I gave these definitions in 991010 MMD:

>[ tremolo - the tone (frequency) is varied, but the amplitude is
>[   mostly constant.
>[ vibrato - the amplitude is varied, but the frequency is constant.

MMDers are already writing to me expressing disagreement; please let me
know where up-to-date definitions are published by music educators and
the music industry.  In the meantime I'll stick with the old Italian
meanings which are taught at music schools and which have appeared in
printed classical music for hundreds of years.

 - - -

Vocalists in different lands sing with differing amounts of tremolo
or vibrato.  I've heard church and opera soloists who seems to quaver
up-and-down at least a whole note, and I've also heard soloists with no
quavering (notably in Russian men's choirs).  On the radio one can hear
the fast quaver of the Middle East, or the slow quaver of a blues
singer.

The violinist moves the finger to-and-fro along the fingerboard to
create a tremolo in imitation of the human voice.  The movement of the
finger can be broad or narrow, fast or slow.  I don't think it matters
if tremolo in a mechanical violin is created by altering the length of
the violin string or by altering the tension.

The steel guitar (Hawaiian slack-key guitar) has no frets, and the
speaking length of the strings is adjusted with the heavy steel bar
held in the left hand of the player.  He can also create a nice tremolo
by wiggling his hand.

Among the wind instruments, the trombonist moves the slide of the
trombone to change the length of the resonant air column.  Reed instru-
ments are different.  The long conical chamber of the saxophone is only
broadly resonant, and so the saxophonist can produce tremolo by changing
the pressure on the vibrating reed at the mouthpiece.  In contrast,
it's quite difficult to make a tremolo on the clarinet of the symphony
orchestra, wherein the long cylindrical air column seems to have more
control of the frequency than does the vibrating reed.

The first mechanical (or mechanically aided) instrument to produce a
vibrato (varying sound intensity) is possibly the Deagan Vibraphone,
played by jazz artists like Lionel Hampton and Red Norvo.  This
instrument resembles a marimba built entirely of metal, with metal bar
chimes (like a glockenspiel) and metal tubular resonators.

Deagan fitted a motor-driven rotating butterfly valve at the top of
the resonant tube, such that the acoustic coupling between the vibrating
bar and the resonator is continuously varied.  The vibration of the
bar itself is unaffected, but the rotating valve at the open end of the
resonator greatly affects the audio energy transmitted to the listener.

Robbie Rhodes