[CAUT] Lack of low-frequency response

[Richard Brekne]

Hi Ron

Thanks for the thoughtful reply.  Correct me if I am wrong, but what I 
am <<hearing>> is that if there is not enough cross grain stiffness in 
the middle of the soundboard... or better said out in front of the bass 
bridge and tenor bridges and a bit left of center pehaps... then the 
board will acoustically break up into many smaller vibrating areas and 
vibrate poorly as a single whole at its fundemental. And perhaps the 
same can be said of the first couple low vibrational modes of the 
panel.  If this happens a thin, nasal like sound can occur because 
essentially you've filtered out the low end... and perhaps boosted some 
midrange frequencies, maybe even higher frequencies... depending on the 
over all stiffness elsewhere in the panel.

I'm assuming that in just about any panel the along the grain stiffness 
is more or less a given and cross grain placement of ribs wont alter that. 

What you say about old panels pans out perfectly with what I am 
observing as well... and indeed this particular instrument I'm dealing 
with has been fascinating from several points of view.  Every one of the 
pianists who period pianos for a living who has played this thing has 
loved it.  Some have understood my own less then enthusiastic comments 
about the thinness of the sound... but insist this is how they are 
<<supposed>> to sound.  Myself.. I have to think by mid 1850's piano 
sound was well on the way to approaching that Steinway sound that 
established itself some 20 years later.  Looking at the ribs... 
listening to the sound.. I have to think along the lines you suggest... 
that the cross grain stiffness was in the panel itself because of 
compression.

This jives well with the killer octave problem of today as well if I am 
not mistaken.  Its harder to identify a lack of sustain in the longer 
strings perhaps... but that thin weak tone development is akin.

If this is right then... it suggests a way of improving sound on any 
kind of instrument that suffers this kind of problem.  Indeed the 
appearance of the so called riblets for the killer octave goes exactly 
in this direction, just applied to a different problem and a different 
area of the soundboard.  One could either install rib height extensions 
or extra ribs as needed.

It also raises several questions on both sides of the CC, RC, and RC&S 
discussion I'd like to see hashed through again.  I cant help but think 
that a gut feeling I've had for many years now ends up being true.  That 
at least while the elastic strength of the wood survives the compression 
strain the cross grain stiffness in the panel is more homogeneous then 
in boards with little compression used... hence creating a somewhat 
different overall sound picture.  This would explain also what old 
panels that are carefully removed and turned into rib crown and 
supported panels apparently  dont sound quite as <<new>> as one might 
expect.  The trade off being longevity of that original character. 

Actually.. theres a lot I'd like to see discussed over again about all 
this. 

Cheers and thanks for the post...
RicB


    Richard and all,

    This is the line I was waiting for someone to contribute, and uncle
    Jim was the one who did it. Thanks Jim.

         >. . . We try to make the board stiffer in the middle and more
         >flexible around the edges so it will vibrate as a unit at low
        frequencies
         >instead of breaking up into standing waves. . . .
         >Jim Ellis


    Several commentators have said that they believe the board is too
    stiff. But the further qualification which we need to answer is, ". .
    . too stiff where", which may lead us to conclude that the soundboard
    might not be stiff enough. I've looked at images of the piano around
    which this topic is focused, and to me it looks like the board very
    likely has insufficient stiffness in the middle area of the panel to
    act as an effective low frequency driver.

    Now some might say, why then did the original maker build the
    soundboard with these dimensions? Well this design probably was
    originally built as a compression crowned panel. So it would have
    somewhat-worked for a time. I say somewhat worked, because I believe
    that almost everything I've seen which was built between 1860 and
    1880 had grossly insufficient panel stiffness when compared to some
    of the more effective later designs (its always easy to be wise in
    hindsight). Again, some will claim that these older boards have lost
    their stiffness with time. Yes they will have. But if you build a
    replacement panel for instruments of this period, using the same
    physical dimensions, they will exhibit a quite similar tonal quality,
    in spite of the fact that the soundboard is new. I believe it comes
    down to insufficient belly stiffness. Sure the rim weight/stiffness
    will play a part also, but the soundboard design along with the
    choice of hammer will be, above all else, the two principle factors
    which determine the overall tonal outcome.

    When thinking about the desired physical characteristics of a sound
    board assembly, remember that while the very best low frequency
    speakers have a flexible perimeter region, the cone itself is
    reinforced to increase stiffness and reduce standing waves. Watching
    standing wave patterns form on a driven panel might look very
    interesting, but the propensity of the panel in itself to form
    standing waves is not helpful. We really should be trying to reduce
    them. I believe an effective design can help to minimise their
    influence over the tonal outcome. Considering where to make a board
    stiff and where to make it flexible, would seem to be a major
    priority.

    Much of the sound board tone building equation comes down to a
    relationship between area/stiffness/mass per unit area. I don't know
    of anyone who has a magic formula for determining how much of what is
    desirable, but there are answers out there in waiting as the 'circle
    gets smaller'.

    Ron O.
    -- 
    OVERS PIANOS - SYDNEY
        Grand Piano Manufacturers