Audio Transmitter

Need to design an electronic circuit capable of transmitting inaudible sound in frequency range of 17 kHz to 19 kHz within a geographical area of radius of 70-80 meters. The sound signal would be carrying some predefined digital bit sequence (you can use any modulation technique as long as its mathematical demodulation is possible). Sound signal should be detectable by any normal smartphone available in market. Circuit should consume as much less power as possible. Heat dissipation, if any should be taken care. Let us know how would you do it & how much time would you need for this.