Multi-Channel Acquisition and Visualization of the Dolphin Echolocation Beam – Instrumentation Design and Bioacoustic Results

Sammanfattning: Toothed whales (Odontocetes) use echolocation to navigate and find food in dark or murky waters. A wide range of innovative studies has since the 1960:s been used to characterize their echolocation beam and map out their echolocation skills. In order to render high quality recordings of the sounds emitted by these animals, advanced measurement systems are required. There are still interesting envisioned studies, which so far have been impossible to conduct, due to the technological complexity of the required measurement systems. However, the recent development of computer based data acquisition technology has opened up new possibilities for the field of marine bioacoustics. This doctoral dissertation describes the design of a multi-channel measurement system enabling visualization and analysis of the cross section of the dolphin echolocation beam, and describes the bioacoustic results obtained from such measurements. The methods and results included in this dissertation span over several disciplines of science such as acoustics, data acquisition technology, hardware design, software design, signal processing, biology and dolphin cognition. The measurement system design allows for recordings of the echolocation beam cross section at 47 points simultaneously with a sample rate of 1 MS/s. The employed burst mode sampling technique enables longer recording sessions than previously described systems and also makes run-time visualization of the echolocation activity of dolphins possible, even in highly reverberant surroundings. The system can also be set up as an acoustically operated touch screen, controlled by the dolphin’s echolocation beam. It is suggested that the presented run-time as well as post-processing data visualization modes offer the generally visually orientated human a better opportunity to grasp the dynamics of the echolocation beam than before, when echolocation recordings have been made with just a few (1-7) hydrophones. Measurements of the beam cross section show that the beam is dynamic and at times can have a single dominant peak, while at other times have two forward projected primary and secondary peaks, spatially separated and each with different frequency contents and frequency bandwidths. It is hypothesized that the acoustic “pressure valley” in between these two peaks can be capitalized on to optimize pray localization, a hypothesis congruent with the echolocation strategy previously observed in Egyptian fruit bats.