Ringing middle string

[Arlie D. Rauch]

On some old uprights, I've encountered a phenomenon in which the 
middle string does not damp while the outer two do.  This may happen 
on individual notes but sometimes on many notes.

Recently I replaced the damper felt in an old upright, and the middle 
strings kept right on ringing (as they had done before), certainly 
throughout the mid range of the piano.  Was the piano designed this 
way to make it seem more alive?  I don't prefer it that way.  What is 
the suggested fix, if there is one?

On some occasions on an individual note like that I have sanded the 
damper felt so that it is thicker in the middle than on either side. 
It works, but it would take a fair amount of time when many have this 
characteristic.

Thanks for any input.

Arlie D. Rauch
Glendive, MT