Weak Reflectors Research Articles

Ultrasonic Lamb wave detection using a fiber-optic quasi-distributed acoustic sensing system.

The Lamb wave represents one of the most utilized forms of ultrasonic guided waves for flaw detection. A novel approach, to the best of our knowledge, for Lamb wave sensing using a fiber-optic quasi-distributed acoustic sensing (QDAS) system is proposed. Sensing elements constructed with optical weak reflectors are designed and analyzed, demonstrating the theoretical feasibility of establishing an ultrasonic sensing array comprising over 300 points. An empirical mode decomposition method is applied to this system to remove harmonic components in a frequency division multiplexed interrogation system. Verification experiments are carefully designed, employing compact 2.5 cm-sized sensing elements to capture 1 MHz Lamb wave signals on an aluminum plate. These findings significantly broaden the applicability of QDAS in the field of ultrasonic flaw detection.

High-spatial-resolution quasi-distributed acoustic sensing with phase noise suppression based on φ-OFDR

A quasi-distributed acoustic sensor with high-spatial-resolution based on phase-sensitive optical frequency domain reflectometry (φ-OFDR) was demonstrated. The source of noise that affects the accuracy of phase demodulation was analyzed. Two low noise linear frequency sweeps (LFSs) with different sweep ranges obtained by injection-locking method were used to achieve high-spatial-resolution and long-distance sensing. The phase noise caused by laser, intensity and fading noises was effectively suppressed by further combining a femtosecond-laser-inscribed weak reflector array (WRA) and a phase index method (PIM). Vibration signals located a long distance of over 1 km were successfully demodulated with a spatial resolution of 2 cm through the use of 12 GHz LFS and a WRA with an interval of 2 cm. In addition, the 5 mm ultra-high spatial resolution capability of the system had been demonstrated through the use of 52 GHz LFS and a WRA with an interval of 5 mm.

Vibration measurement technique by using OFDR with in-line interferometers

Due to their compact size, electromagnetic interference immunity, and compatibility with embedding in composite materials, optical fiber sensors have emerged as a versatile and integral solution for structural health monitoring (SHM) in various structures. Among the key contributors to the evolution of this field is the significant progress in optical frequency domain reflectometry (OFDR), a technology that offers unparalleled high resolution and sensitivity in distributed strain sensing, thereby establishing itself as an indispensable tool in the realm of SHM. Through the implementation of OFDR, we devised a distributed fiber optic sensor tailored for the precise measurement of vibration or strain within structures. In previous works, vibration monitoring has been achieved using an OFDR with an array of 20 in-line interferometers, serving as weak reflectors that not only facilitated vibration monitoring but also furnished valuable strain information. In this study takes a step further by introducing an innovative technique wherein in-line interferometers are strategically arranged in a non-uniform fashion along the optical fiber. To validate the practical utility of this technology, extensive simulations and experiments were conducted, utilizing a cantilever beam as a representative structure. The results obtained from these endeavors showcase the tangible potential and reliability of optical fiber sensors in the practical implementation of SHM across various applications. By enhancing the precision and scope of structural monitoring, optical fiber sensors equipped with non-uniform interferometer arrangements pave the way for more effective and comprehensive structural health assessment in diverse settings.

Least-squares reverse time migration of simultaneous sources with deep-learning-based denoising

