Article about bridge agraffes - function, types

[Calin Tantareanu]

> -----Original Message-----
> From: pianotech-bounces at ptg.org 
> [mailto:pianotech-bounces at ptg.org] On Behalf Of RicB
> Sent: duminică, 19 noiembrie 2006 01:08
> To: pianotech at ptg.org
> Subject: Article about bridge agraffes - function, types

> I'm not entirely sure what exactly you mean in the below and the 
> corresponding part of your article.  What I'm hanging onto is 
> the basic 
> impedance match between string and the bridge.  This is a pretty 
> complicated affair when you start mixing in the relative strengths of 
> partials.  Perhaps its just the wording that trips
> 
>      "which are much stiffer and transmit high frequency vibration
>     better") 
> 
> The inference being that the wood surface damps higher frequencies 
> earlier.   That would mean that if you constructed two 

Ric,
What I tried to say is that a wooden bridge cap, as found in a normal piano,
is, when compared to the metal bridge agraffe, much softer.
Now the frequencies of strings (including partials) at the top treble goes
into the thousands of cycles per second. At those frequencies, anything that
is somewhat flexible is likely to absorb them rather than  transmit them
down to the soundboard .
Keeping this in mind, you will also note that the string tends to dig into
the wood cap, creating an unclean termination which further decreases
efficiency. The bridge agraffe has a metal edge which allows for a very well
defined termination. In addition, it has a much larger footprint on the
wooden bridge, so rather than digging into the wood, the agraffe moves the
whole bridge when it vibrates.
The result is that the bridge agraffe transmits high frequencies better to
the bridge and soundboard. It's actually quite simple.
You might say this is just theory, but the pianos with bridge agraffes have,
among other qualities, a sensibly longer sustain.




> bridges of equal 
> mass... the only difference being the stiffness of each... then you 
> could observe the same result.  I'm not really sure this is 
> the case... 
> but then I havent really thought about it much either. In anycase I'm 
> not comfortable with the word transmit here.  The stiffer and 
> more massy 
> a termination is... the less it transmits of vibrational 
> energy through 
> it.  It will rather reflect this energy back through the input source 
> (strings) typically increasing sustain and lowering output amplitude.

Well, not always so. It's more about aviding energy losses anyway. The
bridge agraffe simply directs more of the strings energy into the soundboard
than a wooden bridge cap does.

> 
> I'd assumed the filter was to minimize the audible effects of loss of 
> energy to the termination itself... much like braiding a 
> front duplex to 
> quite capo noise.

No, the filter (= felt in the agaraffe) actually induces a controlled energy
loss, especialy in high partials.
Because the bridge agraffe is so much more efficient at transmitting high
frequencies, it makes very high partials audible which might not be
desireable given the piano tone we are used to and like to hear. That's why
those filters are used, to tune the harmonic content of the note.
A duplex segment, of which there is a tiny one in the bridge agraffe itself,
always has an effect on the speaking length itself. Muting it influences the
harmonic spectrum of the speaking length too.

Calin Tantareanu
http://calin.haos.ro
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