As is the case for all instruments, tube instruments start with a vibrating sound source and use resonance to amplify the desired frequencies. For woodwind instruments a single or double reed vibrates. For flutes a fipple turns a smooth air flow into an oscillating flow which becomes the initial vibration. Woodwind instruments use holes to change the effective length of the tube to get different fundamental frequencies. Keys activated by springs and levers extend the reach of the human hand in opening and closing holes in the instrument. The cross sectional shape of the tube changes the overtone frequencies, giving the particular instrument its timbre. Brass instruments start with a pair of buzzing lips. The length of the tube is changed by either a sliding tube arrangement or a valve that shunts the air through tubes of different lengths. The impedance matching provided by the bell of a brass instruments enables them to produce more sound and also shift their overtone frequencies to give them their unique timbre.