Studying Sonar Beam Forming in Porpoises With 48-Channel, High-Speed System

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"Using the PXI platform with LabVIEW software meant that we could meet the requirements in terms of development time and system performance. "

- Kristian Beedholm, Department of Zoophysiology, Aarhus University

The Challenge:
Determining whether echolocating toothed whales (porpoises) exert control over the width of their acoustic sonar beam during a target approach, a technique known in other areas as beam forming.

The Solution:
Utilizing the PXI Express platform to record acoustic data for 48 synchronized channels at 500 kHz per channel, to determine how the click waveforms appear within a plane close to the target.

Author(s):
Kristian Beedholm - Department of Zoophysiology, Aarhus University
Danuta Wisniewska - Aarhus University

Whale Bioacoustics—An Interdisciplinary Field of Research

At the Zoophysiology Section of Aarhus University (AU) and the Biological Department of University of Southern Denmark (SDU) we have studied the acoustics involved in echolocation in both bats and whales for several decades. The Fjord&Bælt (F&B) facility in Kerteminde provides access to trained harbor porpoises, which gives a unique opportunity to study the animals’ behavior during semi-natural scenarios.

We must research the acoustic behavior of harbor porpoises to gain the insights needed to mitigate the problem of bycatch of these animals. During their foraging, porpoises often get attracted to fish caught in gill nets and become entangled themselves, which leads to death by suffocation. To design nets and deterioration devices that fishermen can use to reduce bycatch, we must determine the challenges that these animals face.

The leverage we get from many years of experience in animal training in the F&B facility combined with the technical and programming skills of employees of the universities of AU and SDU provides a unique opportunity to undertake a highly specialized and focused study into the dynamics of ultrasonic echolocation. This cooperation resulted in a system capable of capturing the shape of the acoustic beam the porpoises emit when engaging a target.

Parallel Processing Is Paramount in Providing Fast Data Streaming

Our system consists of 48 custom-built small piezo-element hydrophones arranged in a plane to form a recording matrix (Figure 1). We performed signal conditioning by a custom-made 48-channel charge amplifier with built-in band-pass filters. An NI PXIe-1078 chassis equipped with three NI PXIe-6358 modules, each with 16 differential analogue channels each sampling at 500 kHz per channel, formed the backbone of the recording system. We used an additional USB-6356 device to simultaneously send out a trigger signal for the 48-channel PXI Express system and provide an acoustic synchronization signal. We transmitted the signal, an FM sweep from 160 to 210 kHz (outside the hearing range of porpoises), into the water and so it could be picked up by the hydrophones of a DTAG-3 data logger device attached with suction cups to the animal itself. The DTAG-3 is visible in Movie 1, showing porpoise approaching a target.

Figure 1. The porpoise approaches the target, more or less perpendicular to the plane of the grid of 48 custom-built hydrophones.

The overall system combines DAQ hardware and LabVIEW software. In our LabVIEW code, three loops run in parallel during data collection and communicate through two data queues. One loop read the data from the DAQ hardware, one reduced the data samples to 16 bit, and the last stored the data on an SSD drive. Compared to other solutions, we could make the parallelization of the process extremely simple due the inherently parallel nature of the LabVIEW programming environment.

The Simplicity of LabVIEW Provides Flexibility in a Multiparty Project

The synchronized multichannel PXI Express system can stream all of the high-speed data directly to disk if provided enough flexibility in the offline analysis that the shape of the individual click’s beam can be fitted onto the observed data. Using the PXI platform with LabVIEW software meant that we could meet the requirements in terms of development time and system performance. We carried out the early stages of planning the experiment prior to investing in the equipment in close collaboration with the technical staff at NI Denmark.

The data collection software remained in constant development throughout the project. LabVIEW suited a project of this type because many of the participants could make adjustments to parameters, which reduces the number of choices to be made by the user. Also, the lead developer could walk other participants in the project through the correction of minor bug fixes over the phone.

Results

The scientific results indicate that the porpoises and possibly also other toothed whales possess a hitherto unknown degree of flexibility over their sonar beam and are able to widen the beam by a factor of about 2.5. This occurs when the porpoise is close to the target (Figure 2), presumably to avoid the prey escaping the whale’s field of view at the final stages of the hunt.

Figure 2. The dynamics of the porpoise’s sonar beam. A: Illustration of the fact that an increase in the angular subtense of the beam corresponds to a much larger increase in the volume ensonified at above a given sound pressure level. B: Relative increase in the area above 3 dB from the peak pressure of click as a function of the distance to the target. At close ranges (distance below 0.5 m) the beam width is much more variable, and at a given distance it may span up to almost three times the area it does at longer ranges.

The system provided sufficient data for the research project to result in a scientific publication in a high-impact journal with the resulting media interest from the BBC. The PXI Express system was sufficiently fast and enabled us to continuously stream the complete data series to disk and analyze them in depth offline. The LabVIEW platform was simple enough to use for the development to be finished within weeks, and for onsite changes to be implemented without significant delays to the projects.

Author Information:
Kristian Beedholm
Department of Zoophysiology, Aarhus University
Tel: +45 25 38 36 85
kristian.beedholm@bios.au.dk

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