TONE and MIDDLE OF THE TONGUE POSITION
A common impediment to good tone especially in beginners is having the middle/back part of the tongue too high. It results in a nasal, closed off sound. The fix of this is simply to lower the tongue position in the mouth. Try to make an “Oe” formation (as in Tone) with the inside of the mouth, as though you had a jaw breaker in your mouth. The tongue position does become higher as you go higher and lower as you go lower. In the range below middle C and low G, you can think of having a golf ball in your mouth, because your mouth should open even farther still.
I encourage you to slowly open the back of your tongue until it is too low and you “fall off” the pitch. I find just before that point is usually about right.
It also is good to have the corners of your mouth not too spread for good tone. This may requite a shift from an “Ee” position to more of an “Oo” or “Oe.” Having the jaw dropped down close to as far as it can go without losing the note is also a help for getting good tone.
The three aspects of lower tongue position, corners of the embouchure not too spread and the lowered jaw are all related. One aspect influences the other. If you are needing the pitch to become lower, those three components as well as dropping the front of the tongue position down are things that help the pitch to become lower.
In general, we want the inside of the mouth pretty open as well as focused toward the aperture (the opening in the embouchure). Playing long tones, long, held out notes, for working on tone is very good for this – I would recommend doing it with a tuner. Something like arpeggios through the range of the horn is a good for this. It’s important to constantly listen for your tone and make it as beautiful as you can.
The physical condition of your lip muscles has a lot to do with good tone. Consistency in practice and playing is one of the biggest helps. I also consider having whole body physical exercise as a huge help for good tone as well as good control, notes speaking more easily and good pitch. Personally I like to swim, but biking, running etc. certainly works. The day before I have something important to play for, it is a high priority for me to get in good exercise. If its done the same day as playing for something that takes very much endurance, sometimes the lip muscles are tired out. It’s important to get a feel for the amount and rhythms of whole body exercise as well as the workout for your chops (lip muscles) that is most beneficial to you. This helps your tone, endurance and lip responsiveness in general.