ALUMINIUM DIAPHRAGMS.

[Marco Gilardetti]

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.

OK, this makes sense, except for the part concerning the piston movement, as - although I perfectly get your point - this is not what in mechano-acoustics is usually referred as "piston movement".

A driver acting in piston-movement range is supposedly moving back and forth as a single, rigid unit. This happens for example with the cone of a woofer loudspeaker at low frequencies. I think that the shape of the ridges in picture, instead, would give more elasticity to the central part, which when excited by the needlebar would facilitate the creation of waves travelling from the center to the outer edge, rather than having the whole diaphragm moving more or less as a one-piece rigid body.

This is not anything necessarily bad, as confirmed by the fact that a flat diaphragm has been described as "poorly sounding". Also loudspeaker cones act as pistons only at lower frequencies; at higher frequencies, harmonic 2nd and higher order vibrations of the cone - that can have both circular nodal points as well as radial nodal points, that also combine with each other in very complex shapes - have an important role in sound emission, which actually gets of primary importance in wideband or full range speakers. The overall mass of the cone could never be moved as a single rigid unit at the speed needed to efficiently emit frequencies as high as 10 KHz, so the residual flexibility of the cone gains importance as the frequency gets higher.

This said, a cone is an inherently rigid shape by geometry, so it is relatively easy to have it acting like a piston, at least at low frequencies. Not so for a flat aluminium foil, obviously. Should you really want to experiment with a piston-moving diaphragm (perhaps you don't ) my educated guess is that you shoud try to have radial reinforcing ridges at the center, and then a pair (or more) of circular ridges all around the outer edge. I also wonder, just for the sake of curiosity, if you also tested a diaphragm with a flat central area and then ridges around the outer edge (somewhat similar to a flat-driver loudspaker), or with other shapes.

[Inigo]

I've been tempted to try that, but as far as I always have in mind the orthophonic design, I believe it wouldn't sound well. Once I tried a mica diaphragm with a very flexible edge. It was on the Juwell Electro soundbox, a German mimick of HMV no4 but with larger diameter. I stuck a thin rubber sheet (from a toy balloon) to a smaller mica diaphragm, and installed it on the soundbox, so the diaphragm acted as a piston, provided that the edge was so flexible in comparison to the mica glass. It was pretty difficult to get the edge rubber tensioned all around, but eventually I got it.
Guess what ..! It sounded dreadful! No treble at all! Finally I guessed that the edge must better not be so flexible... Besides that, the weight of the soundbox biased badly the piston to one side... There's a mechanical function also herein, for the diaphragm must be springy enough to support the reaction of the weight of soundbox on the needle. Maybe if I had used a counterweight of some sort....
Anyway, I place the HMV 5a in the podium as the winner. I'd like to be able to make something that sounds so well.... Even my Meltrope III sounds a bit dead in comparison, for it doesn't have the crispness of the 5a. In small horns (HMV 127) you don't notice it, compared to the no4 soundbox, but in the 194 it sounds a bit dead and tubby. BTW, after using the Meltrope for some months in the 127, mainly for the strong bass, I've reverted to the no4, which in the end is also a bit better on the treble. Besides that, I felt the Meltrope was too strong for such a small horn. Once I tried there a 5a, but I felt it yet stronger than the Meltrope. Not pleasant at all. The tiny machine seemed at the point of exploding! These strong soundboxes need a horn providing more load (intuitively).

[Marco Gilardetti]

I've been tempted to try that, but as far as I always have in mind the orthophonic design, I believe it wouldn't sound well.

Yes, you're possibly going again through all the trials that ultimately led to the Orthophonic diaphragm design, and re-discovering why they didn't work any better.

Guess what ..! It sounded dreadful! No treble at all!

By mere speculation, it's somewhat what I expected. The thick central part would kill higher-order vibrations of upper frequencies, while the more pronunced piston movement would perhaps deliver more energy to the bass, but the bass is exactly what goes missed first in a horn of insufficient size. So basically, for one reason or another, the entire sound spectrum happened to be restricted.

These strong soundboxes need a horn providing more load (intuitively).

This also mekes sense: there's nothing worse than a driver insufficiently loaded by an inappropriate horn.

[leels1]

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...

So much better explained than I did! I guess that's why you're the engineer . So in your opinion, is there a tangible difference between the design of the edges on a 5a vs a 5b... and if not, why might they have change the design?

Also, if EMG are considered the "pinnacle" of sound reproduction, I wonder why they kept a relatively simple diaphragm design compared to the orthophonic? The 5a/b design is supposed to be balanced with with a re-entrant horn, but they also used them on exponential horned machines which theoretically has no extra benefit?

[mrrgstuff]

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

I've had a go at making aluminium diaphragms:

https://youtu.be/mK1bGhanqf4

as part of experimenting with 3D printed soundboxes

I've also tried flat metal:

https://youtu.be/aMrX8tn-tYI

And various types of plastic

https://youtu.be/XqAlJVpN3ZM

Very interesting topic of experimentation and great to see lots of other people investigating it too!

[anchorman]

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.

OK, this makes sense, except for the part concerning the piston movement, as - although I perfectly get your point - this is not what in mechano-acoustics is usually referred as "piston movement".

A driver acting in piston-movement range is supposedly moving back and forth as a single, rigid unit. This happens for example with the cone of a woofer loudspeaker at low frequencies. I think that the shape of the ridges in picture, instead, would give more elasticity to the central part, which when excited by the needlebar would facilitate the creation of waves travelling from the center to the outer edge, rather than having the whole diaphragm moving more or less as a one-piece rigid body.

