Ceiling Fans

[Bill Ballard]

On 8/1/96, Ronald R Shiflet rote:
<<it needs to be brought up more often.  I tuned a piano 2 weeks ago
that was so close, I was able to spend my time fine tuning.  Last week
the owner called back to complain that it sounded terrible.  I asked her
to first check the sound with the fan on and then off.  This has happened
before.
	I wonder how many people get frustrated with us and go elsewhere
when the fan is the real problem.  A good lesson in communication.
>>

A strange but true phenomenon. Kent Weeb wrote about it in Baldwin's
"Sounding Board". I recounted my own acquaintance with it in the 2/91
"Granite Action" (NH Chapt):

"Close Encounters of the Salad Master Kind"

I knew something was wrong as soon as I started in on the tuning of this
5-year old Yamaha U1, and I could already feel condolences for its
owners. The tone in the third and fourth otcaves was anemic, washed out.
Worse yet, a subtle flutter was showing up in the middle partials (3d
through 7th) on notes in this region, maybe one or two partials per note.
The partial number affected changed from note to note, and this flutter
was independent of the tunings usual beat rates. Was this the reason I
was called in on a pianos not badly out of tune? I listened around the
room for other sources of pitch which might be interfering with the piano
sound. The loudest electrical appliance was the ceiling fan which
whispered innocently.

I could still tune and so I did, until halfway through when I decided to
shut off the fan, just on a hunch, mind you. Like magic, the sound turned
froma cold drizzle to warm sunshine. This was one for Dr. Science, my
neighbor, who is a black belt, 12-star radio engineer with a labor rate
for servicing the region's electronic facilities twice my own. He's never
forgotten a word he's read, and his hearing is so acute that on a visit
to NYC American Museum of Natural Science's gem room, he compained to the
guard that the ultrasonic security system was giving him a headache.

Yes, Ira knew. Sound transmitted by or received from moving objects
produces a Doppler effect in which frequency or phase is modualted. For
all practical purposes, I the listener was stationary, as was the plane
in which the turning fan blades moved. However the blades themselves were
attatched to the hub with a slight pitch so as to push air. Consequently,
sound from the piano was faced with a constantly changing distanceto the
blades which would reflect them. Had the fan been unnecessarily fancy,
the blades might have had a curved rather than flat cross section., which
would have made theDoppler modulation non-linear. Regardless, the phase
modulations were a product of the distance fromthe piano to the blades
(fluctuating periodically within a fixed range), the motor's rpm, and the
number of blades (these latter constant). All that was left was for these
factors to combine in consonance with some partial on the note currently
played. And I had noticed that as I moved from note to note the flutter
would roost on a different partial (or partials). Shut off the fan and my
headache was gone.

Bill Ballard RPT
NH Chapter

"No one builds the *perfect* piano, you can only remove the obstacles to
that perfection during the building."    ...........LaRoy Edwards