This is a booster for large shank mouthpieces, designed to be used with the type of mouthpiece (classic) shown in the image, made by Denis Wick. I used it with a 4AL. When attached, it gives the impression that the directionality of the sound coming from the bell is increased, without the vibration being diffused by the mouthpiece receiver. This is just my personal feeling, but I could feel the difference. It has been used and is a used item, so please refrain from purchasing if you are sensitive to such things. Denis Wick classic mouthpiece booster trombone megatone megatone