On Jan 23, 2002
Phillip L Ford wrote:
> > Although the statement itself is unambiguous, Den Hartog immediately qualifies
> >it by urging care be taken in its application which has to do with a system in which
> >a resonant frequency approaches the "vibration absorber frequency".
>
> Sorry, I don't know what the vibration absorber frequency is.
>
> It is the frequency at which the mass most efficiently acts as a vibration damper,
> although this is not greatly germane in this discussion. I merely quoted this to
> emphasize that Den Hartog immediately insists that care be taken in generalizing from
> this statement.
>
> Can you propose an
> experimental means of verifying directly on a piano that motion does not occur
> or that if it does it has nothing to do with sound production?
>
> You misconstrue my assertions in regard to motion as I have not maintained that is
> does not occur. Rather, I say that substantial motion at the string bridge interface
> induced as a result of the bridge attempting to follow the so called cyclic loading of
> the board by the string during its excursion is detrimental to the sound as it will
> displace the node and thereby confuse the frequencies produced by the system. The
> actual motion of the bridge is elastically induced through the development of standing
> waves in the bridge/soundboard and, depending upon the particular piano, may be
> substantial or otherwise. These are two different matters. I maintain that the
> "cyclic pressure" proponents mistake one for the other.
I can indeed propose an experimental method to separately guage these two
components. Judging, from the strident and self- satisfied tenor of the response made to
my post last evening some may not have the objectivity to consider fairly the
implications of the results of this simple experiment which I first tried several years
ago, although upon reflection the results are most instructive. You can easily
replicate this yourself for a little money and acquire in the process a useful tool.
> There exists a device called an "incidence meter" which is used to adjust the
> angular relationships of the wings and other surfaces on a model airplane and can be
> used to guage the nature and relative strengths of the motions under discussion as
> they exist in a piano. I use it as a downbearing guage on occasion in the shop.
> Basically, this device is a balance and pointer encased in a small box with a scale and
> mirror. The mirror is used to negate the effects of parallax in reading the guage
> which will indicate the angular measure of incidence to a level surface. This device
> is, in effect, a kind of plumb line encased in a box in which the pointer is free and
> will orient itself vertically due to the balance mechanism. The displacement from
> vertical will be indicated by a guage. Should the base of the meter be sat down on an
> unlevel surface the guage will indicate the angulur incidence of the surface to level.
> In all cases, regardless of the orientation of the box in which it is placed and to
> which the guage is attached, the pointer will indicate the vertical direction after
> equilibrium is reached.
> When this device is placed upon the bridge or bridge pins and a note is struck one
> can observe the needle move about. AH HA Motion of the bridge - the cyclic
> pressurists must be right! The motion of the needle indicates motion of the surface
> upon which the incidence meter rests. It is possible, by placing this device upon the
> bridge, plate, soundboard, lock and other surfaces to acquire a visual indication of
> this motion and its relative strengths occuring as a result of motion where the
> incidence meter is placed. You can, indeed, do the same thing simply by holding it in
> your hand, moving it and observing the results at the guage. FINALLY, I have seen the
> light and am now a "conventional thinker"! By placing the meter upon the bridge and
> striking the key with various levels of force one can plainly see the needle respond in
> proportionate measure to the excitation of the key. However, there is more to the
> story!
The Cyclic Pressurists belive that the flexing string lifts, pushes and pulls upon
the bridge as a result of a force produced during the excursion they take the standing
waves to be. They have been repeatedly explicit on this point, indeed, posting just
today declarations as to these operations and, in particular, contending again the
irrelevance of loading. Well, let us test this point. We have seen the Incidence Meter
indicates motion when the string is struck and is vibrating BUT it indicates virtually NO
MOTION when the string is pressed down or lifted up by a finger or tool. This occurs
even though the forces applied to the strings be made much greater than those applied
by the hammer. This can be seen by deflecting the strings to markedly greater
displacements than occur during the vibration of a string. It makes little difference
whether you deflect three strings or four or five unisons. THERE IS NONE OR ONLY THE
VERY, VERY, SLIGHTEST SIGN OF MOTION IN THE INCIDENCE METER. Whatever the results, they
certainly exhibit little similarity to those obtained from the string when it is
vibrating. This much is unquestionable, I believe, to any fair observer.
I am sure the certain opinions of the high school physicists on this list will
instruct us in that which we are, presumably, too simple to comprehend: that the device
does not indicate motion simply because its limits of resolution are such that it cannot
detect the relevant motions that are there. This may well be so, but in taking recourse
to such observation, if they do, which would be very characteristic of their
commentary, they will indeed fail to grasp the larger point: The meter readily displays
motion at the bridge for many unisons when the string is struck, yet with the same
unisons one is unable to cause the meter to indicate substantial motion of any kind by
deflecting the string, much less the requisite merely similar response their system
implies. This is a subtlety, among the many, which will probably be cavalierly
disregarded in the attempt to maintain the claimed veracity of the simple one degree of
freedom system upon which the attempt is made to develop a comprehensive view of a very
complicated, oscillating sytem such as the piano.
> If there are indeed, any left reading this, then I suggest you not take my results
> as authoritave. Buy the device for yourself, try it and analyse the results. There
> are many fertile observations to be drawn from testing the piano with it. It can be
> found in most hobby shops.
Regards, Robin Hufford
>
>
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