An embodiment adjusts a position of a piezoelectric generator within a hollow cylindrical tube, the position adjusted to cause the tube to vibrate at a first resonant frequency in response to an acoustic stimulus, the piezoelectric generator configured to close one end of the tube, the tube further comprising an open end disposed at an opposite end of the tube from the piezoelectric generator. An embodiment adjusts, by applying a voltage to the piezoelectric generator, a resonant frequency of the piezoelectric generator, the adjusting changing the resonant frequency of the piezoelectric generator to the first resonant frequency.