Directional UltraSound

Hello guys and gals, i need someone that is good at audioprocessing. Im need a piece of software that create two carrier ultrasound tracks that contains a audible messages, heard of lrad ? Search the net for directional speakers, lrad .... The principle is that if you have a sound wave of 40khz and one of 41khz aimed at a distant subject you will hear the 1khz humm .. So i need a software that takes two carrier tones and modulate them with a audio track, mp3 or wav. Read up on the subject before bidding, so you need what to do to make a good product that does not make the audio sound like a dalek, or a toad. Marten