Distance estimation of hidden objects based on acoustical holography by applying acoustic diffraction of audible sound

Occlusion is a problem for range finders; ranging systems using cameras or lasers cannot be used to estimate distance to an object (hidden object) that is occluded by another (obstacle). We developed a method to estimate the distance to the hidden object by applying acoustic diffraction of audible sound. Our method is based on time-of-flight (TOF), which has been used in ultrasound ranging systems. We determined the best frequency of audible sound and designed its optimal modulated signal for our system. We determined that the system estimates the distance to the hidden object as well as the obstacle. However, the measurement signal obtained from the hidden object was weak. Thus, interference from sound signals reflected from other objects or walls was not negligible. Therefore, we combined acoustical holography (AH) and TOF, which enabled a partial analysis of the reflection sound intensity field around the obstacle and hidden object. Our method was effective for ranging two objects of the same size within a 1.2 m depth range. The accuracy of our method was 3 cm for the obstacle, and 6 cm for the hidden object.