The ultrasonic receiving system of a Pacific white-sided dolphin: Part 2. Multi-input/two-output system

Abstract

According to anatomical investigations, measurements of the receiving sensitivity distribution and investigation of ultrasonic sound wave propagation paths, each ear may be constructed as a multi-input/single-output system in the head. These two multi-input/single-output systems may be considered to act together as one multi-input/two-output system. Here, the input points are highly sensitive areas lying just inside of the lower mandibula. The two output points are the tympanic bones in each mandibula. All input points are connected acoustically to both tympanic bones by the the digastrici, mylohyoidei and sternohyoid muscles. Mathematical simulations were carried out using this multi-input/single-output system, applying impulse response functions previously measured from the right tympanic bone to each of the measuring points on the dolphin’s head surface. From the results of these simulations, it is considered that the highly sensitive areas located in lines along both mandibula act as line arrays, so the beamwidth of the receiving system can be narrow. Furthermore, the spectrum of the echo signal is changed by the target direction, and human beings can recognize this difference when frequency components of the echo signal are shifted down to the range of human hearing.