Broadband Chirp Signals Research Articles

Damage imaging in plates by evaluating local entropy in guided wavefield data

This paper presents a new algorithm called broadband entropy-based two-stage damage imaging (BE2DI) for the detection and imaging of defects in multi-layer plates.The method differentiates defects from either local scattering phenomena of propagating Lamb waves or locally increased vibrational activity for stimulated global plate resonances by exploiting the higher vibrational activity of full-field broadband vibration signals at defective regions and the distinct spatial behavior of defects and artifacts. Using these features, first, a candidate damage map is constructed by means of iterative thresholding based on descriptive statistics of broadband data. Afterwards, an adaptive regional damage index based on local entropy is introduced, which utilizes operational deflection shapes to process previously identified candidate regions and subsequently reveal both shallow and deep defects. BE2DI can be seen as an automated local defect resonance identification approach in the case of shallow defects. Multiple plates having various defect types with different sizes and depths are investigated. Broadband chirp signals are used to excite the plates using low-power piezoelectric actuators, and vibrational data is registered by 3D scanning laser Doppler vibrometry. The obtained results quantitatively and qualitatively demonstrate that the proposed method can effectively detect different kinds of damage, regardless of being shallow or deep.

Monitoring of mechanical changes in a pipe assembly with complex geometry using multi-mode acoustic signals

Guided wave ultrasonic testing has been widely used for nondestructive and long-range evaluation of diverse mechanical structures, such as pipes and plates. In general, a large number of guided wave modes exist even in the simplest geometries, and they exhibit highly dispersive characteristics. Thus, sophisticated excitation methods and elaborate modal analyses need to be implemented to excite, acquire, and process well-defined pure modes as part of such a measurement method. If the interrogated structure has complex geometry, for example, with cross-sectional variations or multiple joints, the utilization of pure modes becomes significantly more difficult. To overcome this difficulty, this paper proposes the use of multi-mode, multi-frequency guided waves for long-range inspection of structures with arbitrarily complex geometries. Using broadband chirp signals and multi-mode time–frequency analysis in a prototypical complex pipe assembly that includes multiple pipe diameters, elbows, and flange connections, we experimentally show sensitive detection of mechanical changes (both material addition and loss). In addition, we demonstrate that the complexity of the structure can be modeled in the digital domain with sufficient detail to allow accurate numerical simulation of the acoustic response using this method.

Impact Damage Detection Using Chirp Ultrasonic Guided Waves for Development of Health Monitoring System for CFRP Mobility Structures.

When impact damage occurs in carbon fiber-reinforced plastic (CFRP) structures, it is barely visible but may cause significant degradation in the mechanical properties of the structure. Hence, a structural health monitoring (SHM) system that can be installed in CFRP mobility structures and is sensitive to impact damage is needed. In this study, we attempted to establish an SHM system based on ultrasonic guided waves, which are generated by inputting a broadband chirp signal into a film-like piezoelectric actuator. The relationship between impact damage size and maximum time-of-flight (ToF) delay was investigated for three types of CFRP plates: woven, non-woven, and hybrid laminates. As a result, it was found that the maximum ToF delay increased linearly with an increase in the damage size for all CFRP laminates. Moreover, the amplitude of the A0 mode was found to be significantly affected by the damage length in the wave propagation direction. Thus, this SHM method using chirp ultrasonic waves can quantitatively evaluate the size and extent of the impact damage in CFRP laminates.

Based on electro-optic phase modulation broadband synthetic aperture imaging lidar

This paper proposes a broadband synthetic aperture imaging lidar based on electro-optic phase modulation, which uses an electro-optic phase modulator to generate a series of broadband chirp signals after the narrow linewidth seed source laser passes through the phase modulator. The de-chirp method is used to achieve pulse compression, and by adopting analogy filters to filter out the interference of multiple harmonics, the near ideal de-chirp signal is attained. The experimental results show that when selecting the first-order sideband signal as the desired signal, we obtain a linear frequency modulated signal with a frequency modulated bandwidth of 3 GHz and a tuning rate of 10 THz/s, and it is achieved the resolution of 0.06 m in the range and 0.02 m in the azimuth.

