Voicing question

[Delwin D Fandrich]

Jim Coleman, Sr. wrote:

> Just recently (a week ago) I installed a "wet" humidistat which centers
> humiditiy at 48% RH instead of the usual 42%. My customer who has an
> unusually fine Yamaha CF about 20 years old but rebuilt is always looking
> for more power and carrying power. To our amazement the piano went out
> of tune but the carrying power increased another major amount. My customer
> had suspected that the tone got better when we had rainy seasons. I think
> I'm convinced now. The customer momentarily is ecstatic.
>
> What do some others of you think who have experienced this kind of change?
>
> My present thinking is that the board has pushed up a little stronger
> against the strings and that the impedance between strings and board has
> changed enough so that there is more resistance of the board to the
> energy of the string's downward pressure and that this causes the duration
> to increase, but I can't understand the greater power also.
>
> Jim Coleman, Sr.

-------------------------

Jim,

It would take a really huge increase in actual acoustic power to be even barely noticeable
by the listener. There remains, however, the perception -- the "illusion" -- of power.

You're right, by adding moisture to the board, the wood fibers have swollen a bit, and
this has increased the springiness of the soundboard assembly. This changes increases the
"springiness" component of the impedance equation and raises the overall impedance load
presented to the strings. This is the general explanation given to account for the
increase in sustain in these situations.

Working in the "power" increase might appear to be a bit more abstract. I can think of two
possible explanations. The first, and, in my view the most likely, is based on the notion
that the soundboard assembly's impedance load as presented to the strings depends on both
the mass and the elasticity of the soundboard panel. This impedance load does not vary
directly with frequency, however. Depending on the actual frequencies of the energy in the
strings that is trying to move the soundboard, that impedance load can appear to be either
mostly massive or mostly elastic. By increasing only the springiness of the soundboard,
the impedance load presented to the low frequency energy in the strings will increase
somewhat more than it does to the higher frequency energy. This has the effect of
increasing the sustain at the lower end of the frequency spectrum somewhat more than it
does at the higher end. The result of all of this will be that the relative power and
sustain of the individual harmonics within the soundwave envelope produced by the
soundboard will now have more energy content in the lower frequencies. Audibly, this would
give the effect you are hearing. But, it will be just an illusion. Measured with a sound
pressure level meter, there probably is no real increase in the overall power production
of the piano.

This is why soundboards that have lost a substantial amount of their crown to compression
set give off a sound that is percussive and of short sustain. The lower frequencies are
transferred very rapidly into the soundboard and are dissipated very rapidly as sound --
well, some is eaten up as heat due to the friction of the assembly -- leaving only the
higher partials with any energy left in them to sustain its vibrating motion. Since there
is not a whole lot of energy in these higher partials to begin with, it is also quickly
dissipated. This is also why it is not very practical to attempt to "restore" the
compression-crowned soundboard in which crown has collapsed. No matter how good you can
make it look by patching, shimming, etc., there is really no effective way to restore the
requisite springiness that is needed to balance the impedance equation. (For more on this
see the series on soundboards just completed in the Journal)

By the way, when I refer to the "illusion of power," I don't mean to imply that either you
or your client are making any of this up. I mean that when a pianist does not have to work
as hard to achieve the the dynamics and the power level that he or she is striving for
with any particular piece of music, they feel that the instrument they are playing has
good "power." I suspect that the owner doesn't have to work quite as hard to make music
right now.

The second -- and, in my opinion, much less likely -- scenario that I can think of is that
the added humidity has created enough of a change in the characteristics of the hammer to
be audible. I'm assuming that this is a Dampp-Chaser humidistat you're referring to here
and surely it is isolated from the action and the hammers by a belly rail and a keybed.
Anyway, I suppose that it is conceivable that with a higher ambient humidity level, the
hammer could be slightly more massive, though I can't imagine that this increase would be
enough to actually make any audible difference by itself. More possibly, the added
moisture might have made the hammer felt slightly more elastic -- slightly softer, if you
will. This would result in a sound that would be a bit less harsh and strident. I.e., the
hammers would excite the lower partials in the strings just a little bit more than they
did before, adding to the illusion of power.

Assuming that the first explanation is the more likely of the two, the improvement in
performance you're hearing now will be short-lived. Compression set has already reared its
ugly head, and, short of replacing the soundboard,  there is not much you're going to be
able to do about it. You're client should enjoy the extra performance while it lasts.

Regards,

Del