Sound in soundboards

[John Delacour]

At 7:25 PM -0600 12/3/01, Ron Nossaman wrote:
>  >...I borrow hydraulic equipment to make sure
>  >that bridge really can't move.  I haven't tried it, but you suggest
>>the effect will be to kill the bass of the piano as though the
>  >soundboard and bridge were not there


>That should be the case. Just like clamping the driving coil of a speaker
>cone to the magnet. The transducer can't transduce if it can't move.

Yes, according to Del and you it SHOULD be the case but my intuition 
tells me that it will not be the case at all and that though my 
massive clamping structure may absorb a proportion of the energy, the 
effect on the bass of the piano will be slight enough to enable the 
piano to be played and to sound quite satisfactorily.

>  >So far as I know sound travels always as a
>  >compression wave through whatever medium, but you are suggesting
>>something different.
>
>But the soundboard isn't transmitting sound. It's dispersing string energy
>and producing sound. Transducing.

I'm more of a classicist than an electrician and to me transmit means 
"send through..." and transduce would mean "lead through..." if it 
were in Webster's dictionary, which it aint in mine.  At any rate I 
can't visualize the difference in effect.  I think I'll take a gamble 
and stick with "carry" and with my assertion that sound travels ONLY 
as a compression wave in whatever medium.

>  >So the sound, I understand, begins its
>  >journey moving _along_ the surface as far as the boundary and then
>>gets reflected _into_ the soundboard. How does it travel then?
>
>Like a ripple, with the stiffness of the transmission medium (soundboard
>assembly)...

I guess you mean the transduction medium?!

>...determining the energy transfer rate from the string. It's the
>same kind of ripple that propagates along a struck string, and reflects
>from the terminations back into the string - only there are a whole lot
>more driving frequencies and excitation sites with the soundboard, so the
>ripple pattern is much more complex. It's not really a compression wave,
>though I'm not sure what it would be technically called other than a ripple.

So the sound travels as a ripple.  And since I've taken my arbitrary 
stand, I obviously have to be consistent and say that it can't, 
simply because that's not the way sound travels, but _always_ as a 
compression wave.  I say also that the transverse movements of a 
piano string differ from a ripple in that the peaks and notes are 
static, but however that may be the sound produced by transverse 
moments is linear and at right angles to the string.

>  > I'd suggest it travels
>  >wherever the medium exists to carry it and that it travels most
>>effectively in the hard and long-fibred winter growth.
>
>Then why does changing the rib height and feathering have so much more
>effect than thinning the panel? The long grain stiffness has more of a
>constrictive, than a transmission enhancing effect in practice. That's why
>floating the soundboard in the tail enhances the bass in a small piano so
>well. It relieves the built in excessive constriction of long grain
>stiffness, and shifts stiffness control to the ribs, where it is more
>manageable.

Well that's not a contradiction of what I say.  If ultimately the 
soundboard was not induced to move up and down, and to move 
differently at different places, then we'd have no sound at all, 
because to produce the compression waves that reach our ears through 
the air, the air must be amply moved.  When Del says the soundboard 
is not an amplifier, I presume he means that it does not add anything 
to the vibrations it receives and to that extent of course he is 
right.  In the case of an electronic amplifier of course the case is 
different and a great deal of power is added to make the signal 
audible, but that's the problem with all these electrical terms.  We 
are not electricians and the meaning of "amplify" is basically to 
enlarge, to enhance, to make ample.  The sound from the string alone 
is not ample, that produced by conversion of the energy from the 
string to vertical movements of the board is.  In that sense the 
soundboard is most certainly an amplifier, though since I'm not an 
electrician I would never use the term.

JD