Aldo A. Glean

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.

Mode-shape-based mass detection scheme using mechanically diverse, indirectly coupled microresonator arrays

We explore vibration localization in arrays of microresonators used for ultrasensitive mass detection and describe an algorithm for identifying the location and amount of added mass using measurements of a vibration mode of the system. For a set of sensing elements coupled through a common shuttle mass, the inter-element coupling is shown to be proportional to the ratio of the element masses to the shuttle mass and to vary with the frequency mistuning between any two sensing elements. When any two elements have sufficiently similar frequencies, mass adsorption on one element can result in measurable changes to multiple modes of the system. We describe the effects on system frequencies and mode shapes due to added mass, in terms of mass ratio and frequency spacing. In cases in which modes are not fully localized, frequency-shift-based mass detection methods may give ambiguous results. The mode-shape-based detection algorithm presented uses a single measured mode shape and corresponding natural frequency to...

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.

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...

Quantification of disorder sensitivity of a subordinate oscillator array

Recent research has shown that arrays of small dynamic elements attached to a larger, primary structure can be tuned to significantly alter the time and frequency response of the system. Often referred to as “fuzzy structures,” such subordinate oscillator arrays have applications including damping, radio frequency filtering, energy harvesting, micro electromechanical systems, and chemical vapor sensing. This work shows that an attached array adds damping and demonstrates a distinct rejection band in the spectral response. The subordinate system here is an array of cantilever beam attachments with a range of isolated natural frequencies that surround a target band. Small errors in the distributions of masses and stiffnesses of the attachments can have a significant degrading effect on desired response. Error on the order of 0.1% can be significant. This paper discusses experimental verification of this phenomena. In the experiment, small amounts of mass were added to the tip of both individual and combinat...

Characterization and design of sound absorber materials

Sound absorbing materials are widely used to mitigate noise in indoor environments. Foams and fiberglass are commonly used for passive noise control in the automotive, aerospace industries and for architectural design. The physics of sound absorption in porous materials is not typically included in introductory acoustics courses. However, characterization and design of sound absorbing materials and modeling of their properties can be valuable for students interested in applied acoustics. In this laboratory-oriented course at the Catholic University of America, we cover design of multilayered sound absorbers and experimental procedures for testing such materials. These include an introduction to data acquisition using LabVIEW and post-processing (with Matlab) of recorded sound as well as microphone calibration, and measurement of sound pressure level, and the frequency response function of a speaker. The second part of the course focuses on room acoustics, acoustic properties of materials, impedance tube m...

Impedance tube experiments as a tool to teach acoustics of porous media

As a practical matter, sound propagation in many everyday situations includes interactions with porous materials. Such materials include textiles, foams, gravel, grass-covered ground, asphalt, and trees. Modeling sound propagation in complex environments, such as passenger cabins or urban spaces, with porous materials like these is essential to understanding realistic acoustics problems. The theory behind the sound absorption in porous material is not typically included in introductory acoustics courses. This is, at least in part, attributable to the mathematical complexity of the topic. Despite this barrier, acoustic properties of porous material can be easily introduced to typical undergraduate students using simple experiments to demonstrate the acoustic effects of different materials. This presentation describes acoustic property measurements of rigid-frame porous material with straight cylindrical pores as well as foam and soil included in an “Introduction to Acoustics” class at the Catholic Universi...

Managing property distribution errors in arrays of coupled resonators

Arrays of small attachments can be designed to draw mechanical energy from a primary structure in a manner far in excess of their proportional size. Earlier work has shown that slight variations or errors in the property distributions of arrays of coupled resonators can have a dramatic effect on the response of primary structure to an external force. This work investigates the use of an electro-mechanical approach to making small adjustments in the stiffness of the individual elements of the array to alter the response of the primary. The electro-mechanical coupling is achieved by way of laminated thin piezoactuators mounted on a fraction of the subordinate elements. The piezoactuators are electrically coupled to a switching network that changes the effective stiffness of the subordinate elements. This ability to adjust the stiffness distribution facilitates real time control of the rate at which the energy is transferred into the coupled array. This apparent damping can then be adjusted to draw or reject...