Voxel based method for predictive modelling of solidification and stress in digital light processing based additive manufacture.

A method for predicting the solidification and stress of a digital light processing 3D print process is presented, using a voxel-based, multi-layer model to predict the degree of polymerization of the material at every stage during the print. Additive manufacturing offers extremely short development cycles, making predictive modelling of the complex chemical and mechanical interactions of photo-polymerization during part construction unappealing compared to iterative work-flows. Accurate predictions of stress, and the impact of the print parameters and post-print process upon stress, become increasingly important for 3D printing micro-scale electrical and mechanical systems as we design resonators and conductive layers. The process uses a simple method of printed cantilevers to calibrate the stress from various print processes such as propagation of the polymerization front and polymerization gradient. The model is found to have good predictive value and is capable of stress and solidification prediction from a computer aided design file.

Resilin Distribution and Sexual Dimorphism in the Midge Antenna and Their Influence on Frequency Sensitivity.

Small-scale bioacoustic sensors, such as antennae in insects, are often considered, biomechanically, to be not much more than the sum of their basic geometric features. Therefore, little is known about the fine structure and material properties of these sensors-even less so about the degree to which the well-known sexual dimorphism of the insect antenna structure affects those properties. By using confocal laser scanning microscopy (CLSM), we determined material composition patterns and estimated distribution of stiffer and softer materials in the antennae of males and females of the non-biting midge Chironomus riparius. Using finite element modelling (FEM), we also have evidence that the differences in composition of these antennae can influence their mechanical responses. This study points to the possibility that modulating the elastic and viscoelastic properties along the length of the antennae can affect resonant characteristics beyond those expected of simple mass-on-a-spring systems-in this case, a simple banded structure can change the antennal frequency sensitivity. This constitutes a simple principle that, now demonstrated in another Dipteran group, could be widespread in insects to improve various passive and active sensory performances.

Porpoise click classifier (PorCC): A high-accuracy classifier to study harbour porpoises (Phocoena phocoena) in the wild.

Harbour porpoises are well-suited for passive acoustic monitoring (PAM) as they produce highly stereotyped narrow-band high-frequency (NBHF) echolocation clicks. PAM systems must be coupled with a classification algorithm to identify the signals of interest. Here, the authors present a harbour porpoise click classifier (PorCC) developed in matlab, which uses the coefficients of two logistic regression models in a decision-making pathway to assign candidate signals to one of three categories: high-quality clicks (HQ), low-quality clicks (LQ), or high-frequency noise. The receiver operating characteristics of PorCC was compared to that of PAMGuard's Porpoise Click Detector/Classifier Module. PorCC outperformed PAMGuard's classifier achieving higher hit rates (correctly classified clicks) and lower false alarm levels (noise classified as HQ or LQ clicks). Additionally, the detectability index (d') for HQ clicks for PAMGuard was 2.2 (overall d' = 2.0) versus 4.1 for PorCC (overall d' = 3.4). PorCC classification algorithm is a rapid and highly accurate method to classify NBHF clicks, which could be applied for real time monitoring, as well as to study harbour porpoises, and potentially other NBHF species, throughout their distribution range from data collected using towed hydrophones or static recorders. Moreover, PorCC is suitable for studies of acoustic communication of porpoises.

Active Hearing Mechanisms Inspire Adaptive Amplification in an Acoustic Sensor System.

Over many millions of years of evolution, nature has developed some of the most adaptable sensors and sensory systems possible, capable of sensing, conditioning and processing signals in a very power- and size-effective manner. By looking into biological sensors and systems as a source of inspiration, this paper presents the study of a bioinspired concept of signal processing at the sensor level. By exploiting a feedback control mechanism between a front-end acoustic receiver and back-end neuronal based computation, a nonlinear amplification with hysteretic behavior is created. Moreover, the transient response of the front-end acoustic receiver can also be controlled and enhanced. A theoretical model is proposed and the concept is prototyped experimentally through an embedded system setup that can provide dynamic adaptations of a sensory system comprising a MEMS microphone placed in a closed-loop feedback system. It faithfully mimics the mosquito's active hearing response as a function of the input sound intensity. This is an adaptive acoustic sensor system concept that can be exploited by sensor and system designers within acoustics and ultrasonic engineering fields.

