Del,
The nature of motion itself is, no doubt, one the principal bones
of
contention in this discussion. In my first post on this question last
Sunday I
went into substantial detail as to what these are - I suggested some of
these
were translation, rotation, and stress. I used the word disturbance
advisedly
to avoid use of the already highly charged term wave. In answer to
your
question and perhaps, to concede a little to the exasperated proponents
of the
moving bridge theory, if one wishes to advocate that strain or
deformation of
the bridge is motion then, sufficiently qualified, I would not dispute
this,
although this seems rather a trifling point. Obviously oscillating
motion on a
molecular level traveling as a wave through a medium is motion
indeed. But it is not
the motion of a flexing, rocking bridge, however phrased. That a stress
wave is a kind
motion, I have never disputed, in point of fact I have argued this kind
of behavior is the
case at the string/bridge interface. Substantial energy can be so
transferred. Heat is motion, although we would not say that a hot pan
sitting
on the table is in motion simply because it is hot. The pan is at rest.
Similarly, strain is not necessarily motion although indeed molecular
motion
exists. As the case my be strain may or may not be demonstrative of
motion So
it is with a bridge. It is not, however, visible perceptible motion
and is not
analogous to that motion which the end of the string viewed as somehow
moving
up and down or pushing and pulling could be thought of as causing in the
bridge. I understand how one could think this to be so, I simply think
there is
a better explanation which I have taken great pains to offer. In my
view, I
believe any such rocking motion, were it to occur to any appreciable
degree, would be
substantially detrimental to the transfer of energy from the string to
bridge. It is most definitely not necessary to postulate such motion to
account
for energy transfer from string, through bridge to soundboard. Labelled
however one wishes, the stress transduction method I described below
can do
just that.
Delwin D Fandrich wrote:
> ----- 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.
> What standard wave motions?
> > 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.
> In trying to demonstrate the sense of this postulation, if any, and I believe there is a substantial amount, I make no reference to ignorance. Nor do I think it in any measure appropriate.
>
>
> > 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?
>
> I have given my views on this above.
> > 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?
> I don't need to be reminded that tuning forks are not piano strings. There are useful analogies
to be drawn, however, to those that will contemplate them. Parts of
> the fork are physically, substantially, moving. Others parts transfer strain
> energy. Speaking molecularly, as I described above, everything is moving.
> As you hold the fork and move it closer it is obviously moving. You know, as
> well as I do, that no molecules are jumping the gap. This is addressed in
> detail above.
>
> >
> > 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
I'll come back to these troublesome questions later. As you say the
words
"solenoid model" are not yours, I will, after having re-read one of your
posts
modify and offer your phrase "diaphragm in a loudspeaker". I think
it is plain, in
the context of the list what both refer to.
Regards, Robin Hufford
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