Human auditory system does not perceive the sound signal of all frequencies with equal loudness. Based on equal loudness curves, it is known that a low-frequency signal needs to be produced with a higher-power level to have the same loudness as the middle frequency part. There are two ways to overcome this problem; either boosting the power of the low-frequency part or utilizing the psychoacoustics effect called the missing fundamental. There are many works which have been done utilizing psychoacoustics to enhance the low-frequency perception of audio signal. These works are implemented to normal audio systems. Normal audio systems are comprised of a loudspeaker as the audio reproduction end. It is proposed to implement the low-frequency enhancement to a parametric array loudspeaker. The use of a parametric array loudspeaker to generate highly directional audible signal has been reported in several literatures. However, this audible signal lacks low-frequency content. One of the reasons is the relatively low-power level produced by the existing parametric array. By utilizing the nonlinearity property of the air itself, it is proposed to psychoacoustically enhance the perception of the low-frequency part.

Using a parametric array of ultrasonic transducers, directional sound can be generated from inaudible ultrasonic as a result of nonlinear interaction of ultrasonic wave in air. A finite amplitude ultrasonic wave that can be amplitude modulated by any audio signal is radiated from the array [Yoneyama et al., J. Acoust. Soc. Am. 73, 1532–1536 (1983)]. As a result, an audio signal is produced because of the self-demodulation effect of the modulated sound wave. This, however, generates a large distortion that is impractical. Another method [T. Kite et al., J. Acoust. Soc. Am. 103, 2871 (1998)] uses a preprocessing technique, which utilizes double integration of the square-rooted input. The resulted distortion is greatly reduced but at the expense of very wide bandwidth requirement due to the square-root operation. Various techniques to reduce the distortion by preprocessing have been discussed in previous literature. In this paper, we propose a psycho-acoustic masking scheme, coupled with a suitable modulation that will provide further reduction in distortion. The psycho-acoustic masking focus on the distortion occurred within the critical bands. Simulation and subjective testing have been done to verify the system.