Frequency doubling by active in vivo motility of mechanosensory neurons in the mosquito ear.

Across vertebrate and invertebrate species, nonlinear active mechanisms are employed to increase the sensitivity and acuity of hearing. In mosquitoes, the antennal hearing organs are known to use active force feedback to enhance auditory acuity to female generated sounds. This sophisticated form of signal processing involves active nonlinear events that are proposed to rely on the motile properties of mechanoreceptor neurons. The fundamental physical mechanism for active auditory mechanics is theorized to rely on a synchronization of motile neurons, with a characteristic frequency doubling of the force generated by an ensemble of motile mechanoreceptors. There is however no direct biomechanical evidence at the mechanoreceptor level, hindering further understanding of the fundamental mechanisms of sensitive hearing. Here, using in situ and in vivo atomic force microscopy, we measure and characterize the mechanical response of mechanosensory neuron units during forced oscillations of the hearing organ. Mechanoreceptor responses exhibit the hallmark of nonlinear feedback for force generation, with movements at twice the stimulus frequency, associated with auditory amplification. Simultaneous electrophysiological recordings exhibit similar response features, notably a frequency doubling of the firing rate. This evidence points to the nature of the mechanism, whereby active hearing in mosquitoes emerges from the double-frequency response of the auditory neurons. These results open up the opportunity to directly investigate active cellular mechanics in auditory systems, and they also reveal a pathway to study the nanoscale biomechanics and its dynamics of cells beyond the sense of hearing.

Extreme call amplitude from near-field acoustic wave coupling in the stridulating water insect Micronecta scholtzi (Micronectinae).

Underwater acoustic transducers, particularly at low frequencies, are beset by problems of scale and inefficiency due to the large wavelengths of sound in water. In insect mating calls, a high call volume is usually desirable, increasing the range of signal transmission and providing a form of advertisement of the signaller's quality to a potential mate; however, the strength of the call is constrained by body size and by the need to avoid predators who may be listening in. Male crickets and water boatmen avoid some of the limitations of body size by exploiting resonant structures, which produce sharply tuned species-specific songs, but call frequency and volume remain linked to body size. Recently, the water boatman Micronecta scholtzi was found to circumvent this rule, producing a louder mating call than that of similar, but much larger, Corixa The resonant structure in Corixidae and Micronectinae is believed to be the trapped air reserves around the insect as it dives, driven by a stridulatory apparatus. However, the method by which energy is transferred from the striated area to the bubble is unknown. Here, we present modelling of a system of near-field coupling of acoustic sources to bubbles showing an exponential increase in sound power gain with decreasing distance that provides a simple solution to the stimulus of the air bubbles in Corixidae and Micronectinae and explains the discrepancy of M. scholtzi's extreme call volume. The findings suggest a possible route to engineered systems using near-field coupling to overcome size constraints in low-frequency (less than 500 Hz) underwater transducers, where the input efficiency of a piezoelectric device can be coupled through the hydrodynamic field to the high radiative efficiency of a near-ideal monopole emitter.

Listening to the environment: hearing differences from an epigenetic effect in solitarious and gregarious locusts.

Locusts display a striking form of phenotypic plasticity, developing into either a lone-living solitarious phase or a swarming gregarious phase depending on population density. The two phases differ extensively in appearance, behaviour and physiology. We found that solitarious and gregarious locusts have clear differences in their hearing, both in their tympanal and neuronal responses. We identified significant differences in the shape of the tympana that may be responsible for the variations in hearing between locust phases. We measured the nanometre mechanical responses of the ear's tympanal membrane to sound, finding that solitarious animals exhibit greater displacement. Finally, neural experiments signified that solitarious locusts have a relatively stronger response to high frequencies. The enhanced response to high-frequency sounds in the nocturnally flying solitarious locusts suggests greater investment in detecting the ultrasonic echolocation calls of bats, to which they are more vulnerable than diurnally active gregarious locusts. This study highlights the importance of epigenetic effects set forth during development and begins to identify how animals are equipped to match their immediate environmental needs.