Broadband nonlinear elastic wave modulation spectroscopy for damage detection in composites

A non-destructive testing procedure is proposed for damage detection in composites using full wavefield measurement obtained using a scanning laser Doppler vibrometer. Vibrations are excited using two low-power piezoelectric actuators leading to nonlinear elastic wave modulation at the defect. One actuator is supplied with a broadband chirp signal and the other actuator is supplied with a single-frequency sine signal. First, a time–frequency filtering method is proposed to extract specific nonlinear components of interest (e.g. second higher harmonic and first modulation sideband) without the need for multiple excitation sequences. Next, damage maps are constructed using broadband bandpower calculation of the filtered nonlinear components. It is demonstrated that the modulation sidebands provide an exclusive imaging of defect nonlinearity, and are not affected by potential source nonlinearity. The proposed damage map construction procedure is applied for various carbon fiber reinforced polymer test specimens with different damage features: (1) coupon with quasi-static indentation damage, (2) coupon with artificial delaminations, (3) bicycle frame with impact damage, and (4) stiffened aircraft panel with partially debonded stiffener. The obtained results indicate the high performance of the developed procedure for detection of various defect types in curved and/or stiffened carbon fiber reinforced polymer components.

Novel instrument for inspecting rock bolt integrity using ultrasonic guided waves

Design of an advanced instrument for inspecting rock bolt integrity by means of ultrasonic guided waves is presented. Issues related to features of multimodal and dispersive ultrasonic guided waves used by the instrument are addressed. Application of chirp compression technique in the form of broadband chirp signals capable of exciting long range guided waves is proposed and successfully verified using a new, portable instrument for rock bolt inspection. Performance of the instrument is illustrated by means of signals collected from real rock bolts.

Optimizing Thermoacoustic Characterization Experiments for Identifiability Improves Both Parameter Estimation Accuracy and Closed-Loop Controller Robustness Guarantees

ABSTRACT This article examines the degree to which optimizing a Rijke tube experiment can improve the accuracy of thermoacoustic model parameter estimation, thereby facilitating robust stability control. We use a one-dimensional thermoacoustic model to describe the combustion dynamics in a Rijke tube. This model contains two unknown parameters that relate velocity perturbations to heat release rate oscillations, namely, a time delay and amplification factor . The parameters are estimated from experiments where the system input is the acoustic excitation from a loudspeaker and the output is the pressure response captured by a microphone. Our work is grounded in the insight that optimizing an experiment’s design for higher Fisher identifiability leads to more accurate parameter estimates. The novel goal of this paper is to apply this insight in the laboratory using a flame-driven Rijke tube setup. For comparison purposes, we conduct a benchmark experiment with a broadband chirp signal as the excitation input. Next, we excite the Rijke tube at two frequencies optimized for Fisher identifiability. Repeats of both experiments show that the optimal experiment achieves parameter estimates with uncertainties at least one order of magnitude smaller than the benchmark. With smaller parameter estimate uncertainties, an LQG controller designed to attenuate combustion instabilities is able to achieve stronger robustness guarantees, quantified in terms of closed-loop structured singular values that account for parameter estimation uncertainty.

Multiscale Entropy Analysis of Instantaneous Frequency Variation to Overcome the Cross-Over Artifact in Rhythmic EEG

Generally, for healthy adults, the entropy of electroencephalogram (EEG) signals gradually decreases from wake to sleep stages N1, N2, to N3, and increases during REM. However, some researchers found that multiscale entropy curves of sleep and wakefulness intercept, a cross-over phenomenon whose origin remains unexplored. The objective of the present work is to trace the origin of the cross-over phenomenon and to propose a workaround strategy. We simulated EEG by generating 1/f broadband signal and chirp signals with continuously varying frequencies. We then retrieved the rhythmic component from simulated EEG and real-world EEG and conducted MSE analysis of the instantaneous frequency variation (IFV) of the rhythmic component. The simulation revealed that this interception was ubiquitous in the MSE analysis of simulated EEG with rhythmic components of different frequencies. The cross-over point moved toward larger scale factors with the increasing sampling rate. We found that the MSE curve of IFV from real-world EEG for the wakefulness group was higher than that for sleep, showing no interception. These results suggest that (1) for a rhythmic signal like EEG, MSE analysis of the raw signal is highly affected by the rhythmic component, presenting artificial cross-over curves in sleep EEG study, (2) frequency variation of rhythmic components are complex signal which differs between wakefulness and sleep, in accordance with the complexity loss theory.

Photonic fractional Fourier transformer for chirp radar with ghost target elimination.

For the first time, to the best of our knowledge, we propose a photonic fractional Fourier transformer (PFrFTer), which is used in chirp radar for detecting multiple non-cooperative targets. Based on photonic rotation of the time-frequency plane, the optimal fractional Fourier domain is formed, and the received broadband chirp signals are projected on it, where they behave as impulses. Moreover, through manipulating the fractional Fourier transform spectrum, the PFrFTer contributes to the cancellation of two ghost target sources, so that the ghost targets in multiple-target circumstances are removed. The simulation and experimental results show that the proposed PFrFTer can adapt to multiple non-cooperative targets environments and is immune to ghost targets at optimal working conditions, which agrees well with the theoretical analysis.

