Impedance problem

[Ron Nossaman]

>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