Sound booster for musical instruments using a rotary valve - powerful! As shown in the example, open the cap of the rotary valve, place the booster, and then close the cap. The cap won't close completely, but it's okay to tighten it firmly in this state. The difference from the gold & silver versions also on sale is that this one is incredibly powerful and resonates with a deep, robust sound. In the case of a bass trombone, the sound changes depending on the valve you insert. It might sound a bit dubious, but I think you'll feel some kind of effect, so please give it a try. *Please note that the perceived effects may vary from person to person.