From Miller book: effect of clamping force on diaphragms

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Schmaltz
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From Miller book: effect of clamping force on diaphragms

Post by Schmaltz »

I'm treading cautiously here since the book I've been reading, "The Science of Musical Sounds" by Dayton Clarence Miller (first published in 1916, with this second edition following in 1922), is fully on-line at Google Books and its status as a public domain work is uncertain. Nonetheless, it's a wonderful book with some great nitty-gritty info about acoustic recording and playback.

There was some discussion on one of these forums, probably by Greg Bogantz, about the effects of gasket tightness on the resonance qualities of a diaphragm. The graph below, from the Miller book (figure 118), shows the effect of loose versus tight gaskets on the natural resonance of a diaphragm.

Image

The graphs to pay attention to are "c" and "b," the lower and middle: the diaphragm being tested was glass, 31 millimeters in diameter, and loosely held in the gaskets in "c," the lowest of the three frequency response curves. The gaskets were tightened (how much is not said) and the measurements taken again for the middle graph "b." Personally I was surprised by this one: not only was the amplitude reduced, as I'd expected, but the natural resonance peak of the diaphragm actually shifted higher; something I just didn't think about.

The top-most graph ("a") is the same tightness on the diaphragm but the resonating instrument (a set of pipes) further away from the horn. From the looks of that, it appears that better fidelity resulted since the natural resonance peak of the diaphragm didn't color the sound so much.

I can post more on this topic, as this book is full of goodies like this, but the readership here is probably better served by going to Google Books and looking through the original book. Lots of good stuff in there.
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Re: From Miller book: effect of clamping force on diaphragms

Post by WDC »

Thank you for pointing out this book, a very interesting test. I makes perfectly sense to what I have experienced with adjusting Edison reproducers. I generally tighten the diaphragm just as little as necessary to avoid some bad resonances or vibration. These schemes actually proof it that a less tightened diaphragm has more bass response.

richardh

Re: From Miller book: effect of clamping force on diaphragms

Post by richardh »

This is interesting...I have 2 diamond B edison reproducers and they sound very diffiernt. One has a very pleasing deep sound with no shrill tones and the other is just the opposite with no bass at all! I need to look at the latter...when I can get it out of the carraige (and no Zeppy - I wont be cutting it in half! :D )

RJ 8-)

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Re: From Miller book: effect of clamping force on diaphragms

Post by gregbogantz »

That's correct - bass response is improved by using less clamping force on the diaphragm ring. As you can see from these graphs, it's mostly because the diaphragm resonance is lower in frequency than when the clamping force is increased. Also, less clamping force will improve the damping of the diaphragm which reduces the peakiness of the resonance, provided that elastic gaskets are being used. I always recommend that users experiment with the clamping force when fitting my diaphragms to their reproducers because a fairly minor change in the clamping can cause a pretty noticeable change in the sound. A change as little as 1/8th turn of the clamping ring can make an audible change in the sound when you are close to the optimum operating point.
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Re: From Miller book: effect of clamping force on diaphragms

Post by saxymojo »

Hi
So how tight should it be? would you wind the locking ring down to touch the gaskets then add a ¼ turn? I have always tried not to clamp them down too hard, but I have no scientific method.
Regards Marcel

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Re: From Miller book: effect of clamping force on diaphragms

Post by gregbogantz »

The tightness of the diaphragm clamping ring used on Edison reproducers should be determined mostly by personal taste. In the rebuilding instructions that I furnish with the diaphragms that I sell, I offer this suggested method: Start by making the clamp ring just snug. Test for airtightness by gently blowing into the output tube of the reproducer. Note that the diaphragm is flexing. Adjust the ring tightness just until the air leak is sealed. This is the starting point which will yield the best bass response. Test the reproducer and listen for blasting or rattling. If any rattling is heard, the clamp should be tightened a little and retried. The tighter you make the clamp, the more the resonance frequency is raised and the less the damping becomes. The increased resonance and decreased damping will produce a noticeable shift in the peakiness of the sound with some loss of bass. Too much clamp pressure will result in shrill sound, probably accompanied by blasting (mistracking distortion). You will find that very small changes in the rotaton of the clamp ring will produce noticeable changes in the sound. Make your adjustments in increments of about 1/16 turn (about 20 degrees) of the clamp ring. I find that the optimum setting is usually just a little tighter than the minimum required to seal the air leak. I have found that this adjustment is pretty stable over time, and I usually don't have to secure it. But if you are concerned that the clamp ring might easily get bumped out of position, you can add a dab of finger nail polish to one or two small areas at the threads to secure the ring to the body. Nail polish is good for this application because it is easily seen upon later inspection and easily removed later with a knife or a scribe if you want to do another rebuild or change the setting.
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Re: From Miller book: effect of clamping force on diaphragms

