[CAUT] String Coupling / SB and Bridge stiffness

[Ed Sutton]

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.


----- Original Message ----- 
From: "Richard Brekne" <ricb at pianostemmer.no>
To: <caut at ptg.org>
Sent: Wednesday, June 10, 2009 6:01 PM
Subject: Re: [CAUT] String Coupling / SB and Bridge stiffness


> Hi... no bites on this so I thought I might try phrasing it a different 
> way and see if I could get any feedback.
> If the string terminations were infinitely stiff, then there would be no 
> mechanism for string coupling .... yes ? You could in theory have a piano 
> unison, excite the strings in the usual way with the hammer.... but the 
> infinite stiff terminations would not allow for any coupling, there by the 
> strings would each vibrate completely independent of one another.  Ok.... 
> so add a set of non infinitely stiff bridge pins and leave everything else 
> about the terminations infinitely stiff. You still have the same 
> situation, tho the bridge pins become some form or another of an extension 
> of the string.  Perhaps this scenario would open for false beats on 
> individual strings at best, but there is still no mechanism for the three 
> strings to couple given the infinite resistance to vibration by the bridge 
> itself.  So no pitch drop due to coupling is possible as no coupling is 
> possible to begin with.  Yes ??... So at some point the bridge becomes 
> non-stiff enough to allow for this coupling.  And the more non-stiff the 
> bridges becomes... the greater the ability for the strings to couple, 
> hence the greater their influence on each other... and hence the more 
> evident (greater degree ?) of pitch drop.... Or what ?
>
> Cheers
> RicB
>
>    Got to thinking abit about string coupling and the mechanisms that
>    are at work that account for the pitch drop and got to wondering if
>    it is viable to think of the degree of pitch drop when 2 and 3
>    strings are coupled in on a unison as an indicator of  SB/bridge
>    stiffness at any given note.
>
>    Or... is this another one of my rabbit holes ?
>
>    Cheers
>    RicB
>
>