Vibrato
Vibrato and tremolo are two terms that are often used, misused, and interchanged, and different people have different ideas about the distinctions and similarities between the two terms, techniques, and effects. I have decided to take the essence of these definitions from the "Schaum Dictionary of Musical Terms". They are:
Tremolo is pulsations of air pressure that cause a wavering effect to the loudness of the sound.
Vibrato is slight fluctuations of pitch produced by a slight wavering movement of part of the embouchure affecting the air stream.
The effective difference between the two terms is that tremolo refers to loudness and vibrato refers to pitch. Tremolo is oscillations in volume and tone, vibrato is oscillations of pitch.
In practice, however, both vibrato and tremolo can be applied to the same note at the same time. Then what do you call it? I call it vibrato. In my experience, the term vibrato is used more than the term tremolo for the harmonica to describe both of these techniques (whether used at the same time or not). I'll use "vibrato" whenever a pitch change is present, and tremolo when only volume oscillations are present, the pitch staying constant.
In addition, on the harmonica it is possible to produce oscillations of tone or timbre as well as volume and pitch by manipulating, principally the hands and tongue. This effect can affect volume in addition to timbre, and so becomes more difficult to properly classify. For example, hand effect "wah wah"s are commonly heard on the diatonic. Each "wah" is an opening of the hands, which results in a louder note with a different timbre, but essentially the same pitch. Since there is no pitch change present, I classify this effect as a tremolo.
Effective utilization of vibrato and tremolo is one of the most crucial elements of good tone.
Diaphragm Tremolo
Diaphragm tremolo, as the name says, is a wavering of a note caused by playing pressure fluctuations created by gently pulsing the diaphragm to produce volume oscillations.
If you gently say "ha ha ha" you can feel a pulsing of your diaphragm. It is this motion that drives the diaphragm tremolo.
If you say "ha ha ha" while breathing in you may feel a tugging or pulsing at the back of your throat. To keep the tremolo without pitch variations, open up your throat to keep the air stream from being pinched. If you pinch your throat to fluctuate the air stream, you are probably adding some pitch wavering, which means you are using a throat vibrato.
Throat Vibrato
Throat vibrato is one of the most important diatonic harmonica techniques to master. It adds color and variety to your notes in the same way that it does for a singer, and it is intimately associated with good tone. Throat vibrato may be the most emotional of the vibratos and tremolos because it can engage the diaphragm to make use of both pitch changes as well as volume changes.
Throat vibrato is primarily associated with draw notes on holes 1-7. The diaphragm is used to provide the playing pressure, the throat is used to smoothly pulse the air stream, and the throat works with the tongue to control the note pitch and the depth of the pitch oscillations.
The key to throat vibrato is a smooth oscillation of pitch and volume.
The classic way to get throat vibrato is to imitate a rapid fire machine gun (eh eh eh eh eh), like when you were a kid. Then do it inhaling instead of exhaling. It's the same throat motion that gets the throat vibrato. Work on it until all the chunkiness is gone, and it sounds as natural in your play as it does in a singer's voice. There is a tendency for throat noise to be heard during aggressive throat vibrato, especially when a mic is used and the sound is amplified. You should record yourself and listen to determine if the throat noise is audible or intrusive, and seek to minimize it.
A good way to get the feel of throat vibrato is to try put vibrato or even tremolo on the 3 draw 1/2 step bend. Your throat is involved getting the bend initially, so there's some feel there before you go for the vibrato. Breathing from the diaphragm will help control the vibrato. There's always an interaction between the throat and diaphragm when doing a vibrato, since each is involved in controlling the air stream.
For throat vibrato, obviously the emphasis is on the throat--but you'll probably notice some involvement of the diaphragm as well. As you pulse the diaphragm for tremolo on this note, the change in playing pressure coupled with the throat pulling the bend will tend to add a pitch fluctuation vibrato to the diaphragm tremolo.
Try playing the 3 draw 1/2 step bend amplified, but play softly. Practice very soft draw bends. At some point you'll notice what feels like a direct connection between your throat and the note. Every little nuance of throat motion will be reflected in the sound. Work on playing softly, with good note formation and support from your diaphragm. Then put some vibrato on the note by pulsing the air stream with your throat. The pitch will change up and down because you've got a hold of the bend with your throat. Keep at it, it's worth it!
How much should you use throat vibrato? Different players like different things, so it all depends. Often it is good to emulate what singers do, and start a note straight, then add vibrato toward the end of the note. Some players think there's no such thing as too much vibrato, they love it that much. Other players like to use it more sparingly, to add variety and contrast. I think you should use it a lot, but not all the time. Throat vibrato is one of the best things you can do to improve your tone, your control over the notes you play, and your focus on the connection between your playing and the response of the reeds.
Vibrato on Draw Bends
Vibrato can be placed on draw bend notes, either full bends or intermediate bends. Smooth control of this vibrato makes use of a subtle, coordinated interaction between the throat, tongue, and diaphragm.
The diaphragm provides the playing pressure support for the note, while the tongue and the throat work together to produce the desired pitch. In the case of bent draw notes, given a stable embouchure mouth, tongue, and throat position, the pitch of the note can be changed by changing the playing pressure.
A gentle pulsing of the vocal tract below the throat can be caused by a nearly imperceptible pulsing of the diaphragm. Any more than VERY gentle nearly imperceptible movement of the diaphragm can cause an unacceptably large chunky vibrato that isn't smooth.
