The answer to this question (http://www.straightdope.com/mailbag/mvibrato.html) was pretty good but some of the details are sketchy. The repeated use of the term “tremelo” is the first thing that bothers me. You describe tremelo as being intensity vibrato, while in fact tremelo is actually a rapid alteration of pitches, usually played by the strings in an orchestra. This can be confirmed by any common, non-music, dictionary. When I play saxophone, I use a pitch vibrato, as is tradition. When, I play flute, an intensity vibrato is appropriate. Singers use an intensity vibrato. A good vocalist will tell you that if you’re doing everything else correctly, then vibrato is natural. It may also be important to note that when using pitch vibrato, you only go below the pitch, not above.
-Travis, jtp8736@tntech.edu
Here’s the link.
http://www.straightdope.com/mailbag/mvibrato.html
Cecil Adams doesn’t claim to be an expert in musical terms–he’s just the smartest man in the world.
The details are sketchy because it’s intended to be an amusing, general interest column, not a doctoral dissertation.
BTW, it’s spelled “tremolo”.
“Why, sometimes I’ve believed as many as six impossible things before breakfast!” - the White Queen
And it isn’t Cecil, of course; the column in question was written by Ian of the Straight Dope Science Advisory Board, Notthemomma. But you seem to have been having a bad day today. 
You’re right, DS, I did have a bad day, but in my defense I would like to say that not only was JTP’s post a pointless technical quibble, it was also wrong.
First, I don’t see why we should have to ask a “non-music” dictionary to define a musical term. Nevertheless, here is Merriam-Webster on the subject. They seem to be under the impression that tremolo is produced by either alternating pitch or increasing and decreasing volume. They also use the word “vibrato” in their definition of “tremolo” and use the word “tremulous” in their definition of the word “vibrato”. Feels like they’re going around in circles, but then they aren’t aiming for a technical, musician’s definition–they’re only interested in the general conversational definition, in which “vibrato” and “tremolo” are quite often used interchangeably, as Ian’s column points out.
So now let’s go to a technical, musician’s definition. Here are two definitions from the sweetwater.com-sponsored Word for the Day dictionary: http://www.sweetwater.com/insync/wftd/wftd_t.html
I play the recorder and flute; my sister plays the clarinet. She also sings in the Sweet Adelines, which is the lady barbershoppers. We know a lot of string players.
Tremolo is not a “rapid alternation of pitches”. Tremolo is a rapid alternation in the volume of sound. When string players play “tremolo”, they alternate pressing harder and softer with the bow while holding the left-hand finger steady on the same note, to produce a variation in loudness. When string players play “vibrato”, they keep the bowing pressure steady on the note, but they vibrate the left-hand finger on the string, producing a variation in pitch.
When you’re playing the flute, what you call an “intensity vibrato” is actually a tremolo. You are breathing in and out harder with your diaphragm to make the sound come and go. You are not forcing the pitch up and down by changing the shape of your embouchure rapidly. You are thus actually creating a tremolo, not a vibrato. It is very difficult to change the pitch on a side-blown flute to begin with, and if you were trying to create a “vibrato” effect by shifting your embouchure so quickly, you’d have a cramp in your whole face almost immediately.
On an end-blown flute, also known as a “recorder”, it is completely impossible to change the pitch at all with your embouchure, as the mouthpiece functions as a permanent embouchure. Therefore, all “vibrato” effects that are produced on a recorder are, by definition, “tremolo” effects, produced by changing the pressure of the column of air supported by your diaphragm. (By the same token, if your recorder is flat, you have to overblow to bring it up to pitch. Recorders that are sharp are hopeless. You can bring a side-blown flute up to pitch by tightening your embouchure and blowing more sharply into the hole.)
When you’re playing the saxophone, a reed instrument which functions like a human larynx, what you call a “pitch vibrato” is in fact a true vibrato. You press on the reed with your lips and embouchure, forcing it to vibrate differently to produce the “vibrato” effect, supporting it from a column of air from your diaphragm. The pitch is going up and down, your breath is also going in and out, so I suppose it’s a sort of combination of vibrato and tremolo. A larynx vibrates the same way. The vocal chords vibrate, being supported by a column of air from the diaphragm. So I suppose that an opera singer’s “vibrato” could be considered as a sort of combination of “vibrato” and “tremolo”. But in technical terms, it’s generally referred to as “vibrato”, because of the “vibration” of the “reeds” of the larynx.
The “tremolo” stop on an organ produces a flute-like tone which alternates in volume, in a rather unearthly way. The “vox humana” stop produces a reed-like tone which alternates in pitch, also in a rather unearthly way.
“Why, sometimes I’ve believed as many as six impossible things before breakfast!” - the White Queen
Upon further research, I concede the fact that tremolo can also mean excessive vibrato. However your statement that it is not the rapid alteration of two tones is absolutely wrong and it is in fact the more common usage of the word.
from Dictionary.com:
trem·o·lo
n., pl. trem·o·los. Music
A tremulous effect produced by rapid repetition of a single tone.
A similar effect produced by rapid alternation of two tones.
A device on an organ for producing a tremulous effect.
A vibrato in singing, often excessive or poorly controlled.
Furthermore, it is not only very easy to change the pitch on a “side-blown flute”, it is essential. Playing a wind instrument requires adjusting the pitch of certain out of tune notes. The fluctuation of intensity on flute IS vibrato. Masters such as Galway and Rampal aren’t wrong! The vibrato on saxophone doens’t include any change of intensity as you described. The air stream is constant and certainly not inward. I’m not saying this trying to cause a conflict. As a professionally trained musician, I just want to clarify the meanings of these terms. This isn’t something I just started looking into, but something that I have been studying for years. Your are now more than welcome to flame me and call me names.
Notthemama’s definitions conform to the ones I’m familiar with.
The rapid alteration of two distinct pitches, without portamento between them, is what I would call a trill.
Microtonal repeated variations in pitch around a certain note I would call vibrato, although I know a lot of guitarists refer to their “Whammy Bar”-type vibrato arms as “tremolo arms”, so obviously there is some variation in the usage.
I’ve never heard vibrato applied to variations in volume.
But I’m no musicologist.
Close. It is a trill if the notes are a minor or major second apart. In Baroque music the trill would be approached from the top pitch. Otherwise it would start on the written, or lower, pitch. There is a great description of vibrato in Kyle Horsch’s essay “Saxophone techniqe” in "The Cambridge Companion to the saxophone. Claude Delangle, professor at the Paris Conservatory also describes pitch and intensity vibrato in the same book. 
Don’t worry, JTP, I never flame musicians, just on general principles. And you have to do a lot worse than disagree with me on a simple technical point to get called names.
But if you want, we can repair to the BBQ Pit and I will dredge up all the nasty things I can think of to say about saxophone players, and you can insult my mother, and we can just have ourselves a high old time! 
[Note: this is a joke, OK?] :rolleyes:
I’d just like to say that personally I don’t have much use for dictionaries that use the very term they’re defining in its definition. I had a whole succession of English teachers who wouldn’t allow me to get away with that; why should dictionary.com and Merriam-Webster?
It has already been pointed out that the terms “vibrato” and “tremolo” are commonly, if mistakenly, used interchangeably. You can go ahead and call it “vibrato” or “tremolo” or “pitch intensity” or “that thing where it goes in and out”, whatever you want, it’s fine with me.
I don’t want to get into a big esoteric musicological debate here. Life’s too short, you know?
Bye.
“Why, sometimes I’ve believed as many as six impossible things before breakfast!” - the White Queen