Least-squares reverse time migration (LSRTM) is currently one of the most advanced migration imaging techniques in the field of geophysics. It uses least-squares inversion to fit the observed data, resulting in high-resolution imaging results with more accurate amplitudes and better illumination compensation than conventional reverse time migration (RTM). However, noise in the observed data and the Born approximation forward operator can result in high-wavenumber artifacts in the final imaging results. Moreover, iteratively solving LSRTM leads to one or two orders of computational cost higher than conventional RTM, making it challenging to apply extensively in industrial applications. Simultaneous source acquisition technology can reduce the computational cost of LSRTM by reducing the number of wavefield simulations. However, this technique also can cause high-wavenumber crosstalk artifacts in the migration results. To effectively remove the high-wavenumber artifacts caused by these issues, simultaneous source and deep learning are combined to speed up LSRTM as well as suppress high-wavenumber artifacts. A deep residual neural network (DR-Unet) is trained with synthetic samples, which are generated by adding field noise to synthesized noise-free migration images. Then, the trained DR-Unet is applied on the gradient of LSRTM to remove high-wavenumber artifacts in each iteration. Compared to directly applying DR-Unet denoising to LSRTM results, embedding DR-Unet denoising into the inversion process can better preserve weak reflectors and improve denoising effects. Finally, the proposed LSRTM method is tested on two synthetic data sets and a land data set. The tests demonstrate that the proposed method can effectively remove high-wavenumber artifacts, improve imaging results, and accelerate convergence speed.

Cascaded Weak Reflector Coaxial Cable Structure for Point and Distributed Large-Strain Sensing

Cascaded Weak Reflector Coaxial Cable Structure for Point and Distributed Large-Strain Sensing

Upper limit of the solar wind protons backscattering efficiency from Comet 67P/Churyumov-Gerasimenko

Context. Solar wind ions backscattering is a fundamental plasma-surface interaction process that may occur on all celestial bodies exposed to the solar wind and lacking a significant atmosphere or magnetosphere. Yet, observations have been limited to the regolith-covered Moon and Phobos, one of the Martian moons. Aims. We aim to expand our knowledge of the process to include comets by investigating the backscattering of solar wind protons from the surface of comet 67P/Churyumov-Gerasimenko. Methods. We used one of the ion spectrometers on board ESA’s Rosetta spacecraft to search for evidence of backscattered solar wind protons from the cometary surface. The signal of interest was expected to be very weak and several statistical treatments of the data were essential to eliminate any influence from background noise and instrumental effects. Due to limited knowledge of the signal location within the observed parameter space, we conducted a statistical analysis to identify the most probable conditions for detecting the signal. Results. No significant solar wind backscattered protons were ever observed by the instrument. The statement applies to the large spectrum of observation conditions. An upper limit of the backscattered proton flux is given, as well as an upper limit of the backscattering efficiency of 9 × 10−4. Conclusions. The surface of comet 67P/Churyumov-Gerasimenko distinguishes itself as a notably weak reflector of solar wind protons, with its backscattering efficiency, at most, as large as the lowest observed backscattering efficiency from the lunar regolith.

Real-time ultrasound phase imaging

The Correlation-Based (CB) imaging method is characterized by its high spatial resolution capabilities, but it is known to require heavy computational resources due to its high complexity. This paper shows that the CB imaging method can be used to estimate the phase of the complex reflection coefficients contained in the observation window. The resulting Correlation-Based Phase Imaging (CBPI) method can be used to segment and identify different features or tissue elasticity variations in a given medium. A Numerical validation is first proposed by considering a set of fifteen point-like scatterers on a Verasonics Simulator. Then, three experimental datasets are used to show the potential of CBPI on scatterers and specular reflectors. In vitro imaging results are first presented to show that CBPI allows retrieving phase information on hyperechoic reflectors, but also on weak reflectors such as elasticity targets. It is demonstrated that CBPI helps distinguishing regions of different elasticity, but of same low-contrast echogenicity, which is otherwise impossible with standard B-mode or Synthetic Aperture Focusing Techniques (SAFT). Then, CBPI of a needle in an ex vivo chicken breast is performed to show that the method works on specular reflectors. It is shown that the phase of the different interfaces associated to the first wall of the needle are well reconstructed using CBPI. The heterogeneous architecture used to enable real-time CBPI is presented. A Nvidia GeForce RTX 2080 Ti Graphics Processing Unit (GPU) is used to process the real-time acquired signals from a Verasonics Vantage 128 research echograph. Frame rates of 18 frames per second are achieved for the whole acquisition and signal processing chain on standard a 500 × 200 pixels grid.

