Soundboards: Thickness and Area

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Östen Häggmark" <43-2@chello.se>; "Pianotech" <pianotech@ptg.org>
Sent: October 21, 2003 8:23 AM
Subject: Re: Soundboards: Thickness and Area
>
> Well that may be, but virtually everywhere I look, including the 5
> lectures, the stated purpose of banging on the soundboard is to provide
> an excitement for all soundboard frequencies at once, which
> accelerometers placed about the bridge and board use to identify the
> vibrational mode frequencies of the assembly.

In the most common modal analysis test setup one accelerometer is used and
the soundboard is struck at a series of precisely located points with a
modal hammer. The hammer impulse is broadband in nature. The soundboard
then responds by vibrating (oscillating) in many different ways and at many
different resonant frequencies. How it vibrates in response to a specific
impact is picked up by the accelerometer and recorded by a computer. With
enough impacts over the face of the soundboard, and after suitable
processing, the result will be a picture of how the soundboard vibrates at
its various resonant modes. Striking the soundboard with the hammer simply
generates broadband noise.


>
> And the stated method for
> finding the patterns is to exite the disired mode of the piano by
> vibrating the panel at the corresponding resonant frequency.

When conducting a Chladni test the object is not to find or excite some
desired mode, it is to find out what those modes are. Once those modes are
know they can be studied and analyzed and appropriate action can be taken.
For Chladni patterns they are generally found by driving the soundboard
with a frequency generator/shaker combination of some sort. While this has
the ability to focus the energy to one specific frequency for more exacting
testing and measurement, these patterns can also be seen -- though less
distinctly -- by driving the soundboard with a broadband noise generator or
by striking the soundboard with a hammer will inject the same frequencies.
When being excited with a hammer impact the resonant modes won't be
continuing but will die out as the energy is dissipated into the air in the
form of sound and internally in the form of heat. Striking the soundboard
with a larger and softer plastic hammer will limit the energy injected into
the soundboard panel to the lower frequencies (i.e., the contact time will
be relatively longer). If the hammer is big enough (calibrated modal
hammers are generally quite small) the excursion of the soundboard will
easily be great enough to bounce the sand away from the smaller resonating
modes and move it around to the parameter of the fundamental vibrational
mode of the soundboard. And, as I said earlier, the higher frequency
vibrating modes will be damped out very quickly due to the internal
resistance of the wood. This is not going to be a very precise test.


>
> Yes, most noteably right in front of the bridge. And I wish too he'd had
> time to go more into just what he was accomplishing. But to be sure, the
> sand collects at the places where the soundboard is not vibrating.

And you have, of course, just described a rather crude Chladni test.

Del