Approach for Simultaneous Determination of Thickness and Sound Velocity in Layered Structures Based on Sound Field Simulations

For imaging in NDT or in medical diagnostics, the value of sound velocity is assumed a priori. Interfaces of hidden objects are imaged by the measured time of flight (ToF). The supposed locations and extensions of these objects are incorrect if the actual sound velocity differs from the assumed. For material characterization the thickness of a specimen is determined by mechanical measurements and the sound velocity is determined by ToF-measurements. For multi-layered structures the mechanical determination of the thickness of the different layers is impossible non-destructively. It is necessary to determine both quantities simultaneously to get information about the thickness and the material of the different layers. A variation method is introduced in [1] allowing the simultaneous determination of sound velocity and thickness of up to two layers by focusing with an annular array at a fixed position. It works by varying the focus positions and the assumed sound velocity, which is used to calculate the delay times for each control mode by means of FERMAT’s principle. The amplitude of the echo signals is determined as a function of the control mode. Because the sound field depends on the sound velocity of the medium and the control mode evaluating the amplitude of the echo signals yields additional information besides the time of flight. Alternatively, in [2], a fast and efficient method for a simultaneous determination of sound velocity and thickness of a two- layered structure has been presented. It analyses the different signal parts of an echo reflected from the examined interface. These signal parts correspond to different propagation paths. The difference in time of flight between the signal parts contains the information about thickness and sound velocity of the layer. These time differences are used as an input for an inverse geometric model. Although an accuracy of over 95% had been reached, increasing this accuracy fails, because in both cases the analysis of the signals only uses a geometric model neglecting the wave properties. A half-analytical method based on GREEN’s functions and point sources synthesis is used to calculate the sound field in the multi-layered structures. The echoes of several interfaces are calculated for each element of the used array. Using the same parameter of specimen and the array as in the experiments the evaluation of the simulated signal yields correct time differences based on the wave propagation. They allow assuming effective, corrected source points for the geometric model. With such an optimization of the geometric model an accuracy of 99% can be reached for simulated signals. Measurements are executed on two-layered structures consisting of a first layer of water and a second layer of steel, cupper or aluminum with a thickness of d = 6 mm, 8 mm, 10 mm and 12 mm. For the second layer a deviation for the combined determination of sound velocity and thickness between 3% and 5% is reached with the geometric model for both evaluation methods. With the corrected source point the accuracy can be improved

Material Characterization of Layered Structures with Ultrasound

AbstractThis contribution presents a method for the simultaneous determination of sound velocity and thickness of a system consisting of two layers. To obtain additional information, beside the time of flight (TOF), the amplitude is evaluated. The amplitude of a reflected sound wave depends on the position of the interface in the sound field of the probe and is maximal, if the focus lies on the interface. The focus position is varied by the use of an annular array and the amplitude as a function of the adjusted focus position is evaluated. Since the sound field of the ring elements of the annular array has distinctly side lobes, transverse waves are excited in the specimen and the sound velocity of the transversal wave can also be determined.The method is demonstrated for the determination of layer thicknesses and sound velocities (longitudinal and transversal) on a two layered system

A third massive star component in the \sigma\ Orionis AB system

We report on the detection of a third massive star component in the \sigma\ Orionis AB system, traditionally considered as a binary system. The system has been monitored by the IACOB spectroscopic survey of Northern Massive stars program, obtaining 23 high-resolution FIES@NOT spectra with a time-span of ~2.5 years. The analysis of the radial velocity curves of the two spectroscopic components observed in the spectra has allowed us obtain the orbital parameters of the system, resulting in a high eccentric orbit (e~0.78) with an orbital period of 143.5 +/- 0.5 d. This result implies the actual presence of three stars in the \sigma\ Orionis AB system when combined with previous results obtained from the study of the astrometric orbit (with an estimated period of ~157 yr).Comment: 7 pages, 3 figures, 2 tables. Accpeted for publication in Ap

The Sound of Surveillance: Enhancing Machine Learning-Driven Drone Detection with Advanced Acoustic Augmentation

In response to the growing challenges in drone security and airspace management, this study introduces an advanced drone classifier, capable of detecting and categorizing Unmanned Aerial Vehicles (UAVs) based on acoustic signatures. Utilizing a comprehensive database of drone sounds across EU-defined classes (C0 to C3), this research leverages machine learning (ML) techniques for effective UAV identification. The study primarily focuses on the impact of data augmentation methods—pitch shifting, time delays, harmonic distortion, and ambient noise integration—on classifier performance. These techniques aim to mimic real-world acoustic variations, thus enhancing the classifier’s robustness and practical applicability. Results indicate that moderate levels of augmentation significantly improve classification accuracy. However, excessive application of these methods can negatively affect performance. The study concludes that sophisticated acoustic data augmentation can substantially enhance ML-driven drone detection, providing a versatile and efficient tool for managing drone-related security risks. This research contributes to UAV detection technology, presenting a model that not only identifies but also categorizes drones, underscoring its potential for diverse operational environments

Approach for Simultaneous Determination of Thickness and Sound Velocity in Layered Structures Based on Sound Field Simulations