Multipath Quasi-Distributed Acoustic Sensing Based on Space Division Multiplexing and Time-Gated OFDR

Quasi-distributed optic sensing system has been widely utilized in various applications for its simple configuration, high sensing resolution, and long sensing range. In large sensing network, multipath sensing arrays are needed to be monitored simultaneously. This paper proposes a multi-path quasi-distributed acoustic sensing system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) and spatial division multiplexing (SDM) technique. The single mode fiber segment between weak reflectors is efficiently used. In experiments, vibrations on two weak reflectors array are detected simultaneously, and crosstalk between pathes is analyzed. This multi-path system does not require additional hardware equipment and the bandwidth of the system is not sacrificed, which has great potential in large-scale sensing network.

Systematic study on the optoelectronic and elastic properties of Cu-based ternary chalcogenides: Using ab-initio Approach

The first principles-based GGA approach is used to investigate the elastic and optoelectronic properties of Cu-based ternary chalcogenides QCu3Te4 (Q = Ta, V, Nb) to explore their potential and significance in photovoltaic applications. The calculated energy band structures using GGA approach were corrected using hybrid GGA + U functional. Our study demonstrates that QCu3Te4 (Q = Ta, V, Nb) are the semiconductors with indirect energy band gaps as CBM and VBM occur at different symmetric points (R-X). The Bandgap values of QCu3Te4 (Q = Ta, V, Nb) are reduced by replacing Ta with V and Nb; thereby, the values of bandgaps are 1.5, 0.49, and 1.41 eV for TaCu3Te4, VCu3Te4 and NbCu3Te4, respectively. Therefore, a detailed examination of the optical parameters is performed to demonstrate the prospect of QCu3Te4 (Q = Ta, V, Nb) for optoelectronic device applications. The calculated results revealed that QCu3Te4 (Q = Ta, V, Nb) could be used as antireflecting coatings as such materials are weak reflectors of incident photons and reflect a maximum of 45% photo radiations in the upper UV region. Since VCu3Te4 is an efficient absorber of incident photo radiations in the infrared region, the TaCu3Te4 and NbCu3Te4 show maximum absorption in the visible and near UV regions. Furthermore, in the elastic properties the Poisson's ratio ‘σ’ indicates that QCu3Te4 (Q = Ta, V, Nb) are covalent compounds and are anisotropic as the values of Zener anisotropy factor ‘A’ is not equal to 1. Also, the Pugh's ratio (B/G) values for QCu3Te4 (Q = Ta, V, Nb) are under 1.75. Thus, these Cu-based chalcogenide materials are brittle compounds that can provide an efficient route for optoelectronic devices. It is evident from the spectra of S that NbCu3Te4/TaCu3Te4 are p-type semiconductors; however, VCu3Te4 is an n-type semiconductor. We can predict that TaCu3Te4 is the most promising thermoelectric material for TE devices.

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Investigating the effect of alkali metals on the structural & optoelectronic properties of hexafluorozirconate red phosphors A2ZrF6 (A = Cs, K, Na) using first-principles calculations: A prospect for warm-white LEDs (w-LEDs) applications

To meet the energy challenges of the near future, the development of environment-friendly and energy-efficient phosphors for luminescence devices like light-emitting diodes (LEDs) is the main quest of researchers nowadays. In this connection, traditional lighting devices like halogen lamps, fluorescent lamps, backlights of liquid crystal displays (LCDs), and incandescent lamps can be replaced by state-of-the-art next-generation lighting technology developed using phosphor-converted white LEDs. Hereby, we have presented a systematic study on the structural and optoelectronic properties of Zr-based phosphors A2ZrF6 (A ​= ​Cs, K, Na) for potential photoluminescence and photovoltaic applications like LEDs. The GGA and GGA ​+ ​U schemes are used for exchange and correlation energy potentials treatment to provide a comparative study on the density functional theory (DFT) based first-principles calculations. The GGA ​+ ​U approach improves bandgap values, confirming the well-known flaw of GGA-based DFT methods in underestimating band gaps in strongly correlated systems. Indirect bandgaps are found with the values 2.05, 6.415, and 7.648 ​eV for Cs2ZrF6, K2ZrF6, and Na2ZrF6, respectively. The ε2(ω) spectra show that Cs2ZrF6 absorbs the maximum number of incident photons in the visible region, whereas K2ZrF6 and Na2ZrF6 absorb the maximum of the incident photons in the near UV region. From the R(ω) spectra, we can note that A2ZrF6 (A ​= ​Cs, K, Na) are the weak reflector of incident photons in the visible and IR regions. In contrast, up to 40% of the incident photons are reflected on higher energies (above 12.0 ​eV).

