Analysis of ultrasound radiation and proposal of design criteria in ultrasonic haptic display for practical applications

Abstract

This paper describes a technology to reduce ultrasound radiation which is caused by normal mode vibration of an ultrasonic haptics display. Reduction and analysis of ultrasonic acoustic radiation from the display were implemented for safety use and coexistence with other devices. The focus on solving the problem of ultrasound radiation is based on the investigation of constructive interference conditions of acoustic waves on the top panel. We successfully demonstrated a practical sound pressure level on the prototype. To reduce radiation, it is necessary to reduce the thickness or the lower stiffness-to-density ratio E/ρ. A thickness of 0.3 mm has practical stiffness in the case of glass and the maximum pressure is 110 dB or less at 30 kHz for the usual size of smartphone. This means that raising the “coincident frequency” higher than the driving frequency by reducing the thickness and lowering the E/ρ, causes sound radiation to be sharply reduced.