Klarinet Archive - Posting 000407.txt from 1997/03

From: Adam Calabrese <adam.calabrese@-----.gov>
Subj: Acoustic effects (was: Re: Pre-WWII Amer. Rep.)
Date: Thu, 13 Mar 1997 11:35:12 -0500

At 09:47 AM 3/13/97 -0600, Gary Young wrote:
>I performed the Cage Sonata once on a hot summer day, and
>found myself seated directly under a rotating ceiling fan which people
>wouldn't let me turn off (too hot, no air conditioning). The fan imparted
>a strange slow vibrato-like effect to the clarinet sound. (I don't
>understand the acoustics of this.) At first I was irritated, but then I
>realized this was the sort of thing Cage would have loved, and really got
>into the sound.

As a tangent, I'd like to take a shot at explaining this...

Vibrato on say, string instruments, is caused by rapid changes of pitch.
Using this definition, the most likely cause for the vibrato was a Doppler
effect from the blades. The Doppler effect arises when either a sound
source, the receiver, or the medium (through which the sound propagates)
moves. In your case, the blades of the fan were acting as a secondary
source via reflection. Because they were moving when the sound reflected
off of them, the fan blades caused the wavefronts to be closer together
(when moving toward you) or farther apart (when moving away from you). This
caused a frequency shift (higher or lower, respectively) in the sound waves
reflected off of the fan. It was the addition of primary (clarinet) and
secondary (reflection off of fan) sound sources by the stationary audience
(and performer) that gave the vibrato effect.

Vibrato in voice can be explained as the "rapid reiteration of emphasis on
a tone" (so my dictionary says), and one might think that phase
cancellations from reflections off of walls (of the same frequency tone)
could cause that effect (phase cancellations would cause the amplitude to
very between a max and a min). But the problem is that there are so many
stationary walls and objects at different distances, causing different
phase shifts for the sound that reaches one's ears. Because of this, the
relected sound is most likely not high enough at the correct phase (180
deg) to cancel the primary source.

Sorry for the side track. Back to regular clarinet talk....

-Adam Calabrese
National Institute of Standards and Technology
Gaithersburg, MD 20899