Detailed analysis of two detected overlaying transient components within the echolocation beam of a bottlenose dolphin (Tursiops truncatus).

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echolocation beam contained overlapping components. Across click trains, the number of overlapping components spatially varied within the echolocation beam. Two overlapping components were found to predominantly occur in the upper portion of the beam, whereas the lower portion of the beam predominantly contained a single component. When components overlapped, the trailing component generally had a higher center frequency and arrived less than 5 µs after the leading component. The spatial relationship of components was consistent with previous findings of two vertically distinct beam lobes with separated frequency content. The two components in the upper portion of the beam possibly result from a single transient click propagating through a geometrically dispersive media; specifically, the slower sound speed of the dolphin melon's core slightly delays the more directional, high frequency energy of the click, whereas the less directional, lower frequency energy propagates through more peripheral but higher sound speed portions of the melon.

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.

Scaled reassigned spectrograms applied to linear transducer signals.

This study evaluates the applicability of scaled reassigned spectrograms (ReSTS) on ultrasound radio frequency data obtained with a clinical linear array ultrasound transducer. The ReSTS's ability to resolve axially closely spaced objects in a phantom is compared to the classical cross-correlation method with respect to the ability to resolve closely spaced objects as individual reflectors using ultrasound pulses with different lengths. The results show that the axial resolution achieved with the ReSTS was superior to the cross-correlation method when the reflected pulses from two objects overlap. A novel B-mode imaging method, facilitating higher image resolution for distinct reflectors, is proposed.