Parametric speakers provide narrow directional audible sounds using modulated ultrasounds. Parametric speakers are employed to build audio communication with privacy protection, which is expected to be installed into various instruments. To respond to this requirement, slim-sized parametric speakers delivering high-quality sound must be fabricated. Ultrasonic transducers usually consist of piezoelectric elements and metal resonators, which have a number of resonant modes. To demodulate audible sounds by the non-linearity of a finite amplitude sonic wave, a large amplitude carrier wave must be emitted. Therefore a predominant resonant mode of an ultrasonic transducer is used to transmit carrier wave. Moreover, frequency band around the predominant resonance is used to transmit modulated Fourier components of signals. While considering these principals of sonic transmission, two issues must be solved to fabricate a slim-sized and high quality parametric speaker, i.e., 1) widening of available bandwidth and improvement of electroacoustic efficiency and 2) optimization of transmitter circuits and modulation method based on characteristics of ultrasound transducers. It is necessary to study parametric speakers as a total system comprised by ultrasound transducers, transmit circuits, and signal processing of modulation, to solve these issues. This paper denotes outline of these issues and the present status of our studies.