Sound waves(The behavior of soundboards)

[Delwin D Fandrich]

----- Original Message -----
From: "Robin Hufford" <hufford1@airmail.net>
To: <pianotech@ptg.org>
Sent: December 19, 2001 1:47 AM
Subject: Re: Sound waves(The behavior of soundboards)


> Del,
>      The matter in question is - does the bridge move directly as a result
of
> the vibrating string and is this the mechanism by which the energy in the
string
> is transferred to the bridge and thence to the soundboard in the form of
some
> kind of ripples?

Yes. And they are not 'some kind of ripples,' they are the standard wave
motions of a vibrating edge-supported diaphragm.


>
> When I first posted on this subject I had not reached in my
> reading  your later post in which you disclaimed, at least to some degree,
this
> description of which apparently you have had,  at least, second thoughts
of some
> kind.

No. I haven't been having second thoughts of some kind. I'll consider any
postulation that makes sense. But to somehow get the soundboard moving
without first moving the bridge just doesn't make sense to me. Perhaps it is
just my ignorance on the subject.


>
> Or is there another perspective?  I submit there is and that my posts,
> which have apparently put some people to sleep, along with those of J
Delacour,
> which perhaps have kept others awake,   have explained it or at least
attempted
> to do so.
>      In the interests of  amicable discussion I would have to say however
that
> as the members of this list are at least able to operate computers and
are,
> evidently, literate, it is not likely  they misconstrue what an
accelerometer is
> or what it can do although in point of fact the  motion itself is not what
is
> measured but rather the time rate of change of velocity, that is the
> acceleration which is,  as I am sure you know, a second order derivitive -
> velocity being the first.  That a soundboard once in vibration would also
move
> the bridge and both would easily measured as being in motion I am also
sure no
> one in their right mind  would question and JD has been very explicit on
this
> point.

The question, of course, is, how does one initially get the soundboard to
vibrate without physically moving the bridge first?


