Noise Reduction of Airborne Ultrasound Tactile Display on Moving Stimulation Point

Abstract

The author is working on an ultrasonic device that produces tactile sensation in midair by focusing ultrasound. The high-intensity ultrasound at the focal point presses the skin surface. Although ultrasound itself is inaudible, audible noise is radiated when the focal point moves around. This noise is not desired for a tactile display because the noise makes the experience worse. This drawback has been found and reported in previous work, however no solution has been provided. This paper discusses the source of this noise and proposes a method to reduce it. The phase discontinuity of driving signals occurs when the focal point moves to the next position, and that leads to the sudden change of amplitude of ultrasonic waves. This sudden change of amplitude is the source of the noise. There are two ways to reduce this noise: One is making the original and target focal positions close and the other is changing the phase gradually. The former is effective only when the frame rate is high. Therefore we explore the latter. The behavior of the focal point during the phase shift is studied by simulation. An algorithm is developed and implemented into the current prototype device. The experimental results show the effectiveness of the proposed method.