This is not anything necessarily bad, as confirmed by the fact that a flat diaphragm has been described as "poorly sounding". Also loudspeaker cones act as pistons only at lower frequencies; at higher frequencies, harmonic 2nd and higher order vibrations of the cone - that can have both circular nodal points as well as radial nodal points, that also combine with each other in very complex shapes - have an important role in sound emission, which actually gets of primary importance in wideband or full range speakers. The overall mass of the cone could never be moved as a single rigid unit at the speed needed to efficiently emit frequencies as high as 10 KHz, so the residual flexibility of the cone gains importance as the frequency gets higher.

This said, a cone is an inherently rigid shape by geometry, so it is relatively easy to have it acting like a piston, at least at low frequencies. Not so for a flat aluminium foil, obviously. Should you really want to experiment with a piston-moving diaphragm (perhaps you don't ) my educated guess is that you shoud try to have radial reinforcing ridges at the center, and then a pair (or more) of circular ridges all around the outer edge. I also wonder, just for the sake of curiosity, if you also tested a diaphragm with a flat central area and then ridges around the outer edge (somewhat similar to a flat-driver loudspaker), or with other shapes.

I think the point of the rings towards the center is to encourage the high frequency reproduction. the lighter mass of the center of the diaphragm is maybe then able to move more freely than if it were stiff all the way across. More weight of the diaphragm and needle bar assembly is going to impede high frequency reproduction. I'm kind of baffled that steel was the material of choice for the needle bar, and hope to experiment with lighter materials at some point.

[anchorman]

I've been tempted to try that, but as far as I always have in mind the orthophonic design, I believe it wouldn't sound well. Once I tried a mica diaphragm with a very flexible edge. It was on the Juwell Electro soundbox, a German mimick of HMV no4 but with larger diameter. I stuck a thin rubber sheet (from a toy balloon) to a smaller mica diaphragm, and installed it on the soundbox, so the diaphragm acted as a piston, provided that the edge was so flexible in comparison to the mica glass. It was pretty difficult to get the edge rubber tensioned all around, but eventually I got it.
Guess what ..! It sounded dreadful! No treble at all! Finally I guessed that the edge must better not be so flexible... Besides that, the weight of the soundbox biased badly the piston to one side... There's a mechanical function also herein, for the diaphragm must be springy enough to support the reaction of the weight of soundbox on the needle. Maybe if I had used a counterweight of some sort....
Anyway, I place the HMV 5a in the podium as the winner. I'd like to be able to make something that sounds so well.... Even my Meltrope III sounds a bit dead in comparison, for it doesn't have the crispness of the 5a. In small horns (HMV 127) you don't notice it, compared to the no4 soundbox, but in the 194 it sounds a bit dead and tubby. BTW, after using the Meltrope for some months in the 127, mainly for the strong bass, I've reverted to the no4, which in the end is also a bit better on the treble. Besides that, I felt the Meltrope was too strong for such a small horn. Once I tried there a 5a, but I felt it yet stronger than the Meltrope. Not pleasant at all. The tiny machine seemed at the point of exploding! These strong soundboxes need a horn providing more load (intuitively).

if you make the diaphragm suspension too flexible compared to the mass of the diaphragm and needle bar, you'll end up with a situation where it does not have enough damping and subsequently too much ringing after an impulse.. amongst other problems. perhaps this would be better with a larger horn that more efficiently and broadly loaded the air column against which the diaphragm is pushing?

[anchorman]

P.s. I still want to see the overstall tooling!!!

[Inigo]

in your opinion, is there a tangible difference between the design of the edges on a 5a vs a 5b... and if not, why might they have change the design?

In my opinion, the change between 5a and 5b was not drastic. In the diaphragm I've only noticed the different edge corrugations. The triangles in the 5b provide the same effect, but seem more easy to die-press than the tiny corrugations of the 5a, so maybe the change was only for technical production matters. I've always tempted to think that the 5a is more flexible, but this could be a biased opinion due to the fact that my 5a diaphragms were old, while my 5bs were brand new. I never noticed any change in sound between both.
Apart from that, the only other difference I've found is that the metal front cover of the 5b has four support points beside the screws (acting as tiny legs) that the 5a didn't have. I suppose they are made to improve the cover support when you screw it thoroughly. The old design tended to bend it down when screwed tight.

[Marco Gilardetti]

Also, if EMG are considered the "pinnacle" of sound reproduction, I wonder why they kept a relatively simple diaphragm design compared to the orthophonic? The 5a/b design is supposed to be balanced with with a re-entrant horn, but they also used them on exponential horned machines which theoretically has no extra benefit?

I believe that the EMG mostly relied on no-compromise metals and an excellent design of the needlebar pivot. I never had the chance to put my hands on an EMG as they are so scarce, but gramophone enthusiasts that own both an EMG and a 5a/5b and that I look on as honest listeners, won't swear on either of the two, but basically report that any of the two sounds slightly better than the other depending on the gramophone on which it is installed.

Coming to your second point, I'm not really sure that I get your question. Re-entrant horns also happen to have an exponential profile: there is no difference form this point of view, and I don't think that 5a/5bs were specifically designed with re-entrant horns in mind, also because re-entrants don't have any specific parameter or figure that would tell them apart from their straight horn equivalent when seen from the throat of the duct. Re-entrants were basically designed to save space and fit a huge and long horn in a comparatively small cabinet, once it became clear that horns happen to be surprisingly tolerant (I would rather say almost unbelievably tolerant) to sharp bends, splits, quadrisections, and re-joins. However, the 5a/5bs were also widely deployed, obviously with the same success, on many "normal" folded horns, on portables like the highly regarded 102 (which in turn is equipped with a folded but not re-entrant horn), and so on.