>
>      The process whereby energy is transferred from the string to the
bridge is,
> once again what is in question and I propose, as I did earlier, that a
little
> consideration of the behavior of a tuning fork, in isolation and when in
contact
> with some other medium will be most instructive in this regard.  To
briefly
> recapitulate:
>           In one and the same object, that is, a vibrating fork we can see
> visible indications of stress/strain relationships. On the one hand  the
> flexibility of the tines when struck  allows for perceptible,  visible
> displacements in the region of their ends. While at the same time, on  the
other
> the base of the tines and stem of the fork represents an area in which the
> strain energy, visible in the moving tines, has been constrained by the
> progressive  increase in  stiffness  along the tines as they approach the
base
> to such a degree that the strain is now expressed as a molecular stress
> disturbance which propagates through the base and stem and is reflected
back up
> into the rest of the fork.  At the base of the time and in the stem
visible
> displacement is no longer apparent.  Is there energy, periodic behavior?
Of
> course but on the one hand it is molecular and invisibile at the base and
stem
> and on the other, while still ultimately molecular,  it is demonstrably
visible
> as more organized, translatory behavior, that is transverse motion,
flexion or
> whatever.  The visible flexion can be easily stopped by the  merest  touch
of a
> finger but no human hand can exert sufficient pressure to eliminate the
stress
> wave in the base and stem  that is felt as a vibration.   A small, light
fork
> will,  when lightly touched to a surface,  in fact jump up and down if
held
> vertically and this is a visible indication of substantial energy transfer
> through strain on a molecular level which,  when there is a force for it
to
> react against,  will actually propel the fork upward slightly a number of
times.
>
>        The original impact against the tine of the fork set it in motion,
this
> motion is transduced by the effectively increasingly stiff part of the
tines and
> the base to strain energy on a molecular level, that is a periodic stress
wave:
> this then  passes through base of the tines into the stem  of the fork
where it
> is reflected from an unclamped boundary back into the system.     Should
the
> fork be in contact with a surface of sufficient stiffness  or mass
density,  the
> pressure exerted by it be kept light and the fork be kept vertical while
in
> contact with a horizontal surface then it practically  becomes alive as
the
> action/ reaction of the bottom of the base and the surface itself causes
the
> fork to  undergo translation as the two surfaces propel one away from the
> other.  This and gravity make the fork appear to be slightly  jumping up
and
> down. .  It is especially important to note that the jumping action will
be in
> the direction of the long axis of the fork and not the transverse
direction of
> the tines. A plain demonstration of the transduction of the flexural
strain
> energy to a molecular level and its subsequent transduction to translation
has
> been demonstrated and is further emphasized by  the fact that the
translation
> is  now  oriented 90 degrees to the original direction of flexion.  As the
> pressure is increased, and  I believe this corresponds to and is one of
the
> principal functions of  downbearing, although there are others,   the
force
> exerted on the fork increasingly  prevents relative motion, which is
> progressively extinguished.  The transfer of energy is increasingly of the
> nature of internal, periodic,  molecular deformation -  that is periodic
strain
> or a stress wave rather than relative motion of the two parts.   This, in
a
> nutshell is exactly the mechanism of transfer of energy between the string
and
> the bridge/soundboard assembly and does not require motion of the bridge
to take
> place.  Any substantial motion of the bridge is, in fact, an impediment to
the
> efficient transfer of energy.
>      The fact that the bridge may subsequently be moved by standing waves
> induced in the soundboard assembly is self-evident and an accelerometer
would
> indicate this.  The effect of this motion  upon the transduction
efficiency of
> string/bridge contacts is another question but I will state categorically
that
> the idea that the string or strings of a unison is at least somehow
wiggling and
> rippling the bridge and the soundboard  and that  this is essentially the
> mechanism of transfer of energy from string to bridge/soundboard is
entirely
> suspect for many, many reasons.
>
>      Had this been the case then even a relatively light pressure upon the
> bridge should immediately reduce the loudness of the sound emanating from
the
> soundboard as it does with the flexing part of the fork and a variable
pressure
> would introduce variable volumes in the sound.  This is plainly not the
case.
> It is the case, however, that pressure upon the stem and base of the fork
does
> not eliminate the sound; and this is  precisely what occurs when pressure
is
> applied to the bridge.  Obviously, one could say that a pressure
sufficient to
> destroy the system could be easily generated; evidently these effects
would be
> different then and these kinds of pressure are not what I am referring to.
.
>      There are numerous parallels between   fork and the string where
these
> effects are exactly the same.  The   merest touch to the side of the end
of the
> tines  extinquishes the transverse flexion or motion of the tines and its
> subsequent transduction to periodic strain and is easily  sufficient to
stop the
> sound.  The  damper assembly exerting  force against the string does
exactly the
> same thing -  using a mere flexible piece of felt   it stops the
transverse, at
> first visible motions of parts of the wire, readily and easily.  The
subsequent
> transduction is starved and  the driving of the board is  thereby ended
equally
> readily and easily.

Tuning forks are not piano strings. Take your tuning fork and bring it very
slowly toward contact and just before it makes contact you will hear a
slight rattle, or buzz. Is this the result of molecules jumping the gap? Or
because the for is physically moving?


>
>       Surely, no one would argue should the solenoid model be accepted,
that is
> that the string somehow ripples the bridge to any substantial degree,
that the
> extinction of sound occuring when a damper is let down onto the strings
could
> possibly be the result of what would essentially be the reverse of what
you and
> Ron appear to advocate - that the damper is sufficient to operate as a
> counterweight to a rocking and rolling soundboard, particularly with a
flexible
> felt interface moderating  the force and effect of the damper assembly.

Dampers are simply auxillary mass absorbers. They are designed to absorb and
dissipate vibrating energy from whatever they are applied to.


> It
> seems far simpler to suppose that the string is, in fact,  driving the
board  in
> a manner that does not contain the troublesome questions implicit in the
> solenoid model; this is the strain transduction  method described above
and that
> when the transverse behavior or motion in part is extinguished then so is
the
> transduction mechanism that had been driving the board.

I don't find that there are 'troublesome questions' implicit in function of
the soundboard as I've described it. I don't use the 'solenoid model' term.
That was invented by someone else.

Del