A new mechanical index for gauging the human bioeffects of low frequency ultrasound

Low frequency ultrasound has a diverse set of industrial and medical applications ranging from high power industrial ultrasound equipment through to various therapeutic medical applications. In recent years, several speech interface applications have also been developed which exploit the low ultrasonic frequency region to augment human-computer interfacing. These devices tend to operate just above the threshold of human hearing where signals can be generated and detected using off-the-shelf audio hardware components. Mechanical index has long been one of the main criteria used for determining safety limits for human exposure to ultrasound, however it is known to be inaccurate below about 500 kHz. This paper revisits the mathematical and physical foundations of the mechanical index, in particular transient cavitation, and applies these to the low-frequency ultrasound region. Simulations are performed to evaluate the effects on both blood and water. From the results, a new mechanical index formulation is proposed, which extends down to significantly lower frequencies. The aim is to provide a gauge for determining bio-effects of emerging and future low frequency ultrasonic applications operating around 20 kHz to 100 kHz.