Application of conformal map theory for design of 2-D ultrasonic array structure for NDT imaging application: a feasibility study.

Two-dimensional ultrasonic phased arrays are becoming increasingly popular in nondestructive evaluation (NDE). Sparse array element configurations are required to fully exploit the potential benefits of 2-D phased arrays. This paper applies the conformal mapping technique as a means of designing sparse 2-D array layouts for NDE applications. Modeling using both Huygens' field prediction theory and 2-D fast Fourier transformation is employed to study the resulting new structure. A conformal power map was used that, for fixed beam width, was shown in simulations to have a greater contrast than rectangular or random arrays. A prototype aperiodic 2-D array configuration for direct contact operation in steel, with operational frequency ~3 MHz, was designed using the array design principle described in this paper. Experimental results demonstrate a working sparse-array transducer capable of performing volumetric imaging.

Extremely high frequency sensitivity in a 'simple' ear.

An evolutionary war is being played out between the bat, which uses ultrasonic calls to locate insect prey, and the moth, which uses microscale ears to listen for the approaching bat. While the highest known frequency of bat echolocation calls is 212 kHz, the upper limit of moth hearing is considered much lower. Here, we show that the greater wax moth, Galleria mellonella, is capable of hearing ultrasonic frequencies approaching 300 kHz; the highest frequency sensitivity of any animal. With auditory frequency sensitivity that is unprecedented in the animal kingdom, the greater wax moth is ready and armed for any echolocation call adaptations made by the bat in the on-going bat-moth evolutionary war.

Time-of-flight measurement techniques for airborne ultrasonic ranging.

Airborne ultrasonic ranging is used in a variety of different engineering applications for which other positional metrology techniques cannot be used, for example in closed-cell locations, when optical line of sight is limited, and when multipath effects preclude electromagnetic-based wireless systems. Although subject to fundamental physical limitations, e.g., because of the temperature dependence of acoustic velocity in air, these acoustic techniques often provide a cost-effective solution for applications in mobile robotics, structural inspection, and biomedical imaging. In this article, the different techniques and limitations of a range of airborne ultrasonic ranging approaches are reviewed, with an emphasis on the accuracy and repeatability of the measurements. Simple time-domain approaches are compared with their frequency-domain equivalents, and the use of hybrid models and biologically inspired approaches are discussed.

Discovery of a lipid synthesising organ in the auditory system of an insect.

Weta possess typical Ensifera ears. Each ear comprises three functional parts: two equally sized tympanal membranes, an underlying system of modified tracheal chambers, and the auditory sensory organ, the crista acustica. This organ sits within an enclosed fluid-filled channel-previously presumed to be hemolymph. The role this channel plays in insect hearing is unknown. We discovered that the fluid within the channel is not actually hemolymph, but a medium composed principally of lipid from a new class. Three-dimensional imaging of this lipid channel revealed a previously undescribed tissue structure within the channel, which we refer to as the olivarius organ. Investigations into the function of the olivarius reveal de novo lipid synthesis indicating that it is producing these lipids in situ from acetate. The auditory role of this lipid channel was investigated using Laser Doppler vibrometry of the tympanal membrane, which shows that the displacement of the membrane is significantly increased when the lipid is removed from the auditory system. Neural sensitivity of the system, however, decreased upon removal of the lipid-a surprising result considering that in a typical auditory system both the mechanical and auditory sensitivity are positively correlated. These two results coupled with 3D modelling of the auditory system lead us to hypothesize a model for weta audition, relying strongly on the presence of the lipid channel. This is the first instance of lipids being associated with an auditory system outside of the Odentocete cetaceans, demonstrating convergence for the use of lipids in hearing.

