Low Frequency Absorption of Additively Manufactured Cylinders

Attenuating low frequencies is often problematic, due to the large space required for common absorptive materials to mitigate such noise. However, natural hollow reeds are known to effectively attenuate low frequencies while occupying relatively little space compared to traditional absorptive materials. This paper discusses the effect of varied outer diameter, and outer spacing on the 200–1600 Hz acoustic absorption of additively manufactured arrays of hollow cylinders. Samples were tested in a 10 cm diameter normal incidence impedance tube such that cylinder length was oriented perpendicular to the incoming plane wave. By varying only one geometric element of each array, the absorption due to any particular parameter can be assessed individually. The tests confirmed the hypothesis that minimizing cylinder spacing and maximizing cylinder diameter resulted in increased overall absorption and produced more focused absorption peaks at specific low frequencies. Wider cylinder spacing produced a broader absorptive frequency range, despite shifting upward in frequency. Thus, manipulating these variables can specifically target absorption for low frequency noise that would otherwise disturb listeners.

Development and Design of the Dynamic Vibration Absorber Using Magneto-Rheological Elastomer for the Weight and Power Consumption Saving

In this study, a broadband frequency tunable dynamic absorber was designed and fabricated based on the primary design principle of a mass damper. A magneto-rheological elastomer that can change the relative stiffness when an external magnetic field is applied was used to control the natural frequency of the movable mass of the absorber. A coil to generate the magnetic field was also used as a movable mass to decrease the total weight and to create a constant closed loop of the magnetic force. The hammer impact test results show that the present absorber could change its natural frequency with minimal electric power and had a constant damping ratio. Experimental results of vibration absorbing of an acrylic flat plate show that the proposed absorber could change the natural frequency of the movable mass and reduce the vibration over a wide band by constantly applying the optimum current to the coil in the device with a small power consumption (less than 10 W). Therefore, the proposed absorber works effectively. Further, a technique to determine the electric current applied to the coil automatically based on the phase difference of the vibrational acceleration of the movable mass and the vibrating objective was also presented.

Ultrasonic Characterization of the Elastic Constants in an Aging Ti-6Al-4V ELI Alloy

In this paper, we report the experimental data of the elastic properties of the young and shear modulus based on the variation in the ultrasonic velocity parameter during the microstructural evolution in a Ti-6Al-4V alloy with two varying microstructures, bimodal and acicular respectively. The two different initial microstructures, were treated thermally by aging at 515°C, 545°C and 575°C at different times from 1 min to 576hr to induce a precipitation process. Ultrasonic measurements of shear and longitudinal wave velocities, scanning electron microscopy (SEM) image processing, optical microscopy (OM) and microhardness were performed, establishing a direct correlation with the measurements of the ultrasonic velocity and the elastic properties developed during the thermal treatment of the artificial aging. The results of the ultrasonic velocity show a very clear trend as the aging time progresses, which is affected by precipitation of Ti3Al particles inside the α phase. In this way, we can know, in a fast and efficient way, the elastic properties developed during the heat treatment of aging at long times, since the presence of these precipitates hardens the material microstructure affecting the final mechanical properties.

An Investigation Into Structural-Induced Noise in an Electric Motor

The paper presents an investigation into the noise generated by structural vibration of an electric motor used in appliance products using Computational Simulation Approach. In particular, a 3-D numerical simulation model is specifically developed to predict the frequency response of the stator under three different simulation conditions: radial force only, tangential force only and the combination of both forces. The obtained data is used to analyze the acoustic generation in the far-field. Experimental is used to validate the predicted results. It shows the predicted results are very close to experimental results.

An Experimental Approach in Defect Detection of a Single Row Ball Bearing Using Noise Generation Signal

Bearings are critical mechanical components that are used in rotary machinery. Timely detection of defects in such components can prevent catastrophic failure. Noise is generated during the rotation of bearings even without the presence of defects due to finite number of rotating elements to carry the load. Such noise is associated with the change in effective stiffness during rotation, however, a sharp spike is observed in the noise level with presence of local defects. This study uses the noise generation aspect of roller bearings to identify local defect in a single row ball bearing with outer race stationary under radial load. Experimental testing is conducted on two identical bearings. The defective bearing is selected from a diesel engine subjected to 20 years of service. Dissecting the defective bearing revealed pitting and spalling of the inner race and balls, the most two common bearing defects. Both time and frequency analysis of sound pressure generated by the bearings were performed. The results show that there is a clear distinction in the time and frequency spectra between healthy and defective bearings. Findings of this study revealed that using a simple cost efficient in-house experimental setup, local defects can be readily detected.

An Air Suspension System With Adjustable Height, Damping and Stiffness Using No Viscous Dampers

Air suspension is gaining more and more popularity with both the auto industry and drivers. Traditionally the height adjustability aspect of air suspension systems has been their main attracting attribute. More recently, resolving the classic conflict of combining comfortable ride with sport handling in a single suspension setup has become the main attraction of air suspension. An air suspension system has been developed which in addition to height adjustment, can adjust its damping and stiffness in real time with using neither viscous dampers nor any additional actuators. This is done by real-time adjustment air flow to and from the air springs using proportional valves. Measured relative displacement and acceleration as well as estimated velocity of the sprung mass with respect to unspring mass at each corner are fedback, thru their corresponding gains, to create the control signal that adjusts the proportional valve with the goal of controlling the height, stiffness, and damping at that corner. In a numerical study followed by laboratory testing, the effectiveness of the proposed air suspension system in terms of its ability to vary the damping and stiffness as well as the height of the suspension system is demonstrated.