For imaging in NDT or in medical diagnostics, the value of sound velocity is assumed a priori. Interfaces of hidden objects are imaged by the measured time of flight (ToF). The supposed locations and extensions of these objects are incorrect if the actual sound velocity differs from the assumed. For material characterization the thickness of a specimen is determined by mechanical measurements and the sound velocity is determined by ToF-measurements. For multi-layered structures the mechanical determination of the thickness of the different layers is impossible non-destructively. It is necessary to determine both quantities simultaneously to get information about the thickness and the material of the different layers. A variation method is introduced in [1] allowing the simultaneous determination of sound velocity and thickness of up to two layers by focusing with an annular array at a fixed position. It works by varying the focus positions and the assumed sound velocity, which is used to calculate the delay times for each control mode by means of FERMAT’s principle. The amplitude of the echo signals is determined as a function of the control mode. Because the sound field depends on the sound velocity of the medium and the control mode evaluating the amplitude of the echo signals yields additional information besides the time of flight. Alternatively, in [2], a fast and efficient method for a simultaneous determination of sound velocity and thickness of a two- layered structure has been presented. It analyses the different signal parts of an echo reflected from the examined interface. These signal parts correspond to different propagation paths. The difference in time of flight between the signal parts contains the information about thickness and sound velocity of the layer. These time differences are used as an input for an inverse geometric model. Although an accuracy of over 95% had been reached, increasing this accuracy fails, because in both cases the analysis of the signals only uses a geometric model neglecting the wave properties. A half-analytical method based on GREEN’s functions and point sources synthesis is used to calculate the sound field in the multi-layered structures. The echoes of several interfaces are calculated for each element of the used array. Using the same parameter of specimen and the array as in the experiments the evaluation of the simulated signal yields correct time differences based on the wave propagation. They allow assuming effective, corrected source points for the geometric model. With such an optimization of the geometric model an accuracy of 99% can be reached for simulated signals. Measurements are executed on two-layered structures consisting of a first layer of water and a second layer of steel, cupper or aluminum with a thickness of d = 6 mm, 8 mm, 10 mm and 12 mm. For the second layer a deviation for the combined determination of sound velocity and thickness between 3% and 5% is reached with the geometric model for both evaluation methods. With the corrected source point the accuracy can be improved.</p

Investigation of embedded structures in media with unknown acoustic properties

For the nondestructive evaluation of components with ultrasound a priori information about the specimen is necessary. So the time of flight to a defect is measured and, with known sound velocity, it is possible to determine the correct location of the defect. In general, the sound velocity is assumed as known. If it is not known, the sound velocity has to be determined additionally. This can be done, for example, by measuring the time of flight to the backwall with ultrasound and the thickness of the specimen with a caliper gauge. However, this is impossible to realize with single-sided access to the specimen. For determining the size of inclusions, several techniques like the half-value method or the DGS-method (Distance Gain Size) are established. These methods are based on the assumption of (circular) plane reflectors. Therefore, they cannot be applied on the size determination of inclusions with curved surfaces. This contribution presents new methods for the localization and characterization of inclusions in media with unknown acoustic properties by means of annular arrays. The usage of annular arrays allows the variation of the focus position. By evaluating the signal amplitude as a function of the focus position and the measured time of flight, sound velocity and distance to the reflector can be determined simultaneously [1]. For classifying the inclusions, the directional pattern of the reflected sound field is evaluated. The sound pressure distribution of the reflected sound field at the probe surface mainly depends on the shape of the reflector. Evaluating the amplitude difference between two annular elements allows to classify the reflector shape [2]. These methods are demonstrated on a test specimen with in epoxy resin embedded spheres and circular disc-shaped reflectors with diameters between one and seven wavelengths. The sound velocity of the epoxy resin can be determined with the simultaneous method reaching an accuracy higher than 97%. The amplitude difference method allows to distinguish between spheres and circular disc-shaped reflectors. The same method also enables a non-scanning determination of the size of the circular disc-shaped reflectors, even for dimensions smaller than the width of the sound beam. For the size determination of the spheres, several methods are presented and discussed.</p

Investigation of Embedded Structures in Media with Unknown Acoustic Properties

For the nondestructive evaluation of components with ultrasound a priori information about the specimen is necessary. So the time of flight to a defect is measured and, with known sound velocity, it is possible to determine the correct location of the defect. In general, the sound velocity is assumed as known. If it is not known, the sound velocity has to be determined additionally. This can be done, for example, by measuring the time of flight to the backwall with ultrasound and the thickness of the specimen with a caliper gauge. However, this is impossible to realize with single-sided access to the specimen. For determining the size of inclusions, several techniques like the half-value method or the DGS-method (Distance Gain Size) are established. These methods are based on the assumption of (circular) plane reflectors. Therefore, they cannot be applied on the size determination of inclusions with curved surfaces. This contribution presents new methods for the localization and characterization of inclusions in media with unknown acoustic properties by means of annular arrays. The usage of annular arrays allows the variation of the focus position. By evaluating the signal amplitude as a function of the focus position and the measured time of flight, sound velocity and distance to the reflector can be determined simultaneously [1]. For classifying the inclusions, the directional pattern of the reflected sound field is evaluated. The sound pressure distribution of the reflected sound field at the probe surface mainly depends on the shape of the reflector. Evaluating the amplitude difference between two annular elements allows to classify the reflector shape [2]. These methods are demonstrated on a test specimen with in epoxy resin embedded spheres and circular disc-shaped reflectors with diameters between one and seven wavelengths. The sound velocity of the epoxy resin can be determined with the simultaneous method reaching an accuracy higher than 97%. The amplitude difference method allows to distinguish between spheres and circular disc-shaped reflectors. The same method also enables a non-scanning determination of the size of the circular disc-shaped reflectors, even for dimensions smaller than the width of the sound beam. For the size determination of the spheres, several methods are presented and discussed.</p