an electronic modulator, adapted to receive audio signals, wherein the electronic modulator generates a carrier signal to be modulated with the audio signals to produce a modulated signal; and

at least one electro-acoustical emitter having at least two ultrasonic frequency emission zones coupled to the electronic modulator to reproduce the modulated signal, the at least two ultrasonic frequency emission zones each having at least one resonant frequency, wherein a center frequency of the carrier signal is offset from each at least one resonant frequency in view of a rate of change of phase of each emission zone in a vicinity of each at least one resonant frequency in order to increase a phase coherence and combined parametric output of said emission zones.