A method and apparatus for tensioning or relaxing a membrane of a musical instrument, such as a traditional frame drum. A pressurized fluid is guided into at least one variable pressure chamber formed by an expandable hollow body. The hollow body is positioned around the resonator of the instrument. The membrane is subjected to pressure between a membrane support and a membrane fixing device by means of the pressurized fluid in the chamber. Pressure is exerted evenly all around the circumference of the membrane. The membrane is fixed only by a band so that it can be tensed or relaxed very rapidly. The band is arranged to vibrate freely in relation to the resonance body while the membrane is subjected to pressure from the variably pressurized chamber.