Ronald N. Miles

Mechanically coupled ears for directional hearing in the parasitoid fly Ormia ochracea

An analysis is presented of the mechanical response to a sound field of the ears of the parasitoid fly Ormia ochracea. This animal shows a remarkable ability to detect the direction of an incident sound stimulus even though its acoustic sensory organs are in very close proximity to each other. This close proximity causes the arrival times of the sound pressures at the two ears to be less than 1 to 2 microseconds depending on the direction of propagation of the sound wave. The small differences in these two pressures must be processed by the animal in order to determine the incident direction of the sound. In this fly, the ears are so close together that they are actually joined by a cuticular structure which couples their motion mechanically and subsequently magnifies interaural differences. The use of a cuticular structure as a means to couple the ears to achieve directional sensitivity is novel and has not been reported in previous studies of directional hearing. An analytical model of the mechanical response of the ear to a sound stimulus is proposed which supports the claim that mechanical interaural coupling is the key to this animals ability to localize sound sources. Predicted results for sound fields having a range of incident directions are presented and are found to agree very well with measurements.

Directional hearing by mechanical coupling in the parasitoid fly Ormia ochracea.

Sound localization is a basic processing task of the auditory system. The directional detection of an incident sound impinging on the ears relies on two acoustic cues: interaural amplitude and interaural time differences. In small animals, with short interaural distances both amplitude and time cues can become very small, challenging the directional sensitivity of the auditory system. The ears of a parasitoid fly Ormia ochracea, are unusual in that both acoustic sensors are separated by only 520 μm and are contained within an undivided air-filled chamber. This anatomy results in minuscule differences in interaural time cues (ca. 2 μs) and no measurable difference in interaural intensity cues generated from an incident sound wave.The tympana of both ears are anatomically coupled by a cuticular bridge. This bridge also mechanically couples the tympanana, providing a basis for directional sensitivity. Using laser vibrometry, it is shown that the mechanical response of the tympanal membranes has a pronounced directional sensitivity. Interaural time and intensity differences in the mechanical response of the ears are significantly larger than those available in the acoustic field. The tympanal membranes vibrate with amplitude differences of about 12 dB and time differences on the order of 50 μs to sounds at 90° off the longitudinal body axis. The analysis of the deflection shapes of the tympanal vibrations shows that the interaural differences in the mechanical response are due to the dynamic properties of the tympanal system and reflect its intrinsic sensitivity to the direction of a sound source. Using probe microphones and extracellular recording techniques, we show that the primary auditory afferents encode sound direction with a time delay of about 300 μs. Our data point to a novel mechanism for directional hearing in O. ochracea based on intertympanal mechanical coupling, a process that amplifies small acoustic cues into interaural time and amplitude differences that can be reliably processed at the neural level. An intuitive description of the mechanism is proposed using a simple mechanical model in which the ears are coupled through a flexible lever.

Investigation of the response of microstructures under the combined effect of mechanical shock and electrostatic forces.

There is strong experimental evidence for the existence of strange modes of failure of microelectromechanical systems (MEMS) devices under mechanical shock and impact. Such failures have not been explained with conventional models of MEMS. These failures are characterized by overlaps between moving microstructures and stationary electrodes, which cause electrical shorts. This work presents modeling and simulation of MEMS devices under the combination of shock loads and electrostatic actuation, which sheds light on the influence of these forces on the pull-in instability. Our results indicate that the reported strange failures can be attributed to early dynamic pull-in instability. The results show that the combination of a shock load and an electrostatic actuation makes the instability threshold much lower than the threshold predicted, considering the effect of shock alone or electrostatic actuation alone. In this work, a single-degree-of-freedom model is utilized to investigate the effect of the shock-electrostatic interaction on the response of MEMS devices. Then, a reduced-order model is used to demonstrate the effect of this interaction on MEMS devices employing cantilever and clamped-clamped microbeams. The results of the reduced-order model are verified by comparing with finite-element predictions. It is shown that the shock-electrostatic interaction can be used to design smart MEMS switches triggered at a predetermined level of shock and acceleration.

Directionality in the mechanical response to substrate vibration in a treehopper (Hemiptera: Membracidae: Umbonia crassicornis)

Abstract. The use of substrate vibrations in communication and predator-prey interactions is widespread in arthropods. In many contexts, localization of the vibration source plays an important role. For small species on solid substrates, time and amplitude differences between receptors in different legs may be extremely small, and the mechanisms of vibration localization are unclear. Here we ask whether directional information is contained in the mechanical response of an insects body to substrate vibration. Our study species was a membracid treehopper (Umbonia crassicornis) that communicates using bending waves in plant stems. We used a bending-wave simulator that allows precise control of the frequency, intensity and direction of the vibrational stimulus. With laser-Doppler vibrometry, we measured points on the substrate and on the insects thorax and middle leg. Transfer functions showing the response of the body relative to the substrate revealed resonance at lower frequencies and attenuation at higher frequencies. There were two modes of vibration along the bodys long axis, a translational and a rotational mode. Furthermore, the transfer functions measured on the body differed substantially depending on whether the stimulus originated in front of or behind the insect. Directional information is thus available in the mechanical response of the body of these insects to substrate vibration. These results suggest a vibration localization mechanism that could function at very small spatial scales.

