimu, 24 x 14 x 1 inch ("0"),

2 pieces of rimu, 74 x 14 x 1 inch ("D"), 4 triangular pieces rimu, 14 x 14 inches ("BH"), 2 squares of medium-weight linen, one 26 x 26 inches and the other 8 x 8 inches. . 4 13-inch woodscrews ("F"’’). 2 4-inch washers ("G’).

A #-inch 8/32 brass screw and nut ("H’). A package of No. 4 cut tacks: A roll of passe-partout binder. 100z. collodion (obtainable at chemists) or varnish. A balanced-armature Joudspeaker unit with driving rod ("I’’). Proof against Overloading. 2 From the foregoing description ‘it becomes obvious that the weight. or tension of the vibrating diaphragms does not act as a constant load on the driving unit. Therefore, although it might be imagined "hat the highly-taut diaphragms require a great deal of

power to actuate them, even the slightnt movement of the drive pin is transmitted without loss to them; as a result the speaker is unusually sensitive, and operates with. great volume even on moderately strong signals. The new speaker is capable of hand_ling far more power than is required for a ardinary radio purposes. In

one test, for example, it was used in eonjunction with a two-stage trans-former-coupled audio amplifier employing two 210-type power tubes in the last stage. A strong local station was tuned in, and a direct comparison was made between this speaker and another of the coné type, of probably the best design on the market to-day. ¢ The latter instrument started to rattle when a certain volume level was reached ; whereas this linen diaphragm

speaker continued to operate without up to the limit o" the amplifier. Volume with Quality. HE performance ot this new loudspeaker, as judged by the ear alone, is remarkable for its brilliancy and faithfulness over the whole range of musical notes transmitted by a high- -------

class broadcast station, Violin music comes through high and clear, .without appreciable flattening of the high notes; while the sounds of drums and the low tones of a piano boom forth with clarity and realism. The unexpected responsiveness of the speaker to the low notes is somewhat disconcerting at first to a hearer unaccustomed to such reproduction; but in a room of favourable acoustic properties (ie, one furnished: generously with ugs, thick curtains, and similar eeho‘illing materials) the "booming" ef‘vets quickly lose their unpleasantness. The loudspeaker may be mounted in . cabinet with its front covered in apestry. Its performance . depends largely on the use of suitable material for the stretched diaphragms, and on the mounting of these in such 4a way that their tension is correct and will remain so. Some constructors are mounting the small frame with a screwed rod in each corner, so that in order to tighten both diaphragms it is‘ only necessary to tighten a nut at each corner, then adjusting the nuts on the driving spindle. A diagram appears showing the response eurve for the linen diaphragm, and in comparison with the average loudspeaker it is exceptionally good, as

variations in proportional output between about 50 and 10,000 cycles are hardly sufficient to be perceptible to the ear. Resonances which seem to be due to the fact that the natural time periods of iudividual membranes are incompletely suppressed are not so marked as to be in any way objectionable.