Long range intensity fluctuations on the Chukchi continental shelf measured during the year-long Canada Basin Acoustic Propagation Experiment 2017

Analysis of one-year-long broadband transmissions from five deep water sound sources recorded on shelf during the Canada Basin Acoustic Propagation shows strong azimuthal variability. Broadband chirp signals (140–325 Hz) transmitted every 4 h were received on spatially distributed receiver arrays on the Chukchi shelf from October 2016 through 2017. Here, we present spatial, temporal, and azimuthal correlations by four receiver arrays with source-receiver distances ranging from 228.6 to 524.4 km in a cross-the-shelf orientation. Temporal fluctuations in received field are related to water column variability for each source-receiver pair. Received signals from the same source to each receiver on the shelf, separated from 15.8 to 49.63 km in “along-the-shelf” direction show large intensity fluctuation. This is related to the turbulence that signals have experienced in their path. Temporal and spatial variations of temperature profiles were measured on the shelf, however, from deep water sources to the shelf break region, there is little information. Analysis are conducted for three time zones, from fully frozen seas, transition from full ice, and ice-free conditions. The parabolic equation model is used to report the propagation of field for the ice-free condition. Limitations of the acoustic modeling due to spatially under-sampled input environmental parameters are discussed. [Work supported by ONR-322 OA.]Analysis of one-year-long broadband transmissions from five deep water sound sources recorded on shelf during the Canada Basin Acoustic Propagation shows strong azimuthal variability. Broadband chirp signals (140–325 Hz) transmitted every 4 h were received on spatially distributed receiver arrays on the Chukchi shelf from October 2016 through 2017. Here, we present spatial, temporal, and azimuthal correlations by four receiver arrays with source-receiver distances ranging from 228.6 to 524.4 km in a cross-the-shelf orientation. Temporal fluctuations in received field are related to water column variability for each source-receiver pair. Received signals from the same source to each receiver on the shelf, separated from 15.8 to 49.63 km in “along-the-shelf” direction show large intensity fluctuation. This is related to the turbulence that signals have experienced in their path. Temporal and spatial variations of temperature profiles were measured on the shelf, however, from deep water sources to the shelf ...

Design and testing of three-dimensional additively manufactured anisotropic underwater pentamode materials

Three-dimensional (3-D) pentamode (PM) materials are used in underwater applications because they can yield impedance matching with water and eliminate shear modes over a wide frequency range while resisting shear deformation in the static limit. Further, a PM lattice can also yield significant anisotropy in sound speed, which is useful for devices that rely on transformation acoustics [Su et al., J. Acoust. Soc. Am. 141(6) (2017)]. The present work attempts to verify the predicted anisotropic sound speeds of additively manufactured anisotropic 3-D PM samples. A titanium sample approximately 80 × 80 × 60 mm was suspended in front of a plane-wave source in a water tank to measure the time of flight for wavefronts with and without PM samples present. The measurements were conducted using broadband chirp signals generated by the source, while a scanning hydrophone recorded the response in front of the sample at constant depth. Deconvolution methods were used to extract the impulse response at each scan point and changes in the arrival times of the wavefronts were observed. Frequency domain observations were also made to determine impedance matching characteristics and desired quasi-fluid behavior. Experimental and simulation results will be presented and discussed. [Work Supported by ONR.]

ALGORITHMS FOR IDENTIFICATION OF SPACE OBJECTS BASED ON A SERIES OF RADAR SWEEPS OF BROADBAND SIGNALS IN RANGE

The paper considers a method for processing a series of radar sweeps in range, which can be obtained using broadband signals to identify space objects. Variants of the algorithm that numerically implements this method are developed and their modeling and comparison for signals with linear frequency modulation is carried out. In developing the method, a multi‑step Bayesian scheme for solving the problem of estimating the states of a dynamic Markov system from measurement results was used. A model of radar sweeps in range as the complex envelope of a received radar signal at the output of a filter matched with a probe pulse, which is used when developing an algorithm, is described. Analytical expressions are obtained for the likelihood function of complex radar sweeps in range in the variants of incoherent and coherent processing of sequences of discrete samples and corresponding discrete spectral amplitudes. A computer implementation of the variants of the identification of space objects using a series of radar sweeps was performed. The results of modeling the variants of the identification of space objects using a series of radar sweeps of broadband chirp signals in the case of observing axisymmetric objects are presented. The simulation showed that variants of the algorithm with processing of spectral amplitudes are preferred.

