The Exponential Horn

Radio Record, Volume II, Issue 26, 11 January 1929, Page 25

The Exponential Horn

A Successful Adaptation

CORRESPONDENT, "Reproduction," writes: Being interested in pure reproduction from my radio set, I constructed about fifteen months ago the six-foot exponential horn described in ===! the "Radio News" by Mr. and which was recently published in t*e "Record." I found the reproduction from this horn with a good unit surpassed anything I had previously heard at that

time. About nine months later a friend of mine installed a Ferranti pushpull power amplifier with two Mullard P.M. 256’s in the last stage. and the production from this with the 6ft. horn is the best I have ever heard outside a dynamic speaker, and is better than some makes of these which I have heard.

The only fault I had to find with the 6ft. horn was its bulkiness, so I tried to devise a method of reducing and still keep the tonal properties. The photo I enclose shows how I effected this. The tota] height of the speaker is 42 inches, the width 20 inches, aud the depth 22 inches.

= Constructional Details. T is made from a sheet of wallboard 10ft. x 4 ft. This is divided into two pieces, 6ft. x 4ft. and 4ft. x 4ft. respectively. The 4ft. x 4ft. was subdivided into two pieces, 2ft. x 4ft. On one of these pieces a line was drawn

6ft. long, representing the centre of the curved side. This was divided up into 6 1ft. sections. The diameter of the horn was then scribed, using the same meusurements as published in the "IKecord" for the straight horn. This was then cut out with a keyhole saw and used as a template for the other side. which was then cut out. | The 6ft. x 4ft. piece was divided us | in "Radio Record" for straight horn, © thus giving four pieces about 6ft. long — and 20 inches wide, in the wide end. | The length of the outside of the | curved side was then measured and found to be about 7ft. 4in. .This was divided into six parts of 14in. each. A centre line was drawn on one of the pieces 6ft. long, and divided into five sections of 14in. each. The diameter was then scribed with the same measurment as for the straight horn, ouly , using the 1+in. sections instead of the 12in. ditto. To these diams. are added the thickness of the insides. From one of the other six-foot pieces a piece was scribed 14in. long to follow on from the other to complete the outside. These were then cut out with a keyhole saw. The length of the inside of the curved side was then measured and found to be 5ft. 8in. On one of the remaining 6ft. pieces a centre line 5ft. 3in. was drawn and divided into six equal parts of 10}in. Using the same diameters. the inside was marked out, allowing the thickness of the board at both edges so as to keep the inside measurement correct. Finishing the Speaker. THE assembly was then started. The inside was put on in sections and fastened with lin. brads. The whole was then glued from the inside and given several coats of shellac. The outside was then coated witb- shellac op the inner side, and fastened in the same manner, This was put on in

three sections. All cracks were then filled with glue from the outside, and after the horn was completely airtight the whole was given three coats of shellac all over. A piece of conduit was then gtued in the small end, and the unit connected with a rubber connection. As I intend to install the

speaker in a console I left it in this state, but if it is not desired to place it in a cabinet, it may be papered to match the wallpaper of the room. I know of two other horns which have been built from these measurements by a friend of mine, and they are giving every satisfaction. The tonal properties are identical with the straight horn, and has the advantage of taking up far less space.