A stringed musical instrument with a body having at least one hollow space within including one or more vents allowing movement of air into and out of the instrument body. The back includes a back access panel attached to the back allowing access into the body. A threaded fastener extending between the back access panel and a top of the instrument body. The fastener is accessible and movable from outside the instrument body. The back access panel is flexible so that turning in the threaded fastener will deflect the back panel and alter the instrument body.