More CC vs RC questions was RE: Killer Octave & Pitch Raise

[Sarah Fox]

Hi David,

Interesting questions, but I have to admit that you've gone far beyond my 
realm of expertise.  You're talking practice, while I'm talking theory.

All I can offer is a few general insights.  In rather oversimplified 
terms...

The hammer delivers an impulse to the string.  Depending on the softness of 
the hammer and its velocity of impact, there will be a certain function of 
force over time.  If you do a spectral analysis on that impact function 
(e.g. a Fourier analysis), you'll get information about which frequency 
components are present in the impact, and in what amplitude.  This impact is 
a source of broadband energy.  It lacks periodicity and harmonicity and 
therefore is not a "note."  However, it may be centered around a certain 
frequency range and may have a general "pitch," used in a loose sense.

When this impulse is delivered into the string, the string vibrates, but it 
only vibrates to the spectral components of the impulse corresponding to its 
free resonant frequenies -- the fundamental and its harmonics.  Thus, the 
string is a filter, storing some of the energy from the impulse.  The rest 
of the energy is dissipated, including into surrounding strings 
(acoustically coupled through the bridge).  This corresponds to the "whack" 
of the hammer.  (Everyone knows that a "whack" to any part of a piano will 
excite the strings throughout the instrument.)

Then the string vibrates the bridge and SB.  Depending on the mass and 
stiffness of these elements, energy will be bled from the vibrating string 
to the panel and into the air, with much of the energy of course being lost 
to friction.

Now, the magic of making all this work is to have the proper relationship 
between spectral components.  The hardness of the hammer should be such as 
to deliver the correct spectrum of impulse into the strings, to excite the 
SB with the optimum balance of harmonics.  The tighter the match between the 
free resonant frequency of the string and the free resonant frequency of the 
underlying SB/bridge, the faster energy will be dissipated through the 
system.  That means higher amplitude and shorter sustain.  It is this 
interface that *must* be inefficient.  How inefficient?  Well, that's where 
piano building becomes an art, not a science.

If you want to know how the SB responds to vibration, just deliver an 
impulse to it.  Literally, unscrew a hammer and whack the top of the bridge 
with it -- or perhaps just beside the bridge.  That tells you what is in the 
board, when the element of energy storage (the string) is removed.  You can 
get fancy and do a spectral analysis on it, or you can just listen carefully 
and identify the balance of frequency components with your ear.  A SB that 
is less responsive to a given spectral range will of course need more energy 
within that range to excite it, but this can be accomplished.  I can't 
really answer your questions about the spectral properties of RC/S vs. CC, 
except in a general, theoretical sense.  My only point at the beginning of 
this discussion was that I would expect more of a spectral shift towards 
lower frequencies in a CC board throughout the sustain of a note.  I would 
*guess* that a RC/S board would have broader response properties, but only a 
"whack" analysis would answer that question!  ;-)  And I haven't a clue 
about the mechanical properties of hammer felt.  That is what goes into 
shaping the impulse, which is arguably the most important element in a 
piano's tone production.

I would think that if a hammer can produce the desired spectrum in its 
impulse, and if the string can efficiently store that energy, the board can 
then dissipate the energy.  The only question is one of how fast and how 
efficiently.  This question must be addressed separately for each partial of 
a note.  The sustain for each partial will have a different duration, 
depending on the properties of the SB -- which was the underlying point of 
my original post (the higher partials being more subject to damping from 
hysteresis).

To say that the piano is a complicated acoustical device is an enormous 
understatement!! ;-)

Peace,
Sarah


----- Original Message ----- 
From: "David Love" <davidlovepianos@comcast.net>
To: "'Pianotech'" <pianotech@ptg.org>
Sent: Wednesday, February 16, 2005 2:22 AM
Subject: RE: More CC vs RC questions was RE: Killer Octave & Pitch Raise


> One more thought, for now.
>
> We already know about things that could go wrong with a CC board.  Here
> I'd like to speculate about something that could go wrong with a RC&S
> board, and I'd appreciate any comments.  Typically, as I understand it,
> the RC&S panel is dried down to about 6.5% EMC so that the panel
> undergoes relatively less compression and the panel itself is therefore
> somewhat less stiff.  If for some reason, the panel, say, were not to be
> dried down adequately, then the stiffness of the assembly could still be
> achieved because the ribs, bearing most of the load, could still be
> compressed and bear much of the load.  All seems fine.  But because the
> panel was not adequately prepared, the stiffness normally achieved by
> the panel itself would be somewhat less and its contribution to tone
> production would be altered.  A softer panel with altered impedance
> characteristics might require a softer hammer with its own upper
> partials dampening effects.  The softer panel itself might also have the
> effect of dampening the upper partials to some degree.  This combined
> effect of softer hammer, softer panel, might produce a tone that while
> having adequate power (loudness) might not have the balance of partials
> to make the tone interesting, lively, expressive, you choose your
> favorite word.  So control of the panel's EMC, while seemingly not as
> critical for purposes of achieving stiffness in the assembly overall,
> might still be very critical in order to achieve a certain tonal
> balance.
>
> The next question:  How much stiffness is required in the panel itself
> to produce a desirable balance of partials or will cause the necessary
> use of a hammer of enough firmness and density to accentuate the upper
> partials the degree necessary.  Another way to say it is: producing a
> soundboard which requires only a very soft hammer to drive it may limit
> the expressive quality of the instrument.
>
> David Love
> davidlovepianos@comcast.net
>
> -----Original Message-----
> From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org] On
> Behalf Of David Love
> Sent: Tuesday, February 15, 2005 10:33 PM
> To: 'Pianotech'
> Subject: RE: More CC vs RC questions was RE: Killer Octave & Pitch Raise
>
> Actually, I think the RC&S board doesn't seem to require as bright a
> hammer because the panel is less stiff (not the assembly, but the panel)
> and therefore the impedance characteristics are different.  I'm not sure
> that a softer hammer will produce "sparkly highs" it's just that it
> doesn't take so much of a hard hammer to get the volume out of the
> instrument.  That fact that the hammer is softer may, in fact, reduce
> the sparkly highs as a softer hammer tends to damp higher partials, and
> produce the impression of a narrower or more focused tonal spectrum for
> the same volume.  Is that not possible?
>
> David Love
> davidlovepianos@comcast.net
>
>
>
>
>
> Regarding your second question...  If the RC/S board doesn't require as
> bright a hammer to produce sparkly highs, perhaps the voicing technician
> has
> more latitude to voice the hammer to emphasize the fundamentals.  The
> result
> of the RC/S design may well be a richer sound spectrum.  Of course that
> would be my apriori assumption about any design that is more efficient.
> In
> answer to your question, I suspect an RC/S board would be more capable
> of
> producing whatever tonal qualities the voicing technician wished to
> achieve,
> whether it is a darker sound (stronger fund. and weak overtones) or a
> brighter sound (weak fund. and strong overtones).  My point was/is that
> the
> more rapid changes in spectral content throughout a note's sustain on a
> CC
> board might result in a desirable expressive quality to that design.
>
>
> Peace,
> Sarah
>
>
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