HMV No:4 Soundbox question.

[Inigo]

Clear, thanks. Logic results, because not only the diaphragm material and size is involved in the sound, also the details and thickness of the air chamber behind it plays a role. The air chamber in the no4 behind that corrugated diaphragm has a strange shape.
No wonder the Victor orthophonic and HMV 5a/5b are among the best sounding soundboxes when properly restored. They have several refinements, among them the air chamber of thin thickness all constant across the diaphragm area, the phase plug, etc. that altogether make them (arguably) the best soundboxes ever made. Their sound is so well balanced and the range so wide...
I've tested the Meltrope III in several machines against the HMV 5a, and, frankly, I find the HMV much better. There Meltrope has more bass but lack of treble, making it in the end a bit bumpy, and not so well balanced as the HMV. Again, there's a notorious difference among them two in the shape and size of the air chamber behind the diaphragm... and the Meltrope has no spider, an essential element in making the frequency response wider and well balanced...

[leels1]

That’s a good point re the phase plug in the 5 types. I guess that’s the three pronged addition screwed to the backplate? I wonder what it’d sound like why out that and what sort of difference it’d make. Have you seen the aluminium backplates that John Sleep has? They are missing this, and also don’t have a rubber connector to the tone arm so perhaps that would alter the sound somewhat?

[CarlosV]

That’s a good point re the phase plug in the 5 types. I guess that’s the three pronged addition screwed to the backplate? I wonder what it’d sound like why out that and what sort of difference it’d make. Have you seen the aluminium backplates that John Sleep has? They are missing this, and also don’t have a rubber connector to the tone arm so perhaps that would alter the sound somewhat?

The rubber connection is essential to the good operation and to the sound of the soundbox. It attenuates the mechanical load on the record and damps acoustic feedback from the arm. Its contribution to the sound is quite evident when you replace an old hardened rubber back with a new and flexible one.

[Inigo]

I agree on all that.
The three-leg thing with the bullet shaped prong that goes inside the soundbox outlet or neck is the 'phase plug'. The three legs are only the means to support the central bullet, which is the real core of the contraption. This helps channeling the air currents coming from all around the back of the diaphragm into one single current inside the neck. Of course the flow of air is turbulent, goes back and forth and so on... so the conclusion on the benefits of the presence of the phase plug must have been 50% theory... and 50% trial and error! But examining the shape of the neck or outlet that goes from the air chamber to the tonearm, the outer part has a soft curvature, and the inner part has the bullet, which makes it all a conduit in this shape, in cross-section:

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