[CAUT] False Beats and wood

[RicB]

Hi Don

I see you've come to the same tangent on the single string beat problem 
that I have. Seems quite probable that its much more correct to think 
along the lines of the whole bridge/bridgepin termination as the support 
for the string and that any number of reasons for that whole to present 
a springy termination in one or more directions exists. Further that any 
one of these reasons can cause what we call the classic false beat... or 
single string beat. I have also wondered why CA works and wondered 
beyond the tightening idea. I dunno... just seems like there are too 
many holes in the more simplistic loose pin idea.

As for what happens when you increase pressure on the screwdriver... you 
have it right. First the beat rate slows down, and then it stops.. and 
then with more increase in pressure it speeds up again, increasing in 
speed with increase in pressure.

And I agree that this view tends to explain much easier a few other 
things like the ones you mention below, and for example why adding mass 
in close proximity to an offending strings front bridge pin nearly 
always has some impact on the false beat.

Your quote comes from the spot in this paper that got me started 
thinking in this direction... and when I read Jim's old article 
describing the simple support situation that illustrates this 
functionally... well that more or less sealed up my thinking on the 
loose pin bit, at least until something more convincing pops up. If I 
remember that all correctly its possible for the string to behave if its 
either longer or shorter then its actual length depending on the degree 
of massyness / springyness. Which is exactly what happens with my 
screwdriver... more mass to the degree the beat becomes apparent again. 
There was another bit ... something about the phase angle of impedance I 
think somewhere in that article series that caught my eye on this 
subject. Guess I'll have to take another visit to the Swedish site.

Cheers
RicB


    Hi all,

    I was reading through the five lectures again--been a while.

    In the coupled motions article the point is raised that the bridge is a
    springy termination which cause the string to "think" it is longer
    than it
    is lowering the frequency.

    Ric has asked often why a screw driver on a bridge pin may initially
    stop a
    false beat but when more pressure is added the beat may start up again.

    The simple answer may be that adding "apparent" mass by touching the
    screw
    driver makes the pin less able to spring--but as pressure is
    increased past
    a certain level, the frequency will begin to change--causing a return of
    the falseness. If this is the case then as Ric increases the
    pressure the
    beat should change it's speed (that is until the screw driver slips
    off and
    makes a hole through the soundboard *grin*).

    As a side issue it may also offer an explanation of why CA glue
    works. It
    may simply makes the bridge a bit denser--providing a more
    support--making
    the wood less springy. I.E. It hasn't got much to do with the bridge pin
    being loose—it has to do with the wood failing to be dense enough. That
    nicely explains why a bridge pin can be so loose it can be removed with
    just your fingers—and still not produce a false beat, or it can be
    so tight
    that a captive hammer is needed to remove it—and still produce a
    pronounced
    beat.

    Here is the quote (out of context) from the article that got me thinking
    along this direction:

    "If the support is "springy," that is, one which displaces sideways
    in the
    direction in which the string applies a force to it, there will no
    longer
    be an exact node at the support. Instead, the extrapolated node will be
    somewhat beyond the physical end of the string; or, in other words, the
    string will "think" that it is longer than it really is, causing it to
    lower its frequency."

    Comments?
    Regards,
    Don Rose, B.Mus., A.M.U.S., A.MUS., R.P.T.
    Non calor sed umor est qui nobis incommodat

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