Hi Robbie -- Understand that I'm speaking from sixty years of musical
background.  I started playing the pipe organ when I was eight years
old and I'll be seventy next month.  I've spent a lot of time talking
with other musicians as well as studying music.  This is what I believe
most true musicians will agree with:

  vibrato = a pitch variation of a tone;
  tremolo = an intensity variation of a tone;
  tremulant = a combination of the pitch and intensity variation
    due to a single cause.

This is not the same as a vibrato and tremolo caused by two different
means, such as can be found in some Leslie tone cabinets.  In this
case, particularly with sound from the tone wheel generator models of
the Hammond organ, the frequency is altered by a "vibrato scanner"
which alters the phase relationship to create a Vibrato effect which,
when routed through an external Leslie tone cabinet, is further altered
by the rotational movement of the tone source by a set of special
loudspeakers, at least one of which also produces the so-called Doppler
effect.

In many cases, the speed of the rotating speaker, or speakers, is
controllable to the extent that more than one combination of vibrato
and tremolo can be obtained.  Since the two are not produced in
synchronism, I would expect them not to qualify as a tremulant.

Mr. Donald Leslie obtained patents on this system and formed the
Electro-Music Company which for many years manufactured the "Leslie"
tone cabinets and in-built systems as well.  Interestingly, he first
called his system "the Vibratone", and they also came to be called
"Hollywood" speakers due to their place of manufacture.

Here's an interesting observation: if you are directly above a single
Leslie tone cabinet you will hear little, if any, Doppler effect.  This
is due to the rotation being about a vertical axis, so that if you are
in-line with that axis the effect will be lost on you.  This is not the
case with a true vibrato or tremolo or tremulant.

It is not necessarily true that a vibrato will produce an intensity
change, as I can demonstrate with an oscilloscope.  It is likewise
provable, by using my Conn StroboConn or StroboTuner, that a tremolo
does not necessarily produce a vibrato effect.

There is one other effect that has been mentioned but not correctly,
as I understand it.  That is iteration, or re-iteration.  This musical
effect is commonly found on music rolls for pianos and similar instru-
ments.  It is sometimes referred to as "mandolin effect", or "banjo
effect in the case of Nelson-Wiggen and some others.

Obviously, a piano does not lend itself well to either vibrato or
tremolo, so the creators of music for the piano often used iteration
and re-iteration to achieve a pleasant-to-the-ear effect.  Trills were
often used similarly.  These effects help maintain the apparent level
of a particular tone rather than having it decay, as is the normal
effect of a piano tone once struck.

That's the way I see it.

Hal Davis