A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea

A miniature differential microphone is described having a low-noise floor. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure-referred noise floor. In the microphone described here, both the diaphragm thermal noise and the electronic noise are minimized by a combination of novel diaphragm design and the use of low-noise optical sensing that has been integrated into the microphone package. The differential microphone diaphragm measures 1 x 2 mm(2) and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled directionally sensitive ears of the fly Ormia ochracea. The sound pressure input-referred noise floor of this miniature differential microphone has been measured to be less than 36 dBA.

Nonlinear Dynamics of MEMS Arches Under Harmonic Electrostatic Actuation

We present an investigation of the nonlinear dynamics of clamped-clamped micromachined arches when actuated by a dc electrostatic load superimposed on an ac harmonic load. The Galerkin method is used to discretize the distributed-parameter model of a shallow arch to obtain a reduced-order model. The static response of the arch due to a dc load actuation is simulated, and the results are validated by comparing them to experimental data. The dynamic response of the arch to a combined dc load and ac harmonic load is studied when excited near its fundamental natural frequency, twice its fundamental natural frequency, and near other higher harmonic modes. The results show a variety of interesting nonlinear phenomena, such as hysteresis, softening behavior, dynamic snap-through, and dynamic pull-in. The results are also shown demonstrating the potential to use microelectromechanical systems (MEMS) arches as bandpass filters and low-powered switches. An experimental work is conducted to test arches realized of curved polysilicon microbeams when excited by dc and ac loads. Experimental data are shown for the softening behavior and the dynamic pull-in of the curved microbeams.

The Development of a Biologically-Inspired Directional Microphone for Hearing Aids

The development of novel microfabrication techniques for producing a directional microphone for hearing aids is here described. The mechanisms underlying both the structure and function of these unusual microphones were originally inspired by the ears of an inconspicuous insect, the parasitoid fly Ormia ochracea. The structure of Ormia’s ears inspired new approaches to design directional microphones that are more sensitive and have lower thermal noise than that typical of those using traditional approaches. The mechanisms for directional hearing in this animal are discussed along with the engineering design concepts that they have inspired, because they illustrate how basic research can inspire technology development–translational research. However, to realize the potential of bioexploitation this microphone diaphragm concept would have been very difficult to realize without the availability of new silicon microfabrication technologies. Thus, this report can be viewed as an example of what may be possible with the application of new fabrication methods to microphones. Challenges and opportunities provided by the use of silicon microfabrication technology for microphones are discussed.

Tympanal mechanics in the parasitoid fly Ormia ochracea : intertympanal coupling during mechanical vibration

Abstract The acoustic parasitoid fly Ormia ochracea locates its host, a singing field cricket, by means of a pair of small tympanal organs which are less than 2 mm in width. Nevertheless, laser vibrometric evidence shows that this tympanal system is directionally sensitive to sound through the action of a flexible intertympanal bridge that mechanically couples the tympana. Biomechanical data, a mechanical analogue and an analytical model lead to a testable prediction about the vibratory behavior of this tympanal system: if intertympanal coupling occurs, a force applied only unilaterally in non-acoustical conditions should be transmitted, at least to some degree, to the contralateral ear. This paper presents new experiments of direct mechanical stimulation that test the prediction of mechanical coupling. Stimulation on only one side of the intertympanal bridge elicited a contralateral mechanical response. Thus, coupling of the tympanal membranes through a flexible intertympanal bridge is demonstrated by mechanical as well as acoustical stimulation. These experiments also test for the possible presence of a pressure-difference system in O. ochracea. Intertympanal coupling is shown not to depend on the integrity of the air space backing the tympanal system, thus eliminating this possibility.

Beam dampers for damping the vibrations of the skin of reinforced structures

Beam dampers comprising a stiff, lightweight, elongate beam and layer of viscoelastic material located along an attachment flange of the beam are disclosed. The flanges of the beam is attached by the layer of viscoelastic material to the skin of a structure whose skin vibrations are to be damped. While a beam having a cross-sectional I-shape is preferred, other cross-sectional shapes can be used, such as L, Z, U and T-shapes. Regardless of their shapes, the beam acts as a constraining element for the viscoelastic attachment layer. The beam is oriented such that it is stiff in a plane transverse to the plane of the skin, resulting in thickness deformation of the layer of viscoelastic material (rather than shear deformation) converting vibration energy into heat.

High-order directional microphone diaphragm

The invention features a miniature, second-order, microcrystalline silicon microphone diaphragm formed using silicon microfabrication techniques. The diaphragm is composed of two or more rigid diaphragm elements hinged to one another providing second- or higher-order response depending on the number of diaphragm elements used. The response of the differential diaphragm has a response that is highly dependent on the direction of the incident sound. The diaphragms are useful for constructing highly innovative microphones that have far greater directionality, better sensitivity, wider frequency response, and lower noise than is achievable with current technology.