[CAUT] Re: "Mind Bender", etc.

[James Ellis]

I have been out of town for the past 10 days, so I'm late responding to
Michelle Stranges @ oswego about her temperature-and-tuning "mind bender".
Don't be embarrassed, Michelle.  What happened was exactly what I would
expect to happen under those conditions over which you had no control.

Metal does expand as it warms and contracts as it cools.  However, the
steel of piano strings and the cast iron of piano plates have almost the
same coefficients of thermal expansion - not exactly the same, but very
close.  The coefficient of thermal expalsion of most woods along the grain
is less than most metals, but across the grain, it is much more.  I have
not yet mentioned humidity.

While the air temperature is going up, the piano will go flat, because the
strings are exposed to the air, and they are warming much more rapidly than
is the heavy plate with lots of mass.  When the piano stabilises at the new
temperature, it will be back on pitch, or nearly so.  When the temperature
is going down, the strings - exposed to the air - will be cooler than the
plate, and the piano will go sharp.  When it stabliizes at the new
temperature, it will be back on pitch again, or nearly so.

Now comes relative humidity, but that is a much slower process.  The
changing temperature effect is immediate.

Now for the harpsichord.  There's no iron plate here, so it's the thremal
expansion of the strings - vs - that of the wood, and it's going to depend
on the grain orientation.  But what you can depend upon is for the strings
to change immediately with temperature, and the wood to lag behind.  The
change in the effect of relative humidity will lag behind that for several
hours or days.  If you have hot stage lights, that is one more thing to
de-stabilize any tuning.  Not only does the radient heat from the lights
heat the strings and plate unevenly, but it works on the soundboard to make
it expand across the grain from the heat at one rate, but contract across
the grain from drying out at a different rate as the crown is doing crasy
stuff.

Now, you have a hot aid heating system that cycles on and off, and I think
you get the picture.  You are faced with changes going in different
directions at different rates.  When I'm faced with situations like this, I
try to have everything stabilized for several hours at the temperature and
humidity where it's going to be, and then shut everything off while I'm
tuning, so the system does not cycle on and off.

These are no-win situations.  You can get lucky and get the two pianos in
fairly good tune with each other, but the harpsichord is going to go off in
a different direction, and there isn't much you can do about it.

Sincerely, Jim Ellis