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The following article came from an old scrapbook, circa 1930...
Build a Linen Diaphragm Speaker
by John M. Thompson
     Back in the primitive days of radio, when it was a feat worthy of strong men and true to get a station broadcasting music fifty miles distant, loud speakers were something one read about, but seldom saw. If a newcomer wanted to listen to a program, one of the headphones was detached from the headpiece and given him and then, in a very uncomfortable attitude, each sharer of the phone stopped one of his ears with his finger and tried to follow the radio entertainment with the other.
     Then came a day when someone suggested that, if a phone were placed diaphragm-down near the bottom of a bowl, music could be heard all over the room--well, heard nearly all over the room. The truth of the matter is that, as most of us were lucky to have one tube for a detector, and audio amplification was something yet to be dreamt of, it was necessary to place an ear very close to the bowl in order to hear anything at all.
     Then came the simple horns to which a headset could be clamped; then the first electrodynamic speakers, using a horn and requiring lots of power to operate; then the electromagnetic horn speakers and cones, the newer types of electrodynamic cones and, lately, the linen-diaphragm speaker.

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     This type of speaker, an example of which was described in the March issue of RADIO NEWS, has become popular over-night with experimenters, for it is something fairly simple in construction. The frequency-range and tone quality obtainable from this speaker is remarkable and, if the constructor follows the directions presented herewith, he should have a loud speaker of which he can be proud. The total cost of that built by the writer, according to the specifications below, was under $10.
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 (below) shows how the various pieces of cypress 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 gluing, a rattle will be introduced in the speaker that cannot be eliminated without a great amount of trouble. After the pieces A and B have been glued, the corner pieces E are attached. The pieces marked C and D are next joined and screwed to the sides B, making sure that the pieces D are centered.

     While the joints of the wooden frame are drying, the two linen diaphragms are prepared. It is necessary that a hem one-half inch wide be sewed along each side of both the large and small squares; the one being 26 inches square and the other 8 inches.
     When the joints are thoroughly dried, the large square of cloth is placed over the front of the frame, tacking down one edge; be careful to place the tacks fairly close to one another, so that there will be little danger of the cloth 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
     Now locate the exact centers 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 ¼ inch in diameter. Then prevail upon one of the ladies of the family to work a button-hole 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 dry, the diaphragms will be stiff and slightly flexible and, when tapped with the finger, will sound like a drum.
     The small coupling bolt is next prepared. This is an 8/32 bolt, ½-inch in length, through which is drilled lengthwise a small hole, just large enough to take the driving rod of the loud-speaker unit which is to be used. One of the washers is put over the bolt and the head 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 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 color 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 cypress, 24 x 2 x 1½ inches ("A, B");
2 pieces of cypress, 24 x 1½ x 1 inch ("C");
2 pieces of cypress, 7½ x 1½ x 1 inch ("D");
4 triangular pieces cypress, 1½ x 1½ inches ("E");
2 squares of medium-weight linen, one 26 x 26 inches, and the other 8 x 8 inches;
4 1¾-inch wood screws ("F");
2 ½-inch washers ("G");
A ½-inch 8/32 brass screw and nut ("H");
A package of No. 4 cut tacks;
A roll of passe-partout binder;
10 oz. collodion (Obtainable at drug store) or varnish;
A balanced-armature loud-speaker unit with driving rod ("I").
Note: varnish or dope could replace the "collodion" mentioned in the article. Dope is used when covering radio-controlled aircraft models (check out a hobby store). The passe-partout mentioned in the parts list is a colored tape used to hide the tacks. Disclaimer: Working with any chemicals or electrical devices (especially old ones) can be dangerous, and mistakes can be fatal. If you decide to work with such things, it is solely your responsibility to work safely and to know what you're doing. -DJA