An acoustic occupancy sensor includes and acoustic transmitter, acoustic receiver, variable bandpass filter and a controller. The acoustic transmitter transmits acoustic signals into a defined region and the acoustic receiver receives acoustic signals from the defined region. The received acoustic signal is converted to an electrical signal. A pre-amplifier amplifies the electrical signal and the electrical signal is digitized. The digitized electrical signal is demodulated by a demodulator to remove the carrier signal. The digitized electrical signal is filtered by a bandpass filter that sweeps up along a range of frequencies and down along a range of frequencies. The amplitudes of the filtered signals are averaged and analyzed to determine the presence or absence of an occupant within the defined region.