It is also possible to hit the bend using the throat and back part of the tongue to produce the pitch. This leaves the front part of the tongue free to manipulate the air stream and produce a kind of tongue vibrato. Articulations like "yo yo yo" and "oy oy oy" can help you identify the necessary tongue movements. Sometimes repeated "biting" type motions of the jaw or mouth and jaw can also be used to get some vibrato and tremolo type effects on bent notes.
Blow Note Vibrato
Most commonly, blow notes will utilize tremolo instead of vibrato, especially on holes 1-6, because altering the pitch of these blow notes is fairly difficult. However, on holes 7-10 you can do blow bends, so you can definitely use vibrato on these notes. The diaphragm is used to hold the playing pressure steady, or produce slight pulsations in the volume. The throat is also used to throttle the air stream, and the tongue position is used to interrupt the air stream and control any bends and how much pitch variation you get in the vibrato.
Practice smoothly bending the blow notes down and back up over their full range. This will give you the feel of your tongue position, and how much small changes there affect the resulting pitch.
Make sure your notes are fully supported with your diaphragm so you don't lose your tone during bends, or when you waver the pitch by pulsing your throat or manipulating your mouth/tongue position.
Practice blow bend vibrato on holes 2 and 3 by going from the hole 2 and hole 3 full draw bend (2" and 3"') to blow vibrato for that hole. Your control over the air stream for the full draw bends can be maintained for the blow note vibrato by merely changing the breathing direction.
Alternate practicing on holes 7-10 and holes 1-6. Try to bring your tongue techniques from the upper hole blow bends down to your lower hole blow vibrato. With some practice, you should be able to slightly bend the low notes down in pitch, if you have enough diaphragm support. As you achieve some pitch variations on the low holes, bring that diaphragm support up to your high holes to add control to your smooth blow bends.
The high hole draw note vibrato poses a similar and symmetric difficulty to the low hole blow vibrato. First, concentrate on getting a good smooth tremolo on the volume for those notes. Gradually bring your throat and tongue into the tremolo and try to catch the critical pressure necessary to grab the reeds. Tiny movements and subtle control of the high hole draw notes can add a small amount of bend to those notes, and enable a vibrato effect.
Draw and Blow Vibrato Consistency
Since the draw and blow notes in different cells on an unvalved harmonica like a diatonic bend different amounts for each note, you have to consciously adjust your technique to try to maintain a consistent vibrato for the various note types.
One way to practice this is to use hole 7. Hole 7 seems to have some unique qualities. First, you can blow bend it, but only about a quarter tone--less than a full 1/2 step. This makes it a good candidate for blow vibrato, since some pitch variation room is there. The draw note doesn't really want to bend, but can be coaxed into moving a little bit easier than holes 8, 9, and 10. Since the depth of the blow bend is limited with respect to holes 8-10, the two vibratos--blow and draw--can be made to sound very symmetric. Practice getting a consistent, symmetric vibrato for the 7 blow and 7 draw, then move it up to the higher holes.
On the low holes (1-6) the odds are you've practiced and used draw note vibrato much more than blow note vibrato. Try to apply what you do with the draw note to your vibrato on the blow note. Practice going back and forth between a strong and controlled and rhythmic draw note vibrato and a similar blow note vibrato. Altering between the hole 2 draw and the hole 3 blow is effective for getting consistent vibrato because the pitch is the same.. go for the same tone as well.
Hold the notes for a long time. Listen to how you shape the draw note--how the volume swells, when the vibrato begins, how the note pulses dynamically with the beat, how the sound trails off and whether the depth of the vibrato changes over the duration. Try to make the blow note do the same thing.
The muscles, mouth position, and approach to the blow note will be quite different--pushing the pitch down instead of pulling it down--but try to make the adjustments so that the notes are symmetrical and consistent with each other, even if the playing isn't. I find that when I'm supporting the tiny blow bend on the low holes hard with my diaphragm, the tension makes it easy to produce a fast vibrato. So the speed doesn't naturally follow or flow from the draw note to the blow note--it has to be controlled to a slower rate to maintain the pulse and pace set by the draw note.
As you go back and forth between the draw vibrato and the blow vibrato you may find that what you can do with the blow vibrato feeds back into what you do do with the draw vibrato. You may make the pitch change depth more shallow, or you may change the vibrato rate to facilitate a consistent vibrato between the blow and draw. You may decide to use different kinds of draw note vibrato at different times, for different reasons--one type perhaps for consistency of phrasing in certain passages, another type perhaps for trying to rip a very emotional note out of a phrase.
Vibrato on Overblows
If you have a good stable overblow note you can add vibrato to it. Vibrato on overblows is a similar technique to vibrato on the high hole blow notes. First practice and gain control of vibrato on the normal blow notes in holes 7-10. Then work on hitting your overblows on pitch and sustaining them with good tone and diaphragm support. Then, finally, combine the two techniques and work on a smooth stable overblow vibrato. Note that overblows bend UP in pitch instead of down, so you have the opportunity to add some variety to your tonal palette.
Vibrato on overdraws are similar in technique to draw note throat vibrato. First you have to have a good stable overdraw. Then you have to have good subtle control over your throat vibrato. Practice draw note vibrato on the high holes to gain the fine control over those muscles you'll need for putting vibrato on the overdraw notes.