Huber inversion-based reverse-time migration with de-primary imaging condition and curvelet-domain sparse constraint

Least-squares reverse-time migration (LSRTM) formulates reverse-time migration (RTM) in the least-squares inversion framework to obtain the optimal reflectivity image. It can generate images with more accurate amplitudes, higher resolution, and fewer artifacts than RTM. However, three problems still exist: (1) inversion can be dominated by strong events in the residual; (2) low-wavenumber artifacts in the gradient affect convergence speed and imaging results; (3) high-wavenumber noise is also amplified as iteration increases. To solve these three problems, we have improved LSRTM: firstly, we use Huber-norm as the objective function to emphasize the weak reflectors during the inversion; secondly, we adapt the de-primary imaging condition to remove the low-wavenumber artifacts above strong reflectors as well as the false high-wavenumber reflectors in the gradient; thirdly, we apply the L 1 -norm sparse constraint in the curvelet-domain as the regularization term to suppress the high-wavenumber migration noise. As the new inversion objective function contains the non-smooth L 1 -norm, we use a modified iterative soft thresholding (IST) method to update along the Polak-Ribière conjugate-gradient direction by using a preconditioned non-linear conjugate-gradient (PNCG) method. The numerical examples, especially the Sigsbee2A model, demonstrate that the Huber inversion-based RTM can generate high-quality images by mitigating migration artifacts and improving the contribution of weak reflection events.

Quasi-distributed fiber-optic acoustic sensor based on a low coherence light source.

A quasi-distributed acoustic sensor using in-line weak reflectors and a low coherence light source is presented. The dynamic strain is retrieved from the phase change of the two interfering light beams reflected by the same weak reflector. In the experiments, two vibrations at different channels along a weak reflector array are successfully detected simultaneously. A strain resolution of 50 pɛ/H z with 20-m interval is achieved in experiments, and no cross talk is observed. With simple system configuration and low cost, this approach provides a new, to the best of our knowledge, solution for quasi-distributed acoustic sensing.

In-Fiber Integrated Quasi-Distributed Temperature Sensor Array With High Spatial Resolution for Silicon Nitride Igniter

As a key component in the engine system, silicon nitride igniter’s stable and reliable operation is a necessary condition for the proper functioning of the engine system. However, an online high temperature sensor of silicon nitride igniter inside the engine is absent. This paper presents and demonstrates a novel in-fiber integrated high temperature sensor array with high spatial resolution for online temperature field measurement on a silicon nitride igniter. The sensor array with a series of weak reflectors is written by a femtosecond laser in the core of single-mode fiber. The reflectivity of each reflector can be controlled from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> by adjusting the written power of femtosecond laser. Two adjacent weak reflectors form a compact fiber sensor. The size of a single sensor can be as small as 3–5 mm, so it is able to measure the temperature fluctuation in a space down to several millimeters. The five-sensor-array, with the help of a white light interference demodulation system, can measure high temperature up to 1000°C. Due to the advantages of high reliability, compact construction and simple fabrication, this high temperature sensor array can be widely applied in high temperature sensing applications with space constrains.

