[CAUT] String Coupling / SB and Bridge stiffness

[Ed Sutton]

To answer your question you will have to take measurements.
That will require a method that can be repeated with enough precision to 
give useful information.
The bridge movement will vary depending on the energy inputs from the 
strings, so the input must be controlled, as well as the time of 
measurement. Labs would most likely use an electromagnetic drive to move the 
strings.

There are many variables at work, including our need to believe we know what 
we are doing. I have proved this many times.

Ed S.


----- Original Message ----- 
From: "Richard Brekne" <ricb at pianostemmer.no>
To: <caut at ptg.org>
Sent: Thursday, June 11, 2009 9:45 AM
Subject: Re: [CAUT] String Coupling / SB and Bridge stiffness


> Hi Ed, others.
>
> I do not dispute that in a real world piano there is coupling. Obviously 
> both the front and back terminations are not infinitely stiff in real 
> life. I am trying to get at whether there is a connection between the 
> degree of bridge stiffness and  the degree of coupling.  And whether this 
> connection manifests itself in any way in the amount or degree of pitch 
> drop that results from oscillating strings sharing a common support.
>
> This has several implications actually.  One is that since coupling 
> actually DOES occur in real life pianos, the bridge itself by definition 
> becomes an integral part of the strings termination at the bridge and 
> refutes the postulate that it is the bridge pin and the bridge pin alone 
> that terminates the string at the bridge. One way or the other.... if 
> there is interaction between the strings, then there must be a common 
> support/termination for the interaction to take place.
>
> Another implication is that it may be possible to diagnose stiffness 
> conditions by the degree of coupling observed.  If this is true, then it 
> no doubt would be a handy tool to have at hand.
>
> Given what we already know about termination stiffness and sustain 
> characteristics, I would not be surprised about the Sauter you mention.
>
> Cheers
> RicB
>
>
>    If you can hear the string, it's fair to assume the soundboard is
>    moving at frequencies similar to some of those in the string,
>    therefore it's reasonable to assume that the bridge is also moving.
>    Thus it is also likely that when frequencies are very close,
>    coupling will occur. The amplitude of the wave in the string will
>    play a role as well. Perhaps when it becomes too small to move the
>    bridge, the strings de-couple, at least at that particular partial.
>
>    This may not involve all the partials of the unison at all times.
>    For example, in the higher octaves the first partial may become
>    inaudible almost immediately. The Sauter concert grand with titanium
>    armor in the top octaves is remarkable for the clarity and sustain
>    of the first partials in octaves 6 and 7.
>
>    Ed S.
>
>