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Key bounce - Is it the pad, the spring...?

DavidUK

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Furthering my research in to pads I wondered about some key cups having a tendency to bounce or reverberate when opened.

Is this a symptom of the spring being wrongly tensioned or do other factors come into play? I can't imagine the pad itself had a great deal of rebound ability when it hits the tone hole.

I guess damping at the limit of movement by felt or other material is a contributor though. So, just springs and whatever they open against? Are they the only culprits?
 
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It's to do with the weight of the key and amount of cushioning. (Obviously spring tension will play a part, but you want sufficient spring tension to do the job.) Typically low D will bounce if you let it. If you avoid lifting your fingers off it'll stop it.
I've tried various different types of cork or felt and used a chamferred cork which was recommended somewhere online.
The one that works for me is having cork on the key and a felt piece on the body. Generally I try and avoid sticking cork or felt to the body, but for RH keys it is effective and fairly common to see these days.
 
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turf3

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All of the above. It's a spring/mass system; you have the rotational moment of inertia of the key/pad, being driven by the spring. That rotational movement will have a natural frequency. If the stiffness of the bumper material at that point, and the distance from the pivot to the bumper, reinforce the natural frequency of the spring/mass system, it'll bounce multiple times till the damping inherent in the system kills the bounce.

I doubt there's any reasonable way to calculate these things. In general I would expect long heavy key/pad mechanisms with short key feet to bounce the most. As an example of the opposite of this consider the F palm key. The pad is the bumper material (it's an NC key) and the distance of the bumper from the pivot is very long. These keys don't ever bounce when you close them quickly.
 

Dr G

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Turf3 nails it for the win!

FWIW, you can play with the soft materials to determine the ideal balance of “stiff enough to hold adjustment” and “compliant enough to damp the vibration”. A product called “Sorbothane” was developed to do exactly that for running shoes, then adapted for other uses. We used to put it on the feet of high-power optical microscopes to eliminate vibrations from the room. It is available in thin sheets for applications on musical instruments - some of key feet on my tenor have little sandwiches of Sorbothane and felt or cork.

 

jbtsax

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The large D key on tenors is one that is hard to remove the bounce. Increasing the spring tension and adding a thicker buffering material are the answers that have already been given. My choice of materials include a thin layer of synthetic felt between the tops of the key feet and the back bar, natural cork on the bottoms of the key feet, and felt discs on the body under the feet on the lower stack. I have had good success with self adhesive felt discs. They stay in place, and are easily removed without leaving a residue on the body of the saxophone.

The keys requiring the lightest possible spring tension in order to achieve good regulation are the F# (alternate F#), the G#, and the Bis. I lighten the spring till the key begins to bounce and then strengthen it a bit. Sometimes I leave a small amount of lost motion between the bar from the Bis and the adjusting screw so that the "stop" for the key is the felt under the touch of the A key which better reduces any bounce.

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Dr G

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Here’s a pic of the feet on my Borg’.
 

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jbtsax

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Very interesting. Thanks for sharing that. I am assuming the sorbothane is glued to the cork on the foot of the key and not the body of the sax. There is also something I haven't seen before, and that is the material wrapped around the backbar (leather?) in addition to the material (tech cork?) on the top of the key foot. With the thicknesses of all of those different materials, achieving the desired key heights looks to me to be a bit of a challenge. Did the sax come from the factory with that set-up or was that done by Matt Storher as part of an overhaul?
 

Dr G

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Very interesting. Thanks for sharing that. I am assuming the sorbothane is glued to the cork on the foot of the key and not the body of the sax. There is also something I haven't seen before, and that is the material wrapped around the backbar (leather?) in addition to the material (tech cork?) on the top of the key foot. With the thicknesses of all of those different materials, achieving the desired key heights looks to me to be a bit of a challenge. Did the sax come from the factory with that set-up or was that done by Matt Storher as part of an overhaul?

Yes, that was a Matt Stohrer job. The factory Borgani setup uses cork feet and leather wraps. From previous discussions with Matt, I learned that he makes the Sorbothane/cork sandwich, applies it to the foot, then sands the cork to final thickness. There are a few other places on the horn that get a sandwich treatment with Tech Cork/Teflon. His setups hold regulation for a long time and have a great feel under the fingers.
 

DavidUK

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Last night I was watching YouTube videos relating to the material properties of silicone, as one does...

Anyhow, a video showing steel balls being dropped on to various "rubber" sheets had the ball stop dead with no rebound when dropped on Poron.

I've found a comparison of Poron vs. Sorbothane where Poron is preferred, here: Assessment of some shock absorbing insoles | O&P Virtual Library

I did try to find the video from last night but when I searched on YouTube for "balls drop poron" the results weren't quite what I'd expected. Those with a weak constitution should refrain from trying to find said video.

Anyhow, Poron may be of use as a buffer but I know nothing much about it as both it and Sorbothane are alien to me.
 

turf3

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I think that Poron and Sorbothane are two different trade names for materials of the same general class: open cell polyurethane foams of a specific type. Of course, each comes in a huge range of individual types with their own specific properties.
 

Stephen Howard

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For what it's worth, many repairers set the low D far too light - probably labouring under the mistaken belief that all the keys on a stack need to be at the same tension.
 

Dr G

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Last night I was watching YouTube videos relating to the material properties of silicone, as one does...

Anyhow, a video showing steel balls being dropped on to various "rubber" sheets had the ball stop dead with no rebound when dropped on Poron.

I've found a comparison of Poron vs. Sorbothane where Poron is preferred, here: Assessment of some shock absorbing insoles | O&P Virtual Library

I guess it depends on whether you are repairing shoes or saxophones. Shock absorption is not the only property of concern. Sorbothane is proven by horn technicians. It’s your call.

Please let us know what works for you.
 

jbtsax

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A tech I worked alongside for several years learned repair from his father in law who was very "old school". His method of solving the bounce problem on large low D keys was to rig up a bell key bumper felt to touch the key at the top of its motion. He called it a "Debouncer". I suggested to my friend that it should have been called a "D Debouncer". His quick witted reply was "or a Double D bouncer". :p drumroll please . . .
 

Colin the Bear

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Double D bouncer? I thought that was something else. Just off to google it. I may be some time. ;)
 

Ivan

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Here’s a pic of the feet on my Borg’.
At risk of being told, "If you can't see it, you don't know enough to understand it", a circle drawn around the cork/sorbothane combo would help the likes of me see the magic

In my defence I spotted the leather ring and what looks like an oddly hinged and uncomfortable thumb rest
 
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Dr G

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At risk of being told, "If you can't see it, you don't know enough to understand it", a circle drawn around the cork/sorbothane combo would help the likes of me see the magic

In my defence I spotted the leather ring and what looks like an oddly hinged and uncomfortable thumb rest

Look at the key foot contacting the body of the horn directly below the leather wrap.

The thumb hook is a Forza - the most comfortable thumb hook ever.
 

Stephen Howard

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Look at the key foot contacting the body of the horn directly below the leather wrap.

The thumb hook is a Forza - the most comfortable thumb hook ever.
I had one on my TJ RAW tenor for a while - could never quite find the 'sweet spot' with it, so went back to the original hook.
That said, I don't find the RAW hook as comfy as the plain old bit of bent brass they stuck on the Yamaha 23.
 

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