Maintaining tone color as the pitch rises up the strings

Rx: Tone in upper register is dull or notes not speaking well in upper positions.

The instructor, Geth Griffith, has made an interesting reference device that might be useful for beginners (or maybe not beginners, we all get less than ideal tone from time to time).

Even without the bent tubing (I bet you could use a bendy straw too) the concept and technique is a good one.