brass v. iron

Ron Nossaman nossaman@southwind.net
Thu Dec 3 13:10 MST 1998


At 11:32 AM 12/3/98 -0500, you wrote:
>In a fortepiano recital here the other night, the room heated up quickly
just prior to and during the concert. It was already 80 degrees (!) by the
beginning, and heated up more as the audience came in. The piano had had
plenty of tuning attention before hand and was as solid as possible.  But
when I checked the piano at intermission, unisons were generally fine, but
all the bass octaves which involved brass wire were remarkably expanded
(flat 2 or 3 beats) and I had to scramble to attempt to bring as many up as
time (and the crowd noise) would allow. The second half was better because
(I suppose) the temperature topped out and leveled off. the fans were even
off - it was ugly, a full house in a small hall.
>
>My organ builder/technician friend Hal Gober mentions the "coefficient of
thermal expansion" of the differing metals involved here (brass in the bass
and iron in the mid and treble). Is that in fact why this happened - are
they that different? I don't know my metallurgy very well. Just curious if
anyone out there has the technical knowledge to verify this.
>
>Yet one more rude reminder (as if I needed any more) about how sensitive
and temperamental fortepianos can be.
>
>Allen Wright
>Oberlin College
>


Allen,

My Machinery's Handbook lists the linear expansion per unit length per
degree fahrenheit for brass at 0.00000986, and for steel at 0.00000636.
According to this, brass expands at a rate over 1 1/2 times that of steel.
Part of the bass drop is probably accounted for here, the rest being wood
and plate movement. It has a higher specific gravity too, about 8.4 as
opposed to 7.8 for steel, which was why it was used in the bass in the first
place, no? As to determining what accounted for what in this instance...
good luck.  

 Ron 



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