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 will not close completely, but that's okay. The difference from other sound boosters on sale is that this one is incredibly powerful and resonates with a deep, rich sound. Changing the valve will also change the resonance. It might sound a bit dubious, but please give it a try. *Please note that the effect may vary from person to person.