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

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

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

Automatic time-frequency analysis of echolocation signals using the matched Gaussian multitaper spectrogram

High-resolution time-frequency (TF) images of multi-component signals are of great interest for visualization, feature extraction and estimation. The matched Gaussian multitaper spectrogram has been proposed to optimally resolve multi-component transient functions of Gaussian shape. Hermite functions are used as multitapers and the weights of the different spectrogram functions are optimized. For a fixed number of multitapers, the optimization gives the approximate Wigner distribution of the Gaussian shaped function. Increasing the number of multitapers gives a better approximation, i.e. a better resolution, but the cross-terms also become more prominent for close TF components. In this submission, we evaluate a number of different concentration measures to automatically estimate the number of multitapers resulting in the optimal spectrogram for TF images of dolphin echolocation signals. The measures are evaluated for different multi-component signals and noise levels and a suggestion of an automatic procedure for optimal TF analysis is given. The results are compared to other well known TF estimation algorithms and examples of real data measurements of echolocation signals from a beluga whale (Delphinapterus leucas) are presented

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

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

Implementering av seismiska ytvågsmätningar på vägbeläggningar

Inom detta projekt har en ny metod för oförstörande profilerande mätningar av styvhetsmodulen på beläggningar demonstrerats och utvärderats på två kontrollerade vägsträckor. Resultaten visar att det nu är möjligt att mäta upp beläggningens styvhetsmodul och aktuell temperatur, vid en kontrollerad frekvens och töjningsnivå, genom rullande mätningar i trafik. Värdet av denna nya möjlighet är att heltäckande profilerande kvalitetskontroll av både nya och gamla vägar kan göras säkrare, från en vanlig bil, samt mer kostnadseffektivt genom att avspärrningar och stillastående provtagning kan undvikas eller minimeras. För en given beläggningstjocklek är styvhetsmodulen den viktigaste materialegenskapen i bundna asfaltlager och den egenskap som används som indata för att dimensionera beläggningens livslängd och underhållsbehov.  Metoden bygger på kontaktlösa seismiska ytvågsmätningar med hjälp av mikrofoner. Uppmätt ytvågshastighet räknas om till dynamisk styvhetsmodul med hjälp av fundamentala viskoelastiska samband. Uppmätt ytvågshastighet och dynamisk styvhetsmodul har jämförts med konventionella stillastående mätningar och provning av borrkärnor i laboratorium. Resultaten visar på en god överrensstämmelse trots delvis okända temperaturprofiler i beläggningen. Rullande seismiska ytvågsmätningar har utförts i uppemot 50 km/h och ca 6 000 mätningar har samlats in på ungefär samma tid som en stillastående konventionell seismisk ytvågsmätning kräver. Inom detta projekt har endast ytvågshastigheten och styvhetsmodulen analyserats och jämförts med referensmätningar. Det finns dock potential att utveckla den automatiska analysen av rådata så att också beläggningens tjocklek och den viskösa delen av beläggningens styvhetsmodul kan utvärderas