Steady Air Flow
Normally in music there are all sorts of differences in the way we blow the air. We speed up the air to play louder or ascend into the high register, and vary the amount of air according to the type of sound we want to produce. However, the trombonist must be able to blow an underlying steady stream of air for effective slurring, regardless of the action of the slide. For example, to slur from first position F to fourth position G, the same air must be blown throughout the F, "across the grain" of the slur, and throughout the G.
Put another way: to play a steady long tone I must blow air at a constant flow rate, despite the fact that my body is changing every split second of exhalation. The wind must blow with constant speed, regardless of how full the lungs are or how fast the slide is moving.
Perhaps the most common problem I have encountered is the "wah-wahs," where a student backs off of the air every shift, then makes a slight crescendo through each note. I believe it comes from fear of the articulation you get in the sound of a clean slur, and can be solved by exaggerating the opposite. I get the student to do several slurs deliberately making a breath accent, or "Ho" attack, on each note.
Another way of developing good legato is to take some melodious etude and practice it without the tongue, or all glissando. Advanced players, try it on the Rochut Etudes, while younger players can use the phrasing studies by Jaroslav Cimera.
The slide must move as quickly as possible between notes to avoid a smear, but not so fast that the instrument jerks and upsets the embouchure. To get a smooth shift between first and fifth position for example, I like to imagine the slide accelerating to the halfway point-about third position-then slowing down the rest of the way. Control the slide between your thumb and index finger, but keep a loose wrist.
On the trip out from first position, think of the slide accelerating from a standstill to 50 km/hr, with your thumb on the slide brace acting as a throttle, and then the index finger acting as a brake to bring you back down to zero when you get to fifth position. Coming back to first position it is reversed, with the index finger pushing and the thumb slowing down. In this way, you can make a smooth, quick shift, without jerking the embouchure.
The final aspect of great legato is a light tongue stroke at the same moment the slide moves.
Now is probably a good time to tell you that there are two schools of legato tonguing in the world. The two "camps" are diametrically opposed to one another, each accusing the other of causing irreparable harm to the advancement of the trombone. One school says, "tongue everything!" The other says, "tongue the ones you must to avoid a glissando, and slur the rest!"
I'm kidding of course, but these two schools of thought do exist. For the longest time I subscribed to the second idea, only tonguing the slurs which would smear without it. I worked very hard to get the slurs and the tongues to sound the same, with some success, but it was never perfect. Now I think I have changed my mind. It was pointed out to me that you get a much more consistent articulation tonging everything, and after a great deal of practise (I was lousy at it) I have to agree. Oddly enough, I found an improvement in my general technique as well. Try it and see!
Ideally, the tongue stroke should happen about halfway between positions, but in reality it is almost impossible to time perfectly. Let your ear be your guide: Is there a slight gliss in the sound before or after the shift?
By the way, I don't believe the tongue changes at all for different articulations. The tongue stroke for an accent is the same as for a tenuto, it is the quality of air that changes the note shape.
Brad Howland is the Principal Trombonist of the Victoria Symphony Orchestra.