Duplex - a case in point

[John Delacour]

I mentioned in passing the other day the question of that section of 
the string between the hitchpin and the brass duplex bridge and 
suggested it was significant without having anything particular in 
mind.  I now have a case, which I find interesting.

I posted an illustration also of the two upright agraffes that serve 
as the bridges of the monochord I was using.  In a later message I 
had set up a trichord in a similar fashion...


\\----xxxxxxxx----[\]___________[/]-----------O`------------------------//


 From left to right, the strings pass from the wrestpin over a soft 
bearing to the first upright agraffe.  Between this and the second 
agraffe we have the 60 mm speaking length.  The wires then pass over 
the movable duplex bridge to the hitchpins, which are parallel to the 
bridges.  The distance from /] to O is also 60 mm and the distance 
from O to // is 120 mm.  The trichord is tuned to top A.

I strike the note somewhere to the right of [\] (or anywhere) and 
obtain a good clean ringing sound with good sustain.  I now hold my 
thumb between [/] and O to mute the back length of back duplex.  The 
sound now produced is very similar and hardly distinguishable from 
the first.

I now mute the string to both sides of O so that only the speaking 
length can sound.  Again, there is barely any detectable difference.

Now for the interesting bit.  I mute only the section running up to 
the hitchpin //,  ie the section that has twice the length of the 
speaking length.  The effect is calamitous.  Striking the string now 
produces the sort of tone you would get from a thick mug or 
flower-pot with no ring or sustain either in the speaking length or 
in the duplex length.

If I move O to a different position, the note is revived and the 
muting of the right hand length does not have the above effect.

This is a very rough and ready experiment and I not interested in any 
small differences in tone due to the 'duplex' length but only in the 
severe deadening effect of muting the 120 mm length.  The arrangement 
would be unlikely to occur in a real piano, but there may be other 
combinations that would have a similar effect.

Any ideas?

JD