Teresa J. Ryan

Noise sensitivity of a mass detection method using vibration modes of coupled microcantilever arrays

Numerical simulation is used to explore the sensitivity to measurement noise of a mass detection approach that uses eigenmodes of an array of nominally identical micro- or nanomechanical resonators. The mode shapes are perturbed to simulate measurement noise, and resulting errors in identifying variations in mass are quantified as a function of array size, coupling strength, and level of mass variation. Sensitivity to measurement noise is low for lightly coupled arrays of nearly identical elements and increases when mass variation causes significant mode localization. For any mass variation level, an optimal combination of array size and coupling strength minimizes noise sensitivity.

Optimal apparent damping as a function of the bandwidth of an array of vibration absorbers

The transient response of a resonant structure can be altered by the attachment of one or more substantially smaller resonators. Considered here is a coupled array of damped harmonic oscillators whose resonant frequencies are distributed across a frequency band that encompasses the natural frequency of the primary structure. Vibration energy introduced to the primary structure, which has little to no intrinsic damping, is transferred into and trapped by the attached array. It is shown that, when the properties of the array are optimized to reduce the settling time of the primary structures transient response, the apparent damping is approximately proportional to the bandwidth of the array (the span of resonant frequencies of the attached oscillators). Numerical simulations were conducted using an unconstrained nonlinear minimization algorithm to find system parameters that result in the fastest settling time. This minimization was conducted for a range of system characteristics including the overall bandwidth of the array, the ratio of the total array mass to that of the primary structure, and the distributions of mass, stiffness, and damping among the array elements. This paper reports optimal values of these parameters and demonstrates that the resulting minimum settling time decreases with increasing bandwidth.

Synthetic aperture acoustic imaging of non-metallic cords

This work presents a set of measurements collected with a research prototype synthetic aperture acoustic (SAA) imaging system. SAA imaging is an emerging technique that can serve as an inexpensive alternative or logical complement to synthetic aperture radar (SAR). The SAA imaging system uses an acoustic transceiver (speaker and microphone) to project acoustic radiation and record backscatter from a scene. The backscattered acoustic energy is used to generate information about the location, morphology, and mechanical properties of various objects. SAA detection has a potential advantage when compared to SAR in that non-metallic objects are not readily detectable with SAR. To demonstrate basic capability of the approach with non-metallic objects, targets are placed in a simple, featureless scene. Nylon cords of five diameters, ranging from 2 to 15 mm, and a joined pair of 3 mm fiber optic cables are placed in various configurations on flat asphalt that is free of clutter. The measurements were made using a chirp with a bandwidth of 2-15 kHz. The recorded signal is reconstructed to form a two-dimensional image of the distribution of acoustic scatterers within the scene. The goal of this study was to identify basic detectability characteristics for a range of sizes and configurations of non-metallic cord. It is shown that for sufficiently small angles relative to the transceiver path, the SAA approach creates adequate backscatter for detectability.

Conformal scanning laser Doppler vibrometer measurement of tenor steelpan response to impulse excitation

A conformal scanning laser Doppler vibrometer system is used in conjunction with a mechanical pannist to measure the surface normal vibration of the entire playing surface of a C-lead tenor steelpan. The mechanical pannist is a device designed to deliver controlled, repeatable strikes that mimic a mallet during authentic use. A description of the measurement system is followed by select examples of behavior common to the results from three different excitation notes. A summary of observed response shapes and associated frequencies demonstrates the concerted placement of note overtones by the craftsmen who manufacture and tune the instruments. The measurements provide a rich mechanical snapshot of the complex motion that generates the distinctive sound of a steelpan.

A study of complex system dynamics: Metronome synchronization

This work investigates energy exchange within a complex vibrating system. The system is made up of a mass, called the primary oscillator, and a number of attached smaller structures, called the subordinate oscillator array. Specifically, a rectangular rigid foam base is the primary mass and mechanical metronomes are used as the subordinate oscillators. This work explores how the orientation and arrangement of the metronomes on the master structure affects the time it takes for metronome synchronization as well as the resulting amplitude of oscillation of the vibration of the primary mass. A MATLAB based image processing approach is used to measure these system parameter. [This work was supported by the Robert W. Young Award for Undergraduate Student Research.]