No evidence for DPOAEs in the mechanical motion of the locust tympanum.

Distortion-product otoacoustic emissions (DPOAEs) are present in non-linear hearing organs, and for low-intensity sounds are a by-product of active processes. In vertebrate ears they are considered to be due to hair cell amplification of sound in the cochlea; however, certain animals lacking a cochlea and hair cells are also reported to be capable of DPOAEs. In the Insecta, DPOAEs have been recorded from the locust auditory organ. However, the site of generation of these DPOAEs and the physiological mechanisms causing their presence in the locust ear are not yet understood, despite there being a number of potential places in the tympanal organ that could be capable of generating DPOAEs. This study aimed to record locust tympanal membrane vibration using a laser Doppler vibrometer in order to identify a distinct place of DPOAE generation on the membrane. Two species of locust were investigated over a range of frequencies and levels of acoustic stimulus, mirroring earlier acoustic recording studies; however, the current experiments were carried out in an open acoustic system. The laser measurements did not find any evidence of mechanical motion on the tympanal membrane related to the expected DPOAE frequencies. The results of the current study therefore could not confirm the presence of DPOAEs in the locust ear through the mechanics of the tympanal membrane. Experiments were also carried out to test how membrane behaviour altered when the animals were in a state of hypoxia, as this was previously found to decrease DPOAE magnitude, suggesting a metabolic sensitivity. However, hypoxia did not have any significant effect on the membrane mechanics. The location of the mechanical generation of DPOAEs in the locust's ear, and therefore the basis for the related physiological mechanisms, thus remains unknown.

Synchrony through twice-frequency forcing for sensitive and selective auditory processing.

Male mosquitoes detect flying females using antennal hearing organs sensitive to nanoscale mechanical displacements and that harbor motile mechanosensory neurons. The mechanisms supporting neuronal motility, and their function in peripheral sensory processing, remain, however, puzzling. The mechanical and neural responses reveal a transition that unmasks the onset of synchronization between sensory neurons. This synchronization constitutes an unconventional, mechanically driven, process of communication between sensory neurons. Enhancing auditory sensitivity and selectivity, synchronization between mechanosensors in the mosquito arises from entrainment to twice-frequency forcing and is formally analogous to injection-locking in high-power laser technology. This discovery opens up the enticing possibility that other sensory systems, even nonsensory cell ensembles, coordinate their actions through mechanical signaling.

Nonlinear auditory mechanism enhances female sounds for male mosquitoes.

Sound plays an important role in the life history of mosquitoes. Male mosquitoes detect females by the sound generated by their wingbeat. Because female wings are weak acoustic radiators, males have been driven by sexual selection to evolve sensitive acoustic sensors. Mosquito antennae are very sensitive acoustic receivers, featuring up to 16,000 sensory cells, a number comparable with that contained in the human cochlea. The antennal sound receiver exhibits frequency selectivity, input amplification, and self-generated oscillations, features that parallel the functional sophistication of the cochlear amplifier. Although arguably the male antenna is well suited to receiving weak female sounds, the role of active mechanisms in mosquito hearing is far from understood. Previous mechanical studies on mosquito hearing largely focused on the steady-state antennal response to harmonic sounds, mostly evaluating the data through conventional Fourier transforms. Here, we report on the time-resolved mechanical behavior of the male antenna in response to female sounds. Crucially, stimuli were designed to reflect the temporal acoustic profile of a female flying by. With these stimuli, several previously unreported nonlinear features were unveiled, involving amplification, compression, and hysteresis. The time-resolved analysis reveals that, through the active participation of the sensory neurons, the antenna mechanically responds to enlarge its own range of detection. This behavior augments the capacity of the antennal receiver to detect female sounds, enhancing the male's chance to successfully pursue a passing female.