Powerful sound booster for musical instruments using a rotary valve. 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 other sound boosters being sold at the same time is that this one is incredibly powerful and produces a deep, resonant sound. For a bass trombone, changing the valve inserted will alter the resonance. 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 effect may vary from person to person.