Impedance problem

[David Love]

I think there are two issues.  One is impedance matching (or lack of)
between the low end of the tenor bridge and the top end of the bass
bridge.  The other issue is resonant frequencies when there is a
sympathetic match between a certain note and a section of the soundboard
that it excites.

David Love
davidlovepianos@comcast.net 

-----Original Message-----
From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org] On
Behalf Of David Ilvedson
Sent: Friday, January 21, 2005 7:02 PM
To: pianotech@ptg.org
Subject: RE: Impedance problem

So is this like on some small grands and the first tenor note or so kind
of booms out...?

David I.



----- Original message ----------------------------------------
From: Ron Nossaman <rnossaman@cox.net>
To: Pianotech <pianotech@ptg.org>
Received: Fri, 21 Jan 2005 18:31:20 -0600
Subject: RE: Impedance problem



>>I didn't weigh it but it was a very small one that I use for making
>>loops for string knots.  On second thought it seems like not enough
mass
>>to have an influence on impedance so soundboard resonance makes sense.

>Right, and an impedance change wouldn't be so note specific, but rather

>more general to the area.


>>What is it that causes that to happen and how is it generally avoided
in
>>design?
>>
>>David Love

>Consider a soundboard a driven harmonic oscillator. A sprung mass
system. 
>When the resonant frequency of the assembly at the driven point is the
same 
>or close to the same as the driver, the soundboard moves in phase with
the 
>driver, increasing in amplitude and draining the driver's energy
quickly. 
>It sounds loud and short. Lower the resonant frequency of the
soundboard by 
>making it either heavier or less stiff, and something different
happens. 
>The phase of soundboard movement lags behind that of the driver,
opposing 
>movement rather than reinforcing it, so the soundboard moves at a lower

>amplitude, and absorbs the driver's energy more slowly. It sounds less
loud 
>for longer. Avoiding it in design is partly luck, depending on the 
>soundboard construction method, and partly ignoring it when it happens.

>Adjusting it after the fact by mass loading the bridge  is easy enough,
and 
>doesn't otherwise muck anything up.

>Ron N

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