Effect of S, Se and Te replacement on structural, optoelectronic and transport properties of SrXO4 (X= S, Se, Te) for energy applications: A first principles study

Optoelectronic and thermoelectric properties of SrXO4 (X ​= ​S, Se, Te) have been investigated with the help of WIEN2K code by using FP-LAPW method and GGA approximation of the DFT approach. The obtained results show that SrSO4 and SrSeO4 are direct wide band gap semiconductors, while, SrTeO4 is an indirect band gap semiconductor. Based on theoretical study of optical properties, it has been established that the investigated compounds are promising candidates for active optical devices operating in the UV and visible range. These compounds have also been found to be weak reflectors for incident photons as they reflect up to 60% of the incident photons in the upper UV region. The semi-classical Boltzmann transport kinetic equations are used to study thermoelectric properties such as electrical conductivity, thermal conductivity, power factor, Seebeck coefficient and Figure of merit of the considered compounds. The research shows that SrSeO4 and SrTeO4 are promising compounds for thermoelectric devices.

Distributed Acoustic Sensing Based on Coherent Microwave Photonics Interferometry.

A microwave photonics method has been developed for measuring distributed acoustic signals. This method uses microwave-modulated low coherence light as a probe to interrogate distributed in-fiber interferometers, which are used to measure acoustic-induced strain. By sweeping the microwave frequency at a constant rate, the acoustic signals are encoded into the complex microwave spectrum. The microwave spectrum is transformed into the joint time–frequency domain and further processed to obtain the distributed acoustic signals. The method is first evaluated using an intrinsic Fabry Perot interferometer (IFPI). Acoustic signals of frequency up to 15.6 kHz were detected. The method was further demonstrated using an array of in-fiber weak reflectors and an external Michelson interferometer. Two piezoceramic cylinders (PCCs) driven at frequencies of 1700 Hz and 3430 Hz were used as acoustic sources. The experiment results show that the sensing system can locate multiple acoustic sources. The system resolves 20 nε when the spatial resolution is 5 cm. The recovered acoustic signals match the excitation signals in frequency, amplitude, and phase, indicating an excellent potential for distributed acoustic sensing (DAS).

Effect of Nb, Ta and V replacements on electronic, optical and elastic properties of NbCu3Se4: A GGA+U study

Development of efficient, renewable and eco-friendly energy sources is on rise and is the main quest of the researchers to meet the energy challenges of future due to vanishing fossil fuels. In this connection, chalcogenides are the most promising materials to be used in state-of-the-art next generation solar cells for efficient production of clean energy from solar radiations. Hereby, we have presented an organized investigation of the optical and electronic properties of XCu3Se4 (X ​= ​Nb, Ta, V) by using first-principles GGA ​+ ​U calculations for their potential applications in optoelectronic devices such as solar cells. Our calculations show that NbCu3Se4 and TaCu3Se4 are semiconductor compounds; however, VCu3Se4 is intermetallic in nature because it shows semiconducting nature in the minority spin channel and metallic nature in the majority spin channel. NbCu3Se4 and TaCu3Se4 possess indirect band gaps as valence band maxima (VBM) and conduction band minima (CBM) are present at two different axes of IBZ (irreducible Brillouin zone), however, VCu3Se4 possess direct band gap in minority spin channel and shows metallic nature in majority spin channel. Energy band gaps of aforesaid compounds increase by replacing Nb with Ta and V. We have reported a detailed investigation of the optical constants to explore the prospect of these materials for optoelectronic applications. This study reveals that these compounds are weak reflectors of incident photons in infrared and visible regions, however, these compounds show up to 50% reflection of incident photons in the upper UV region. Elastic results reveal that these cubic compounds are elastically anisotropic materials with covalent bonding. Material shows ductile nature if value of (B/G) is greater than 1.75 otherwise it must be brittle. In our case, values of (B/G) or NbCu3Se4, TaCu3Se4 and VCu3Se4 are less than 1.75. Therefore, these chalcogenides are brittle in nature.

