Since the modern flute was developed by Theobald Boehm in the 1860's, cork has remained the material for sealing the end. Cork tends to dry out and then leak air over 6-12 months. Once the seal is lost, the resonance losses containment and sound quality and volume suffer noticeably. While there has been tremendous interest in the mouthpiece of the flute, particularly over the past 25 years, the cork has been largely ignored. Every slight change in the mouthpiece area has a significant effect on the sound of the flute, because slight changes are amplified by the length of the flute tubing. Because the stopper plug is located so near the mouthpiece, it also has a significant effect on the sound quality.
I have tried some of the limited variety of stopper plugs available. Most are made of acetal plastic, like mine, and all seem to add some resonance to the flute. The increase in resonance, however, needs to be balanced with sufficient dampening to control the quality of the sound. Otherwise, you get a booming, brassy sound, and the pitch of the notes can change. While I found a plug or two that was an improvement on cork, I was not satisfied and embarked on a quest to manufacture the best possible flute stopper plug. Remarkably enough I have been successful. Improving the projection and reducing resistance came very quickly, by creating a hollowed out, hourglass type shape. The increased space around the head joint tubing allows forfull vibration of the tubing for a full rich tone with less air hiss. Controlling the pitch while maintaining the increased resonance was my biggest challenge, however. This was solved by the addition of the third "O" ring. This middle ring is not necessary for the seal (there are already 2 rings for that), however it provides the additional dampening to bring the pitch into proper balance and gain a more nuanced control of the tone.Proper resistance allows for more tone control. The increased resonance of my plug reduces resistance and makes it easier to play. You will be amazed at the ease of playing those difficult notes in the third octave, and the clarity and fullness of those notes, without being shrill. No facilitator key is needed to reach high 'e' cleanly with my plug. At the other end, the low notes have more punch and response and need less adjustment of the lip position by the player.