The Talking Machine Forum — For All Antique Phonographs & Recordings

Inigo wrote: Wed Nov 09, 2022 5:54 am 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.

Inigo wrote: Wed Nov 09, 2022 9:52 am 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.

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

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

Inigo wrote: Wed Nov 09, 2022 5:54 am 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...

leels1 wrote: Wed Nov 09, 2022 3:34 am I’d be interested to see other people’s experiments.

Marco Gilardetti wrote: Wed Nov 09, 2022 8:21 am

Inigo wrote: Wed Nov 09, 2022 5:54 am 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 wrote: Wed Nov 09, 2022 9:52 am 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).

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?

leels1 wrote: Wed Nov 09, 2022 12:26 pm 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?