- There's a good chance his tongue is raised and pulled back in the shape of the vowel sound "EE." This causes the air pressure in the body to increase, speeding up the air as it passes through the reed, making it vibrate faster. Additionally, a tongue position like this causes harmonic resonance in the reed, increasing the already too fast vibration. Have him move the tongue forward where the tip of the tongue is aware of the bottom teeth meeting the gum. Spread the tongue out until the sides of the tongue touch the sides of the back molars. The back of the tongue needs to feel like it is melting into the bottom of the mouth. This will feel more like a theatrical or British version of the vowel "AH".
- A raised tongue is usually accompanied by a collapsed oral cavity (which is often the cause) and spread embouchure. He needs to separate the teeth at least a thumb-width. As the teeth spread, the mouth corners need to come to the center. This helps the lip muscles strengthen in the center, where the reed is. By bringing the teeth apart, we reduce the air pressure, allowing the reed to vibrate slower. This also creates space under the reed for the tongue to move forward. With the lips moving to center, the muscles have more control of the vibration so the reed no longer vibrates uncontrolled.
- There is also the small possibility the student is squeezing the reed too hard or blowing too hard. Usually these are not the problem, but are worth mentioning.
- To check these first three items, have the student make a sound on the reed and bocal only. For the lowest notes, we want to vibrate closer to a B natural sound on the reed and bocal. If the student is vibrating higher, most likely the above will solve the problem and bring that vibration lower.
- Often when students move into the lower sounds, they allow their left wrist to move instead of only moving the thumb. When the wrist moves, the first and second finger on the front of the bassoon may slide off the holes.
- If the air passes through the reed without a vibration, the lips are not strong enough against the reed. The reed tip needs to close the corners down slightly to vibrate low sounds and if the lip pressure does not do that, then it will not vibrate.
- Because the bassoon is so long, there needs to be enough air passing through the reed to counter the back pressure of the lowest notes. He needs to imagine blowing up a balloon on his bell.
- The reed itself could be the culprit. If the reed is too thick it will resist lip pressure and fail to close enough. Also, a thick reed will naturally want to vibrate fast, and so if a vibration occurs, it may jump to an upper harmonic. Additionally, if the reed tip is too thin, it might vibrate slow easily, but the too thin tip will collapse.
- Is the instrument leaky? If it is old and hasn't been serviced in awhile, the pads might not be sealing. Also, is the whisper key pad torn? If so, it isn't closing the bocal hole, meaning the sound wave will travel too fast through the instrument. If the bocal hasn't been cleaned in awhile, it could also have gunk in it that speeds up the sound wave.
So, that's my dissertation. Hopefully something in here helps.