Numerical analysis of atmospheric sound propagation in the littoral zone

The propagation of sound over long distances is subject to many effects that are deemed negligible over short distances or in situations where idealized estimates are sufficient. This work describes a numerical model of acoustic propagation in littoral zone. The model therefore must include such effects for the purpose of investigating transmission loss during long-range propagation over the complex sea surface. Prior work by the investigators has presented the basic approach, which is a Crank-Nicolson parabolic equation that accounts for surface geometry, spreading loss, wind, and ground impedance. That work evaluated moderated propagation distances (up to 500 meters) and used a uniform temperature distribution. The current work will investigate considerations required for adding a temperature gradient component to the numerical model and explore the sensitivity of transmission loss to temperature gradient.

Graduate Education in Acoustics at The Catholic University of America

The Catholic University of America (CUA) has a graduate program with a long history in acoustics dating back to the early 1930s. The acoustics program moved to the School of Engineering in the 1960s when there were strong needs in underwater acoustic studies to meet U.S. Naval applications. The end of the Cold War was concurrent with a decline in the CUA’s acoustics education that persisted into the 1990s. However, renewed interests in acoustical engineering, acoustic metamaterial, and environmental acoustic research has revived the acoustics research and education programs at the CUA in recent years. Currently, a variety of graduate level acoustic courses are being offered in the CUA’s Mechanical Engineering Department. Students can pursue a master’s degree or Ph.D. degree with research in acoustics or vibrations. The courses in the program include a two-course sequence in fundamentals in acoustics, and more focused courses in ocean acoustics, atmospheric acoustics, acoustic metrology, marine bioacoustic...

Effects of error in a subordinate oscillator array

Existing research has shown that the response of a single degree of freedom resonant system can be modified by the attachment of sets of substantially smaller resonators. Such arrays of attachments, known as subordinate oscillator arrays, can increase the apparent damping of the primary structure, and the property distributions can be selected such that the collective effects result in a response of the primary resonator that is similar to an electrical band-rejection filter. Other prior work with this system has indicated high sensitivity to disorder in the individual attachment properties such that even 0.1% variation is likely to cause undesirable effects in the overall system response. Such levels of variation well below 1% are easily attributable to typical manufacturing tolerances, environmental influences, and degradation factors. This work presents experimental results of a set of prototype subordinate oscillator arrays produced with high precision additive manufacturing techniques so as to prescr...

Full-field laser vibrometer study of dissipation in a beam with a coupled oscillator array

Coupled oscillator systems can be designed to manage the distribution of mechanical energy in vibration or acoustic systems. A specific implementation of an array composed of small damped mass-spring attachments on a primary is here referred to as a subordinate oscillator array. SOAs and their ability to absorb energy from a primary mass are of continued importance in analysis for both structural and acoustic systems. The ability to dissipate energy in a specific frequency band has many potential applications. This work will use laser Doppler vibrometry (LDV) to evaluate a beam mounted on an electromechanical shaker with and without an attached planar SOA composed of much smaller beams. This study is motivated by earlier work that indicated a high sensitivity to disorder in the form of fabrication error. The apparatus allows for the controlled introduction of various levels of disorder to quantify its effect and compare to a numerical model. We will show full-field forced response from the LDV with and wi...

Numerical analysis on the effect of the sea surface roughness on the acoustic transmission loss in the atmosphere

Atmospheric sound propagation over the sea surface is affected by several factors including wind, temperature profile, and sea state. Preliminary numerical studies using a Crank-Nicolson parabolic equation solution approach have demonstrated that sea roughness introduces excess transmission loss. This excess loss induced by the sea roughness can be represented instead by a flat surface with a compensatory effective impedance. Calculation of transmission loss for the flat surface is computationally more efficient but requires the evaluation of the effective impedances for any given sea state. In this study, a 2-D finite-difference time-domain solver is used to determine the effective impedances for several sea states. The effect of sea roughness on transmission loss is then quantified by using the Crank-Nicolson parabolic equation.