Spatial hearing ability of the pigmented Guinea pig (Cavia porcellus): Minimum audible angle and spatial release from masking in azimuth

Despite the common use of guinea pigs in investigations of the neural mechanisms of binaural and spatial hearing, their behavioral capabilities in spatial hearing tasks have surprisingly not been thoroughly investigated. To begin to fill this void, we tested the spatial hearing of adult male guinea pigs in several experiments using a paradigm based on the prepulse inhibition (PPI) of the acoustic startle response. In the first experiment, we presented continuous broadband noise from one speaker location and switched to a second speaker location (the “prepulse”) along the azimuth prior to presenting a brief, ∼110 dB SPL startle-eliciting stimulus. We found that the startle response amplitude was systematically reduced for larger changes in speaker swap angle (i.e., greater PPI), indicating that using the speaker “swap” paradigm is sufficient to assess stimulus detection of spatially separated sounds. In a second set of experiments, we swapped low- and high-pass noise across the midline to estimate their ability to utilize interaural time- and level-difference cues, respectively. The results reveal that guinea pigs can utilize both binaural cues to discriminate azimuthal sound sources. A third set of experiments examined spatial release from masking using a continuous broadband noise masker and a broadband chirp signal, both presented concurrently at various speaker locations. In general, animals displayed an increase in startle amplitude (i.e., lower PPI) when the masker was presented at speaker locations near that of the chirp signal, and reduced startle amplitudes (increased PPI) indicating lower detection thresholds when the noise was presented from more distant speaker locations. In summary, these results indicate that guinea pigs can: 1) discriminate changes in source location within a hemifield as well as across the midline, 2) discriminate sources of low- and high-pass sounds, demonstrating that they can effectively utilize both low-frequency interaural time and high-frequency level difference sound localization cues, and 3) utilize spatial release from masking to discriminate sound sources. This report confirms the guinea pig as a suitable spatial hearing model and reinforces prior estimates of guinea pig hearing ability from acoustical and physiological measurements.

Effect of broadband chirp signal on the sonic behavior of the /kwa/ fishes inhabiting Posidonia oceanica seagrass meadow

Early experiment to study acoustics of Posidonia oceanica seagrasses (USTICA99) transmitted acoustic chirp signals (0.2-16 kHz, 15.8 s) every minute during 3 days over a meadow. Receiving hydrophones recorded vocalizations from unknown fish inhabitants, which sound like /kwa/. Here, we address the impact of our measurements on this fish population. The authors’ ears analyzed the recordings to identify all fish vocalizations and attempt to recognize /kwa/ individuals based on timbre. A comparative analysis of the /kwa/ sound production within time intervals between and during chirp transmission was made to understand if and how the sonic behavior is affected. As in few similar studies for other anthropogenic noise sources on a limited number of species, there was an observable effect. The major part of the vocalization patterns that were identified within the chirp time interval started near the onset of the frequency sweep, which first spans the range of /kwa/ sound production (0.4-2 kHz). The number of single vocalizations per unit time during that period was close to that during the no-chirp time interval while during higher swept frequencies was markedly smaller. This suggests the fish detection range may extend toward ultrasound to avoid dolphin predators. Quantitative results will be presented.

Deconvolution methods to obtain the impulse response of acoustic metamaterial samples of finite extent

One complication of characterizing the response of acoustic metamaterials is that models often assume the medium is of infinite extent. On the other hand, material property measurement can only be done with samples of finite size. The result is that acoustic field measurements include the effects of edge diffraction and scattering from fixtures. This work presents deconvolution methods to extract the frequency-dependent reflection and transmission behavior of acoustic metamaterial samples. Measurements using broadband chirp signals are obtained and subsequently post-processed using deconvolution techniques to obtain high-time-resolution impulse responses. Additionally, the chirp signals can be modified to compensate for the frequency response of the transducers being used. Examples will be shown for the transmission behavior through a two-dimensional pentamode gradient index lens and the reflection response of a flat square plate. [Work supported by ONR.]

