Frog embryos use multiple levels of temporal pattern in risk assessment for vibration-cued escape hatching.

Stereotyped signals can be a fast, effective means of communicating danger, but animals assessing predation risk must often use more variable incidental cues. Red eyed-treefrog, Agalychnis callidryas, embryos hatch prematurely to escape from egg predators, cued by vibrations in attacks, but benign rain generates vibrations with overlapping properties. Facing high false-alarm costs, embryos use multiple vibration properties to inform hatching, including temporal pattern elements such as pulse durations and inter-pulse intervals. However, measures of snake and rain vibration as simple pulse-interval patterns are a poor match to embryo behavior. We used vibration playbacks to assess if embryos use a second level of temporal pattern, long gaps within a rhythmic pattern, as indicators of risks. Long vibration-free periods are common during snake attacks but absent from hard rain. Long gaps after a few initial vibrations increase the hatching response to a subsequent vibration series. Moreover, vibration patterns as short as three pulses, separated by long periods of silence, can induce as much hatching as rhythmic pulse series with five times more vibration. Embryos can retain information that increases hatching over at least 45 s of silence. This work highlights that embryo behavior is contextually modulated in complex ways. Identical vibration pulses, pulse groups, and periods of silence can be treated as risk cues in some contexts and not in others. Embryos employ a multi-faceted decision-making process to effectively distinguish between risk cues and benign stimuli.

Estimation of acoustic absorption in porous materials based on visco-thermal boundary layers modeled as boundary conditions.

A method for estimating acoustic absorption in porous materials is presented in which the thermal and viscous boundary layers are modeled through boundary conditions to the Helmholtz equation for the acoustic pressure. The method is proposed for rigid-framed porous materials in which vibration of the frame is negligible compared to pressure fluctuations in air. The method reduces computation times by 2 orders of magnitude compared to a full thermoviscous acoustic solver. Furthermore, the method is shown to be highly accurate over geometrical features and frequencies of interest as long as thermal and viscous boundary layers do not overlap and the effects of the sharp changes in curvature are negligible. The method is demonstrated for a periodic sound absorber from the literature as well as a sound absorber with a randomly graded microstructure.

Sound absorption by metallic foam after triaxial hydrostatic compression.

An engineering method for triaxial hydrostatic compression of metallic foam is presented to preferentially alter the foam's microstructure. The method is demonstrated on an assortment of open-cell aluminum foams with varying pore size and porosity. Measurements of acoustic absorption indicate that the compressed samples absorb significantly more sound than the conventional samples of equal thickness in the test range from 0.25 to 4 kHz. The acoustic absorption trends that result from the transformation of foam microstructure in the compressed samples are a function of the initial pore size and porosity. An analysis is presented which links the microstructure properties of compressed foam samples to conventional samples, thereby providing a means to estimate acoustic absorption trends for compressed samples through use of existing models.

Analysis of thermal and viscous boundary layers in acoustic absorption by metallic foam.

A method for estimating acoustic absorption in foams is presented using a combination of micro-computed tomography, finite element analysis, and boundary layer loss theory. In the method, the foam is assumed to be rigid framed and the viscous and thermal boundary layers at the fluid and frame interface are assumed to be small compared to foam dimensions. The boundary layer losses are approximated using an infinite planar model. The method is demonstrated for a commercially available open-cell metallic foam and allows for absorption to be estimated without determination of any intermediate variables that are required in existing methods. Enhancement of sound absorbing properties by selection of foam properties, such as porosity and pores per inch, is discussed. Furthermore, predicted absorption trends agree with other published models and experimental data. A simplified, two-dimensional geometry is presented in which the assumptions of this method are analyzed.

A Mobile Acoustic Subsurface Sensing (MASS) system for rapid roadway assessment.

Surface waves are commonly used for vibration-based nondestructive testing for infrastructure. Spectral Analysis of Surface Waves (SASW) has been used to detect subsurface properties for geologic inspections. Recently, efforts were made to scale down these subsurface detection approaches to see how they perform on small-scale structures such as concrete slabs and pavements. Additional efforts have been made to replace the traditional surface-mounted transducers with non-contact acoustic transducers. Though some success has been achieved, most of these new approaches are inefficient because they require point-to-point measurements or off-line signal analysis. This article introduces a Mobile Acoustic Subsurface Sensing system as MASS, which is an improved surface wave based implementation for measuring the subsurface profile of roadways. The compact MASS system is a 3-wheeled cart outfitted with an electromagnetic impact source, distance register, non-contact acoustic sensors and data acquisition/ processing equipment. The key advantage of the MASS system is the capability to collect measurements continuously at walking speed in an automatic way. The fast scan and real-time analysis advantages are based upon the non-contact acoustic sensing and fast air-coupled surface wave analysis program. This integration of hardware and software makes the MASS system an efficient mobile prototype for the field test.

Wave based analysis of the Green's function for a layered cylindrical shell.

