ALUMINIUM DIAPHRAGMS.

[chunnybh]

Hello All,
I’ve been experimenting with different diaphragms for EMG and Expert soundboxes.
As usual there are several varieties. The profiles and the stylus bars vary. No set rules .
My observations:
Most EMG diaphragms have double rings, the center is flat with the outer edge of the diaphragm. The stylus bars are a basic L shape.

The Expert diaphragms have two rings but one is concave and the other convex again the center is flat with the outer edge of the diaphragm.

Some Expert diaphragms have one ring and a domed center. Here the stylus bar is hooked like a question mark so that once installed the bar is parallel with the body. I have seen several of these diaphragms fitted to L shaped bars causing the whole bar to be at an angle with the body so the needle enters the groove at an angle. I presume these are replacement diaphragms.

Stylus bars again vary. The L, hooked and the elongated L which looks like an L but the L has been bent out almost as if the bar was made too short and had to be straightened a little for it to reach the domed center of the diaphragm.

Perhaps all these varying designs were used for playing different types of records. Is this what they meant by “tuning”.

I was very lucky to have been loaned the Overstall templates for “spinning” diaphragms. There are two designs. One is the center domed variety and the other is an odd one with two very narrow rings and a dome in the center. I have been using them but unfortunately they are both damaged. They are scarred with dinks and where the diaphragms diameter has been cut there are spirals cut into the brass formers.
It’s relatively easy to spin the diaphragms but releasing them and cutting the diameter is the main problem. I can see why they are damaged. Out of 10 that I spun I managed to cut only one neatly. The rest were mushed or the cutting blade caught the edge and ruined them.
I used car body filler to fill all the tiny scars and dinks. After smoothing I managed to form perfect diaphragms but them again when trying to cut them out, they ended up a mess. I even hand spun the wheel when cutting them out. Better results but not perfect. Before long I had cut into the car body filler and everything turned into a mess again.
I’ve formed a few and then thought of trying to cut them out. I used a paper hole cutter with a sharp blade and one with a wheel blade, both left a tiny crease on the edge.

As for the aluminum rolls used. I found the material too soft. The two that I have managed to make and use sound very disappointing. I wonder if there is a way of annealing the diaphragms once formed.
Any advice appreciated.

[Orchorsol]

Fascinating Chunny, many thanks. Well done for taking this important work forward. Hopefully others will have insights and suggestions as to the way forward.

[emgcr]

Good work Chunny---it is the start of a long and time-consuming road I fear !

I do not think annealing after the event would be successful as distortion would then certainly be a part of a new problem. Aluminium or Dural sheets are available in varying states of anneal/alloy etc and I think experimentation with these might be the way to go.

Regarding "parting off", I think I might be tempted to make a very precise high quality steel cylinder with hardened edges which could be accurately mounted in the tailstock and then advanced immediately over the outer diameter of the former, acting much like a guillotine.

[edisonphonoworks]

This is an experiment, this Victrola No. 2 has a five one thousandth inch glass diaphragm. I use glass in my recorders, and automatic Phonograph reproducer.
http://youtu.be/h4j3K_HQdxA

[chunnybh]

Graham, Yes the guillotine idea would work if the aluminum was not folded and clamped over the outside of the former. The fact that it is means that at least a few millimeters need to be removed from the outer edge of the diaphragm. But that has given me the idea of cutting another sharp ring at the diameter and then using a cylinder punch to cut it out.
Anyway, its back to the start as I do not want to alter these formers. Besides they can only produce diaphragms for hooked or bent L shaped stylus bars.
Time ( I could do with a few more hours in a day) to make some new formers

Glass diaphragms. I have tried these on cylinder reproducers and also on an HMV 4 soundbox. They certainly brighten up the reproduction but also amplify the crackle and surface noise. The bass I found was sadly diminished. A very tinny sound.

[anchorman]

Any chance anyone has copies of the pictures for mr. Overstall’s tools? This is on my to do list, to try spinning some diaphragms on the lathe.

I’ve tried coke can material, and it works ok. I didn’t have a decent machine at the time, so was limited in my ability to really work for the best sound from it. The issue with that material is that it is covered with some sort of plastic resin on the inside. Or maybe it’s an advantage?

Wyatt Markus had some dies produced to make victor orthophonic diaphragms, these have now been given or sold to someone else who is making diaphragms still. These are stamped, rather than spun. I’m not sure what state the metal was in, but it was a duralumin alloy.