Analysis of Structural Acoustic Design Variables for a Periodically Stiffened Plate Using the Finite Element Method

Investigating the acoustic radiation of stiffened plate structures is significant to the advancement of aircraft, automobile, and marine vehicle design. Plate and stiffener design variables affect how the global structure vibrates and radiates sound. The objective of this paper is to provide insight into how sensitive a periodically stiffened plate radiates sound in air with respect to its design variables. This paper examines a clamped plate that is periodically stiffened along one direction. Finite element analysis is used to quantify the structural acoustic behavior of the plate subject to a harmonic point load at the plate’s center. Fourier transforms are performed along the plate’s surface to reveal the wavenumber content of the plate. Lastly, radiated sound power from the plate surface is computed. A baseline plate without stiffeners is used for finite element modeling validation. Next, periodically spaced beams used for plate stiffening are inserted and varied in thickness. In addition, the plate thickness is also varied. Varying the plate thickness and the stiffener thickness provides insight to each design variable’s contribution to vibration and radiated sound power. The quantified findings from these parametric case studies serve as an insight into the structural acoustic performance of periodically stiffened structures.

Mechanical Design and Development of a Payload for Structural Health Monitoring Experiments on the International Space Station

This contribution reports design and development of a payload for structural health monitoring (SHM) experiments on the International Space Station (ISS). The payload was designed to operate in low earth orbit (LEO) environment and fit specifications of the Materials International Space Station Experiment (MISSE) module. In particular, LEO environmental factors such as a strong vacuum, thermal variations from −18°C to 60°C [1], and background radiation were considered. The payload is a rectangular multi-leveled structure which houses several SHM experiments, active sensors self-assessment, and electronic hardware with data storage and retrieval capabilities. SHM experiments include guided wave propagation in a metallic structure, monitoring of an imitated crack, assessment of a bolted joint, investigation of structural vibration via electromechanical impedance method, and acoustic emission monitoring. In addition, piezoelectric sensor self-assessment is realised using impedance diagnostics. It is anticipated that the payload will operate for one year in LEO and provide insights on the effect of space environment on SHM of future space vehicles during long-duration flights. This contribution focuses on mechanical design of the payload to support SHM experiment. Specific arrangement of payload elements and implementation of boundary conditions for SHM experiments are reported. Theoretical calculations and examples of SHM experimental data obtained in laboratory tests are presented and discussed in light of expected variations due to LEO environment. Measures to protect SHM hardware from harsh space environment are presented. Perspective applications of SHM as an integral component of future space systems are discussed.

Acoustic Control of a Maneuverable Marine Hydrokinetic Cycloturbine

Marine Hydrokinetic (MHK) cycloturbines exploit tidal currents to generate sustainable electric power. Because of the harsh marine environment, MHK cycloturbines require frequent maintenance and repair, which for current systems necessitates the use of a ship, making the process difficult and costly. A novel MHK cycloturbine system has been designed that uses pitching foils for maneuver, potentially circumventing the costs and difficulties associated with deployment and repairs. The vehicle fatigue is decreased and the vehicle’s acoustic signature underwater is reduced by design of a novel acoustic controller. This controller specifically reduces the tonal noise at blade rate frequency. Each turbine foil radiates noise equivalent to an acoustic dipole at multiples of blade rate frequency, and so the vehicle is modelled as an acoustic multipole. At blade rate frequency, the turbine size compared to its acoustic wavelength allows for the entire vehicle to be treated as a compact source. The effect of turbine clocking on directivity and sound power is shown. The effects of the designed controller to reduce tonal noise at blade rate frequency and multiples are verified experimentally through testing in ARL’s Reverberant Tank facility. Fixing a Subscale Demonstrator (SSD) to a reaction frame provides the ability to measure the integrated loads using load cells. The radiated sound pressure is computed for the load cell data obtained. Acoustic control is implemented using the turbine RPM: turbines are clocked by slowing one turbine relative to another for a short period of time.

Preliminary Results of Microwave Induced Thermoacoustics Imaging in Geological Media

Seismic and electromagnetic imaging modalities are conventionally used in subsurface situational awareness applications. These modalities have been very effective at characterizing the geological media in terms of its constitutive mechanical properties such as density and compressibility, as well as electromagnetic properties such as electric conductivity, permeability, and permittivity. In order to enhance these imaging capabilities, a Thermoacoustic (TA) imaging system is used in this work. TA imaging relies on the coupling of mechanical and electromagnetic waves through a thermodynamic process, and it has the potential to reconstruct thermodynamic constitutive properties such as volumetric expansion coefficient and heat capacity. TA imaging has been mostly used in biological applications; this is due to the low signal-to-noise ratio that can be created with this physical mechanism. This work is aimed at addressing such limitation and exploring the use of TA imaging in geophysical applications. Conventionally, a short microwave pulse excitation is used to create the TA wave; so that the stress confinement condition is met while providing high resolution images. This approach requires the use of expensive high power amplifiers to create a detectable TA signal. This limitation can be addressed by using a frequency-modulated continuous wave (FMCW) excitation, which has been recently proposed as a suitable mechanism to enhance the signal-to-noise ratio of the TA signal generated for a given peak power constrain. This paper discusses and compares both pulsed and FMCW TA imaging in geological media. Preliminary experimental results show the efficacy of this approach to image a rock immersed in an oil bath; thus paving the way towards its future use for subsurface sensing and imaging of fluid flow and transport in porous media.