Cylindrical shells composed of concentric layers may be designed to affect the way that elastic waves are generated and propagated, particularly when some layers are anisotropic. To aid the design process, the present work develops a wave based analysis of the Green's function for a layered cylindrical shell in which the response is given as a sum of waves propagating in the axial coordinate. The analysis assumes linear Hookean materials for each layer. It uses finite element discretizations in the radial coordinate and Fourier series expansions in the circumferential coordinate, leading to linear equations in the axial wavenumber domain that relate shell displacements and forces. Inversion to the axial domain is accomplished via a state-space formulation that is evaluated using residue integration. The resulting expression for the Green's function for each circumferential harmonic is a summation over the natural waves of the shell. The finite element discretization in the radial direction allows the approach to be used for arbitrarily thick shells. The approach is benchmarked to results from an isotropic shell and numerical examples are given for a shell composed of a fiber-reinforced material. The numerical examples illustrate the effect of fiber orientation on the Green's function.

Far-field approximation for a point-excited anisotropic plate.

An analytic approximation is derived for the far-field response of a generally anisotropic plate to a time-harmonic point force acting normal to the plate. This approximation quantifies the directivity of the flexural wave field that propagates away from the force, which is expected to be useful in the design and testing of anisotropic plates. Derivation of the approximation begins with a two-dimensional Fourier transform of the flexural equation of motion. Inversion to the spatial domain is accomplished by contour integration over the radial component of wave number followed by an application of the method of stationary phase to integration over the circumferential component of wave number. The resulting approximation resembles that of an isotropic plate but involves wave numbers, wave amplitudes, and phases that depend on propagation angle. Numerical results for a plate comprised of bonded layers of a graphite-epoxy material illustrate the accuracy of the method compared to a numerical simulation based on discrete Fourier analysis. Three configurations are analyzed in which the relative angles of the layers are varied. In all cases, the agreement is quite good when the distance between force and observation point is greater than a few wavelengths.

Directionality of flexural intensity in orthotropic plates.

This work investigates composite plates and their ability to direct flexural intensity, which has important implications for noise and vibration control. It is well known that a composite plate supports a flexural wave whose wavenumber depends strongly on its angle of propagation. This suggests that a composite plate will direct more flexural intensity in some directions than others. The present work considers a thin multi-layered plate in which each layer is constructed from an orthotropic material and has a chosen orientation relative to the other layers. Such an approach may be used to design highly directive structures. An analysis is presented in which a two-dimensional Fourier transform is analytically applied to the equation of motion, yielding algebraic expressions for displacements and stress resultants. Next, a two-dimensional discrete inverse Fourier transform is applied to compute displacements and stress resultants at discrete locations. Flexural intensity is computed at these locations.

Vibrational signaling in the agonistic interactions of red-eyed treefrogs.

Sensitivity to substrate-borne vibrations is widespread in animals and evolutionarily precedes hearing but, compared with other sensory modalities, we know little about vibrational communication, particularly in vertebrates. For plant-dwelling arthropods, vibrations are likely as important as sound. Arboreal vertebrates excite plant vibrations with most movements, but the behavioral relevance of these vibrations has not been tested experimentally. In playback experiments using a robotic model frog and an electrodynamic shaker, we demonstrate that plant-borne vibrations generated by the shaking (tremulation) display of male red-eyed treefrogs (Agalychnis callidryas) are a vibrational signal, necessary and sufficient to elicit tremulations in response. A trend toward increased aggression during visual playbacks suggests that the visual component of tremulations may also convey information. In male-male contests, tremulations were the most frequent aggressive display, and their use and vibrational characteristics varied with male size and conflict context. Nearly all of A. callidryas' signaling behaviors, including tremulations and acoustic calls, excite strong, stereotyped vibrations that travel through plants and could be informative to receivers. Our results demonstrate that vibrational signals serve a key role in the biology of one well-known arboreal frog and suggest that consideration of the vibrational modality may significantly broaden our appreciation of the behavior and evolution of arboreal vertebrates.

Frequency information in the vibration-cued escape hatching of red-eyed treefrogs.

Incidental acoustic and vibrational cues generated by predators are a potential source of information for prey assessing risk. Substrate vibrations should be excited by most predators, and frequency, amplitude or temporal properties could allow prey to distinguish predator from benign-source vibrations. Red-eyed treefrog embryos detect egg predators using vibrations excited during attacks, hatching rapidly and prematurely to escape. We recorded vibrations in egg clutches during attacks by five species of predators and three common types of benign physical disturbance. We analyzed their frequency distributions to assess if and how frequency properties could be used to discriminate between vibration sources and used vibration playbacks to examine the effects of frequency properties on the escape hatching response. Vibrations produced by predators and benign disturbances generally have broad and overlapping frequency distributions, and all frequencies excited by attacks are also excited by benign disturbances. Decision rules based on the frequency distribution of vibrations alone would therefore result in either high levels of hatching in response to benign vibrations (false alarms) or common failures to hatch in response to predators (missed cues). Nevertheless, embryos hatch in response to predator and not benign disturbances in nature, and our playback results show that vibration frequency information is an important component of their hatching decision. Embryos combine frequency with temporal information to refine their hatching response. Moreover, comparing frequency spectra of predator and benign vibrations suggests that the presence of energy in frequencies outside the range characteristic of attacks might serve as an indicator of benign disturbance.