[leels1]

Coke can material is very good. I’ve measured several diaphragms from Meltropes, to Songster, Columbia etc and they seem to all be in the region of 0.08 to 0.12mm thick, but on average around 0.11mm.

The aluminium of the coke can is too springy - as is some sheet alu I got so it needs to be annealed to remove the springiness I’ve found. In the process any plastic film is burnt away, but you have to be careful to do it just enough so as to not melt the alu, it’s very thin and easy to overheat.

Interestingly the EMG soundbox I have is a very early one and the edges of the diaphragm are rough cut, it doesn’t seem to affect the sound produced though. You could try a circle cutter first (allowing for shrinkage when the ridges are put in)

I used a simple method to add the ridges. Purchased a set of hole saws and drilled into some mild steel bar at varying diameters, traced the ridges out using an old bone tailors awl (used for pulling stitches out) and then put the centre hole in.

If you look at existing diaphragms it’s important a piston action is maintained, which I think after experimenting is dependant on how the ridges are placed. The centre must be quite rigid (which the ridges provide) in order for the piston action to work- think loudspeaker and the ridges on that allowing the vibrations.

It’s good fun experimenting, here’s an example of one I did which sounds quite good- note the deeper recess where the needle bar attaches.

There’s a guy in Japan on the UK gramophone Facebook group who produced an excellent reproduction Columbia diaphragm that sounded great. He used the lathe method.

I’d be interested to see other people’s experiments.

[Marco Gilardetti]

If you look at existing diaphragms it’s important a piston action is maintained, which I think after experimenting is dependant on how the ridges are placed. The centre must be quite rigid (which the ridges provide) in order for the piston action to work- think loudspeaker and the ridges on that allowing the vibrations.

I'm not sure I get this point. In loudspeakers, the ridges are placed exactly at the opposite side (exterior of the cone) compared to the diaphragm that I see in picture, but conversely the "motor" of the "piston" acts in the center in both cases. Also, I would think that the ridges add elasticity to the foil, not stiffness.

In any case: excellent work, that diaphragm looks stunning, and it's to some degree reassuring that it can be made with cheap recycled metal!

[leels1]

I probably didn’t explain that too well.

I tried one without the ridges, which sounded bad. I think the ridges stop any “sideways” movement of the diaphragm as it’s working and allow the centre part to move uniformly to give that piston action.

Best way I can explain, is if you take a piece of alu and bend it, it’ll bend easily. If you add the ridges and then bend along the ridges it’s a lot harder to bend, so I guess it adds front to back elasticity but less side to side movement. Hope that kind of made sense, I’m no metallurgist!!

[Inigo]

Yes (structural engineer herein), the ridges provide stiffness in the circumferential direction (tangent) and flexibility in the radial direction. These ridges in the photo makes it easier for the center of the circumvented part, to do back-forth movement, like a piston.
Other diaphragms have radial ridges, which do exactly the opposite: they make the radiated part stiffer. Usually these go accompanied by circumferential ridges near the edge of the diaphragm, so intended all it to make the whole diaphragm move as a piston. In my opinion these aren't too effective, for the radial ridges provide tangential flexibility, and do the diaphragm moved back and forth like a flexible cone in toto...
But the orthophonic diaphragm is different, and to my understanding, a much better and efficient approach. It's a marvel of engineering. The central dome is completely stiff. It is surrounded by two big circular ridges, one of them attached to the spider, and receiving the movement at his peak circumference. So the dome and ridged areas move easily back and forth as a piston, but bending somewhat at the intermediate ridges. The tiny edge creases at the edge are diagonal, somewhat tangential, and make this part much more flexible on the radial direction, to ease more the piston action of the whole ridged and central area. They work all together, and what matters here is the stiffness of each part compared to the surrounding areas.
Besides that, the mission of the big ridges I think is double. The back of the soundbox replicates the shape of the diaphragm, so we have a very wide and thin air chamber, with zig-zag radial shape, behind the diaphragm, which pumps air into the central outlet very effectively. It produces very strong pressure waves! So this shape also makes the chamber larger in the radial direction than a straight plane diaphragm. It's like an air chamber folded or squeezed in zig-zag form, made to fit into a smaller space; if we made it straight, it would result larger than the soundbox diameter. Maxwell and Harrison were real geniuses! I don't know how many hours of study and how many experiments must have they done to arrive at such a sophisticated design...