ZOOPS- O2: A broadband echosounder with coordinated stereo optical imaging for observing plankton in situ

Here we describe the configuration, calibration, and initial results from the combination of two recently developed underwater instruments that measure acoustic reflectivity and, simultaneously, the location, pose and size of millimeter-sized plankton relative to the sonar beam. The acoustic system, ZOOPS (ZOOPlankton Sonar), uses a broadband chirp signal that operates with a single monostatically configured transducer in the 1.5–2.5 MHz frequency range. We demonstrate that the system can record, with adequate signal-to-noise levels, identifiable reflections from single copepods with lengths as small as 360 μm. To simultaneously identify taxa and measure orientation, a pair of “O-Cam” microscopes were stereoscopically calibrated and geometrically co-registered with the orientation and range-resolved acoustic transmissions of the sonar beam. The system’s capability is demonstrated via the in situ measurement of acoustic reflectivity as a function of orientation for 224 individual pelagic copepods comprising three orders of free-living taxa. Comparison with a well-known model, the Distorted Wave Born Approximation (DWBA), using a spheroidal formulation, yields both differences and similarities between the in situ field data and the model’s predictions.

Algorithms and software of whitening of noise in the reception of broadband chirp signals in the HF communication channel

Scientific substantiation of the adaptive noise whitening algorithm is given in the HF band when receiving a continuous chirp signal. The software implementation of the algorithm is presented. Full-scale experiments were conducted, using the developed device with a continuous chirp signal, based on the technology of software-defined radio systems. It is established that whitening tends to equalize noise level throughout the HF band. Some rare emissions of whitening spectrum do not exceed the range of 10 dB.

Broadband normal mode energy metrics in the presence of internal waves

During the Shallow Water 2006 experiment a J-15 acoustic source deployed from the Research Vessel Sharp transmitted broadband (100–500 Hz) chirp signals 15 km away from a vertical line array. The array was intentionally positioned near the shelf-break front and in an area where internal waves are known to occur. Normal mode decomposition helps provide clues regarding the physics behind signal fluctuations due to internal waves, but often analyses are accomplished at single frequencies. A broadband modal beamformer approach is offered that extracts separate modal arrivals. A method for a mode-dependent matched filter is suggested which helps extract the separate arrivals in a low-signal environment. These methods can be used to compute mode-independent energy statistics which help explain signal fluctuations as an internal wave traverses the source-receiver path. [Work sponsored by the Office of Naval Research.]

Wavefront statistics from measurements made in the Philippine Sea and comparisons to path integral theory

Between April 2010 and April 2011, acoustic transmissions were carried out by six sources moored near the sound channel axis and, which were deployed across a 200-300 km radius in the Philippine Sea. The acoustic sources transmitted broadband chirp signals that spanned frequency bands ranging from 140-200~Hz to 200-300~Hz. The transmissions were recorded by a water column spanning Distributed Vertical Line Array (DVLA) that was placed roughly at the center of the area covered by the sources. The transmission ranges from the different sources to the DVLA varied from 125~km to 450~km. The Philippine Sea is an oceanographically diverse environment, which apart from internal waves also contains energetic eddies and internal tides. The acoustic data recorded by the spatially diverse array is hence an opportunity to quantify the degree of anisotropy in the acoustic propagation. This presentation first discusses the wavefront resolution capabilities of the DVLA. The receptions are then used to estimate narrowband ray statistics such as, mean intensity, scintillation index and, depth coherence. Apart from the narrowband statistics, broadband wavefront statistics such as pulse time spread, frequency coherence and pulse time wander are also estimated. The wavefront statistics for the different propagation paths at the array are contrasted with each other and then compared with predictions from path integral theory.

Online Tissue Discrimination for Transcutaneous Needle Guidance Applications Using Broadband Impedance Spectroscopy

This paper reports on a novel system architecture for measuring impedance spectra of a biological tissue close to the tip of a hollow needle. The measurement is performed online using fast broadband chirp signals. The time domain measurement raw data are transformed into the transfer function of the tissue in frequency domain. Correlation technique is used to analyze the characteristic shape of the derived tissue transfer function with respect to known "library functions" for different types of tissue derived in earlier experiments. Based on the resulting correlation coefficients the exact type of tissue is determined. A bipolar coaxial needle is constructed, simulated by finite element method and tested during various in vitro and in vivo experiments. The results show a good spatial resolution of approximately 1.0 mm for a needle with a diameter of 2.0 mm. The correlation coefficients for the three tested tissue types muscle, fat, and blood allow for a clear tissue classification. Best results have been obtained using the characteristic phase diagrams for each tissue. Correlated to the corresponding library transfer function the coefficients are in the range of +0.96 to +0.99 for the matching tissue. In return, the resulting coefficients for correlation with nonmatching tissues are in the range of -0.93 to +0.81.