Vibrato/Tremolo in Leslie Speakers

[Barry Schaede <barrys@xxxxxxxxxxxxxxxxxxxxxxx>]

Mike Curtis' recent post on vibrato mentioned Leslie speakers as source 
of true vibrato.  While this is somewhat true it's not in an absolute 
sense.  The sonic interactions occurring in Leslies are more complicated than
mere frequency modulation.  What follows is an excerpt from a Hammond 
organ/Leslie FAQ.  The explanation is excellent so rather than attempt to 
reiterate the material in my clumsy way I've just uploaded it to the 
list.  This FAQ was provided to me originally by Harv Andruss of our list 
and is an excellent example of the behind the scenes resource sharing 
that can occurr off list as a result of the synergy of the list.  FJM

THE LESLIE TONE CABINET "Pipe Voice of the Electric Organ"

The Leslie Rotating Speaker, named after its inventor, Don
Leslie, is  designed as a sound modification device, not a hi-fi
speaker.  The  pairing of the Leslie Speaker with another device,
usually a Hammond  organ, constitutes a musical instrument.  It
operates on a simple  principle; a directional sound source
rotates at constant (or  variable) speed around a fixed pivot
point.  The effect at the  listening location, some distance
removed, is quite pronounced. The  characterization of a Leslie
Speaker in an acoustically reflective  listening area is a
complicated proposition at best but at least four  effects are in
operation: amplitude modulation, frequency modulation,  timbre
shift, and apparent motion of the sound.  Since the sound source
is directional, the intensity of the sound to  the listener (or
microphone) is dependent upon, at least, the angular  position of
the rotating sound source. The intensity varies as the  sound
source rotates and the listener perceives a periodic modulation 
of the sound as a function of the rotational speed. This is the 
amplitude modulation (AM) component of the sound and when the 
listening position or microphone is placed closer to the sound
source  it will, in general, increase the AM component of the
sound.  The sound source when rotating is periodically
accelerating toward and  decelerating away from the position of
the listener.  This imparts a  Doppler shift on the source
material and thus a frequency modulation  (FM) to the sound.  As
in other Doppler induced pitch shifts the pitch  is perceived to
rise as the source moves toward the listener and fall  when the
source moves away from the listener.  The directional pattern of
the rotating component is frequency  dependent.  High frequencies
exhibit more beaming than do lower  frequencies, which are
emanated in a more omni-directional pattern. A  shift in timbre
is perceived as the angular position of the sound  source
changes.  The treble component is generally strongest when the 
rotating component is pointed at the listener and weakest when it
is  pointed away.  Finally, due to the multiple reflections of
the listening area and the  rotating sound source, the sound
appears to emanate from multiple  locations imparting a sense of
motion to the sound.