[pianotech] SWELL was What is bloom

[David Love]

As some have said, the damper swell that JD describes seems to happen in all
pianos.  It's just a matter of degree and whether the recycling produces
enough energy to further move the soundboard and thereby add to the tonal
picture.  The last three pianos I tuned today are good examples.  A crappy
Kimball spinet had the effect some and the Boston Grand had virtually none
and an older Steinway O had more than the Boston and about the same as the
Kimball.  None produced an outstanding amount.  Once the energy gets
recycled the ability for the soundboard to respond to the secondary energy
input must depend on various factors.  One might be its mass or stiffness
that inhibit it from moving in the first place.  The Boston is a pretty
heavy assembly judging from the scale and thickness of the panel at the
belly rail.  The other might be the amount of potential energy in the
system.  The old Steinway O has a fairly lightweight system so you might
expect that it would move significantly more with the same kind of input,
but the difference wasn't that significant.  It may be that the internal
tension in that Steinway doesn't exist anymore (due to age) and so the
potential energy ready to be unleashed by even a small input of energy just
isn't there.  Some combination of those two factors are at play in any
system and might be a factor here.  

With respect to bloom (the other subject) I think this is something
different though it might be related to the way the system is operating in
some way.  Imagining what it would look like graphically we typically think
of the tone output graph as an attack spike followed by a drop in amplitude
followed by a decay which takes the form of some exponential curve--the
shape of the curve and the rate may vary.  If we imagine what bloom would
look like, there would need to be a slight increase in the amplitude
following the first drop from the attack phase and then a decay curve.  If
you think of that curve in terms of energy then it's not possible since no
energy is added to the system at any point.  But if you think of that curve
based on the amplitude of some given frequency or frequencies then it is
possible since you may have the coupling effect of the same frequencies
coming together from different sources.  So why would that happen?  It might
happen because the chaos phase of the soundboard produces the same
frequencies but in parts of the board that are reacting out of sync with
other parts of the board.  When they finally coalesce into some unified
movement you get a rise in the amplitude of some of those frequencies that
you hear as bloom or swell, whatever you want to call it (forgive my lack of
technical vocabulary in explaining the physical process==hopefully others
will supply that missing piece).  

So if that's true, that it's something like that, it has some interesting
implications.  It would require that the system be able to go into a chaotic
phase of some type enough to create a disparate set of functioning but like
frequencies that eventually become unified and effectively raise the
amplitude of those specific frequencies.  A very tightly controlled system
in perfect balance that could not be driven into that initial imbalanced
chaos phase might not then have the ability to do that.  The tone would
appear very controlled and clear but not have that sense of liveliness or
swell or bloom or whatever you want to call it.  It is interesting to note
that some of the redesigns that I have been involved with or heard that use
large cut off bars, more centered bridge location, stiffer assemblies have
these characteristics of control and clarity (good qualities) but do lack
that bloom. They are more straight line decay without that liveliness or
bloom suggestive of a soundboard system that for a brief moment when the
energy is first input into the system struggles to get everyone on the same
page, so to speak.  Now I can't say whether that goes with the territory or
whether that's a foregone conclusion or whether I've described, albeit
intuitively, what is actually taking place, but I sure wonder about it.     

David Love
www.davidlovepianos.com