A modulation method referred to as Time Shift Keying (TSK) is used to transmit messages between two devices in a highly energy efficient manner. A message represented by an inaudible audio signal is modulated on a transmitting device. The audio signal is comprised of an array of non-zero amplitude delimiter signals with time periods of zero-amplitude transmission between delimiters. The time duration of the zero-amplitude transmission periods is mapped to a symbol, multiple symbols are then assembled into a message. On the transmitting device, the audio signal is broken into pieces or sequences of bits which are mapped to symbols. On the receiving device, the time durations of zero-amplitude transmission are translated to the symbols which are assembled to the message. The delimiter signals have gradually increasing and decreasing amplitudes and have a length such that make them detectable by the receiving device.