Seismic evidence for crustal architecture and stratigraphy of the Limpopo Corridor: New insights into the evolution of the sheared margin offshore southern Mozambique

The Gondwana breakup along the Mozambique continental margin caused the formation of extensional and/or sheared margins. One of these sheared margins offshore southern Mozambique, the Limpopo Corridor (LC), is a key area for understanding the evolution of the southward movement of Eastern Gondwana with respect to Western Gondwana. However, crustal architecture and stratigraphy in this area have been inadequately studied, mainly due to the lack of deep seismic data.In this study, we present interpretations of new multichannel seismic reflection profiles oriented west to east across the LC and Mozambique Fracture Zone (MFZ). Our results show that the basement of the LC is characterized by dipping and alternating strong and weak reflectors, revealing a thick Volcano-Sedimentary Sequence (VSS). To the west of the LC, a marginal basement high zone extends from north to south, consisting of an eroded and fractured VSS. Besides, the acoustic basement is characterized by an eastward dipping or flat VSS in the LC. Sediments overlying the top of the VSS reveal an onlap geometry against the basement high. Five major stratigraphic units (U1-U5) with well-layered sedimentary sequences are mainly constrained from the western boundary of the basement high to the eastern MFZ, and dated to Neocomian, Middle Cretaceous, Late Cretaceous, Paleogene, and Neogene times respectively.We propose that the VSS in the acoustic basement likely formed in pre-Neocomian time. An uplift event occurred continuously from the Oxfordian-Kimmeridgian to Middle Cretaceous, resulting in a basement high that represents the western limit of the LC. Between this limit and the MFZ, the LC experienced the intracontinental rifting, wrench faulting, and discrepant subsidence associated with the southward strike-slip movement of East Gondwana along the MFZ. The earliest sediments overlying the fractured and deformed VSS consist of a wedge-shaped unit (U1) that deposited along the entire LC during the active transform stage. When the spreading centre passed the adjacent segment of the LC in the Mozambique Basin, the depocentre rapidly shifted to the basin, and the LC entered a passive transform margin stage. We propose that the LC results from three successive stages: (1) intracontinental rifting stage with substantial uplift and erosion, (2) active transform margin stage with strike-slip movement along the MFZ, and (3) passive transform margin with a different sedimentary setting.

Dithered depth imaging

Single-photon lidar (SPL) is a promising technology for depth measurement at long range or from weak reflectors because of the sensitivity to extremely low light levels. However, constraints on the timing resolution of existing arrays of single-photon avalanche diode (SPAD) detectors limit the precision of resulting depth estimates. In this work, we describe an implementation of subtractively-dithered SPL that can recover high-resolution depth estimates despite the coarse resolution of the detector. Subtractively-dithered measurement is achieved by adding programmable delays into the photon timing circuitry that introduce relative time shifts between the illumination and detection that are shorter than the time bin duration. Careful modeling of the temporal instrument response function leads to an estimator that outperforms the sample mean and results in depth estimates with up to 13 times lower root mean-squared error than if dither were not used. The simple implementation and estimation suggest that globally dithered SPAD arrays could be used for high spatial- and temporal-resolution depth sensing.

Echolocating bats detect but misperceive a multidimensional incongruent acoustic stimulus

Coherent perception relies on integrating multiple dimensions of a sensory modality, for example, color and shape in vision. We reveal how different acoustic dimensions, specifically echo intensity and sonar aperture (or width), are important for correct perception by echolocating bats. We flew bats down a corridor blocked by objects with different intensity-aperture combinations. To our surprise, bats crashed straight into large (aperture) walls with weak echo intensity as if they did not exist. The echolocation behavior of the bats indicated that they did detect the wall, suggesting that crashing was not a result of limited sensory sensitivity, but of a perceptual deficit. We systematically manipulated intensity and aperture by changing the materials and width of different reflectors, and we conclude that a coherent echo-based percept is created only when these two acoustic dimensions have certain relations which are typical for objects in nature (e.g., large and intense or small and weak reflectors). Nevertheless, we show that these preferred relations are not innate. We show that young pups are not constrained to these relations and that new intensity-aperture associations can also be learned by adult bats.