Post by larryh »

Bob,

Yes that would verify what I had thought also when I was testing my diaphragms. It seemed to me that the advice so common to tighten the ring down really tight was not the yielding the best sound quality. I often mentioned when I was sending out those test diaphragms that just turning it down till it started to feel snug was usually plenty. I found too that a loss of quality was evident with a tight ring as opposed to a looser one. I think here having a very respondent gasket is also a must. Too hard and it would allow a rattle or buzz unless fairly tight. I wonder though, what little I understand about the Orthophonic types, aren't they just sort of pressed in between two plates? I am not sure on that one as I never understood how to disassemble that one.

Larry

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Re: From Miller book: effect of clamping force on diaphragms

Post by Inigo »

Investigating the forum I found this very old and interesting thread.
I second completely the ideas and the advice of Schmaltz, gregbogantz, etc.
My own experiments with hmv no4 (big mica soundbox) have yielded magnific results with this technique, which in the other hand, is quite intuitive once you discover the physics of these soundboxes. Now all my no4s, 4 in all, are adjusted this way, and the sound is greatly improved over the standard setting. I have to say that this standard setting... today doesn't exist actually, for the gaskets, mica, and the soundboxes little differences, make one sound different from another, even adjusted the same way.
This tuning explained by our wise men I complete adding other features that still render it better.
One thing is to abandon the factory style of gasket. The no4 gasket is a ring of red rubber, plain in the outside, but formed in the inside to receive the diaphragm. The inner part is curved in cross-section, shaped like twin cylinders one against the other, so when installing the mica in between, the gasket touches the diaphragm only in one circle. But this gasket is solid rubber, not too compliant. I substitute this by two typical gaskets made of soft rubber tubing.
After closing the back and adjusting the pressure as explained, I fix the somewhat loose screws with a dab of soft glue (common white vinyl glue, easily removable). As the adjustment of the backplate is also somewhat loose, and the new gaskets and soft adjustment are thicker, I then seal the gap left between the backplate and the front ring all around with this glue. It fixes the backplate and helps with airtightness.
The last thing is to substitute the back rubber connector with the brass insert for a new soft rubber only connector. The one that is sold for the emg and Meltrope soundboxes, of the right size, is ideal. It fits directly and snugly on the backplate hub, no need for the brass insert or any screws. And then the soundbox fits snugly on the tonearm. You'll need to wet slightly the inner surface of the rubber to introduce it in the tonearm and adjust manually the soundbox angle, you know: wet, install and yip, yiip, yiiip... bringing it into the approximate right position.
It helps producing the best sound and avoids unpleasant back reactions against the grooves, so the wear of records is minimal. Having no setting screws allows for free adjustable soundbox angle, so together with the length of the needle, helps in adjusting the best tracking. The no4 and swan neck tonearm with the factory adjustment in the 101 has a bit bad tracking at the start grooves of 12" records. With the adjustable needle length and soundbox angle (say, between 50 and 70 degrees to the record) the tracking can be greatly improved.
I've done a very similar thing with the ortho soundboxes 5a and 5b with similar good results. There I maintain the original thin felt gaskets, or used thin plain rubber gaskets with similar results. But the rest of adjustments I do equally, with astonishing good results. I believe that the slight deepening of the air chamber behind the diaphragm also affects the sound.
I've tried to do this also with the exhibition, but it is much more complex, and I've not yet mastered it. The damping springs are my nightmare, as also the fixed rubber back.
I have great expectations about the new gasket tube and new softer rubber backs from Wyatt... These will help greatly. To be continued...
Inigo

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