Josefin Starkhammar

Frequency-dependent variation in the two-dimensional beam pattern of an echolocating dolphin.

Recent recordings of dolphin echolocation using a dense array of hydrophones suggest that the echolocation beam is dynamic and can at times consist of a single dominant peak, while at other times it consists of forward projected primary and secondary peaks with similar energy, partially overlapping in space and frequency bandwidth. The spatial separation of the peaks provides an area in front of the dolphin, where the spectral magnitude slopes drop off quickly for certain frequency bands. This region is potentially used to optimize prey localization by directing the maximum pressure slope of the echolocation beam at the target, rather than the maximum pressure peak. The dolphin was able to steer the beam horizontally to a greater extent than previously described. The complex and dynamic sound field generated by the echolocating dolphin may be due to the use of two sets of phonic lips as sound sources, or an unknown complexity in the sound propagation paths or acoustic properties of the forehead tissues of the dolphin.

47-channel burst-mode recording hydrophone system enabling measurements of the dynamic echolocation behavior of free-swimming dolphins

Detailed echolocation behavior studies on free-swimming dolphins require a measurement system that incorporates multiple hydrophones (often >16). However, the high data flow rate of previous systems has limited their usefulness since only minute long recordings have been manageable. To address this problem, this report describes a 47-channel burst-mode recording hydrophone system that enables highly resolved full beamwidth measurements on multiple free-swimming dolphins during prolonged recording periods. The system facilitates a wide range of biosonar studies since it eliminates the need to restrict the movement of animals in order to study the fine details of their sonar beams.

Evaluation of Seven Time-Frequency Representation Algorithms Applied to Broadband Echolocation Signals

Time-frequency representation algorithms such as spectrograms have proven to be useful tools in marine biosonar signal analysis. Although there are several different time-frequency representation algorithms designed for different types of signals with various characteristics, it is unclear which algorithms that are best suited for transient signals, like the echolocation signals of echolocating whales. This paper describes a comparison of seven different time-frequency representation algorithms with respect to their usefulness when it comes to marine biosonar signals. It also provides the answer to how close in time and frequency two transients can be while remaining distinguishable as two separate signals in time-frequency representations. This is, for instance, relevant in studies where echolocation signal component azimuths are compared in the search for the exact location of their acoustic sources. The smallest time difference was found to be 20 µs and the smallest frequency difference 49 kHz of signals with a −3 dB bandwidth of 40 kHz. Among the tested methods, the Reassigned Smoothed Pseudo Wigner-Ville distribution technique was found to be the most capable of localizing closely spaced signal components.

Intra-click time-frequency patterns across the echolocation beam of a beluga whale

The echolocation beam of toothed whales has been studied ever since it was first discovered in 1960. Recent studies have focused on the frequency distributions across the cross sections of the beams. Other studies have focussed on describing the entire acoustic field around the animal. However, no one has yet described the timing of each frequency component in the main lobe beam in relation to the other frequency components. Even though the echolocation clicks of broadband click species like the beluga whales (Delphinapterus leucas) are short in time (around 70 μs), previous results have shown indications on a frequency dependence with time, within each click. Little is known about the details of how the signal is generated and transmitted into the water. Investigations of when in time the frequency components occur within each click would give us further knowledge to how the signals are generated. This study takes a closer look at these intra-click time-frequency patterns across the echolocation beam of ...

Frequency‐dependent echolocation beam pattern of the bottlenose dolphin.

Moore and others (2008) previously showed that bottlenose dolphins are capable of beam steering and controlling the vertical and horizontal widths of the echolocation beam. A follow‐on study was performed using the same methods as the previous study, but with a younger animal and a higher resolution diamond‐shaped hydrophone array for characterizing the beam. The dolphin performed a target detection task while stationed on a biteplate with targets placed up to 34 deg to either the left or right of the dolphin’s longitudinal axis. The dolphin was capable of beam steering more than 28 deg to either side, which is a greater capability than previously reported and which exceeded the geometric coverage of the array. Frequency band‐limited beam patterns suggested the presence of two beams, spatially separate from one another and which corresponded to higher and lower frequency energies. The finding is consistent with prior anatomical and acoustic evidence of two echolocation click sound sources in the delphinid...

Objective detection and time-frequency localization of components within transient signals

An automatic component detection method for overlapping transient pulses in multi-component signals is presented and evaluated. The recently proposed scaled reassignment technique is shown to have the best achievable resolution for closely located Gaussian shaped transient pulses, even in heavy disruptive noise. As a result, the method automatically detects and counts the number of transients, giving the center times and center frequencies of all components with considerable accuracy. The presented method shows great potential for applications in several acoustic research fields, where coinciding Gaussian shaped transients are analyzed. The performance is tested on measured data from a laboratory pulse-echo setup and from a dolphin echolocation signal measured simultaneously at two different locations in the echolocation beam. Since the method requires little user input, it should be easily employed in a variety of research projects.

The scaled reassigned spectrogram adapted for detection and localisation of transient signals

The reassigned spectrogram can be used to improve the readability of a time-frequency representation of a non-stationary and multi-component signal. However for transient signals the reassignment needs to be adapted in order to achieve good localisation of the signal components. One approach is to scale the reassignment. This paper shows that by adapting the shape of the time window used with the spectrogram and by scaling the reassignment, perfect localisation can be achieved for a transient signal component. It is also shown that without matching the shape of the window, perfect localisation is not achieved. This is used to both identify the time-frequency centres of components in a multi-component signal, and to detect the shapes of the signal components. The scaled reassigned spectrogram with the matching shape window is shown to be able to resolve close components and works well for multi-components signals with noise. An echolocation signal from a beluga whale (Delphinapterus leucas) provides an example of how the method performs on a measured signal.

Beam patterns of the dolphin demonstrate task-dependent dynamics

The echolocation beam of the bottlenose dolphin was first carefully described by Au and colleagues (1978)[“Propagation of Atlantic bottlenose dolphin echolocation signals,” J. Acoust. Soc. Am. 64, 411-422] using various hydrophone array configurations and targets located in front of the dolphin and along its longitudinal axis. Measured beams were described as vertically elevated with mean vertical and horizontal 3-dB beamwidths of ~10°. The experimental paradigm was later augmented with denser hydrophone arrays, greater spatial coverage of the acoustic field, and the training of target detection with targets presented to the left or right of the dolphin’s longitudinal axis. Utilizing two dolphins, beam steering capabilities and beamwidth control were demonstrated. The two dolphins steered the axis of the echolocation beam up to 18° and 28° in the horizontal plane. Horizontal beamwidths were bimodally distributed in one dolphin, with peaks at 16° and 26°. The other dolphin had a unimodal distribution of be...

The dynamics of the bottlenose dolphin sonar beam.

Much research on dolphin echolocation has focused on animals that have been trained to remain stationary or to carry a device that allows the animals to move but restricts the location of the sonar beam. In such cases, a small number of hydrophones measures sonar characteristics while dolphins solve echolocation tasks. As a result, much is known about the beam axis but relatively little is known about other parts of the beam. One reason for this disparity is that it has been difficult to interpret the results from off axis measurements using a small number of hydrophones (that may or may not sample simultaneously). In this paper, we report results from a system of 47 hydrophones in a 0.75 × 0.75 m2 matrix that allowed measurements to be made at multiple locations in the beam simultaneously, with a sample rate of 1 Msample/s. The system both visualizes and records echolocation clicks in real time across the whole cross section of the beam, hence allowing the full dynamics of the sonar beam to be revealed. ...