Flexural wave dispersion in orthotropic plates with heavy fluid loading.

Orthotropic plates support flexural waves with wavenumbers that depend on their angle of propagation. The present work investigates the effect of fluid loading on this angular dependence, and finds that the effect is relatively small for typical composite plate materials in contact with water. This finding results from an analytical model of the fluid-loaded plate, in which the plate is modeled by classical laminated plate theory and the fluid is modeled as an ideal acoustic fluid. The resulting dispersion relation is a tenth-order polynomial in the flexural wavenumber. Direct numerical solution, as well as analysis at frequencies below coincidence, reveals that the angular dependence of wavenumber is magnified but not significantly distorted by the addition of fluid loading.

Flexible information sampling in vibrational assessment of predation risk by red-eyed treefrog embryos.

Prey assessing risk may miss cues and fail to defend themselves, or respond unnecessarily to false alarms. Error rates can be ameliorated with more information, but sampling predator cues entails risk. Red-eyed treefrogs have arboreal eggs and aquatic tadpoles. The embryos use vibrations in snake attacks to cue behaviorally mediated premature hatching, and escape, but vibrations from benign sources rarely induce hatching. Missed cues and false alarms are costly; embryos that fail to hatch are eaten and hatching prematurely increases predation by aquatic predators. Embryos use vibration duration and spacing to inform their hatching decision. This information accrues with cycles of vibration, while risk accrues over time as snakes feed. We used vibration playback experiments to test if embryos adjust sampling of information based on its cost, and measured latency to initiate hatching in videotaped snake attacks. Embryos did not initiate hatching immediately in attacks or playbacks, and the delay varied with the rate at which information accrued. Embryos started hatching sooner in response to stimuli with shorter cycles but sampled fewer cycles (less information) of longer-cycle stimuli before hatching. This flexible sampling is consistent with embryos balancing a trade-off between the value and cost of information.

Temporal pattern cues in vibrational risk assessment by embryos of the red-eyed treefrog, Agalychnis callidryas.

The embryos of red-eyed treefrogs, Agalychnis callidryas, use vibrations transmitted through their arboreal egg clutch to cue escape hatching behavior when attacked by egg-eating snakes. Hatching early increases the risk of predation in the water, so embryos should avoid it unless they are in danger. We exposed egg clutches to intermittent vibrations with different combinations of vibration duration and spacing to examine the role of simple temporal pattern cues in the escape hatching response. Stimuli were bursts of synthetic white noise from 0 to 100 Hz, including the range of frequencies with substantial energy in snake attacks, and had approximately rectangular amplitude envelopes. Embryos hatched in response to a small range of temporal patterns and not in response to many others, rather than hatching to most vibrations except for certain patterns perceived as safe. Neither cycle length nor duty cycle predicted hatching response, except at extreme values where no hatching occurred; the highest energy stimuli elicited little or no hatching. Both vibration duration and inter-vibration interval strongly affected the hatching response. The highest levels of hatching were to durations of 0.5 s combined with intervals of 1.5-2.5 s, and hatching decreased gradually with increasing difference of either duration or interval from these most effective stimuli. Vibration duration and interval appear to function as two necessary elements of a composite cue, rather than as redundant cues. This increases response specificity and reduces the range of stimuli that elicit hatching, likely reducing the chance of hatching unnecessarily in a benign disturbance. Vibration-cued hatching in A. callidryas embryos offers an opportunity to experimentally assess the behavioral decision rules underlying an effective and costly anti-predator defense.

Development, surface exposure, and embryo behavior affect oxygen levels in eggs of the red-eyed treefrog, Agalychnis callidryas.

Oxygen stress can slow development, induce hatching, and kill eggs. Terrestrial anamniote embryos face a potential conflict between oxygen uptake and water loss. We measured oxygen levels within eggs to characterize the respiratory environment for embryos of the red-eyed treefrog, Agalychnis callidryas, a Neotropical frog with arboreal egg masses and plastic hatching timing. Perivitelline oxygen partial pressure (Po2) was extremely variable both within and among eggs. Po2 increased with air-exposed surface of the egg and declined over the developmental period before hatching competence. Through the plastic hatching period, however, average Po2 was stable despite continued rapid development. Development was synchronous across a wide range of perivitelline Po2 (0.5-16.5 kPa), and hatching-competent embryos tolerated Po2 as low as 0.5 kPa without hatching. The variation in Po2 measured over short periods of time within individual eggs was as great as that measured across development or surface exposure, including sharp transients associated with embryo movements. There was also a strong gradient of Po2 across the egg from superficial to deep positions. Ciliary circulation of fluid within the egg is clearly insufficient to keep it mixed. Embryos may maintain development under hypoxic conditions by strategic positioning of respiratory surfaces, particularly external gills, to exploit the patchy distribution of oxygen within their eggs.