Timothy E. McDevitt

Power flow in coupled bending and longitudinal waves in beams

In complex structures, curvature and impedance discontinuities (e.g., junctions) couple bending and longitudinal waves. Propagation losses for longitudinal waves are often much less than losses for bending waves, and damping treatments often less effective on longitudinal waves. When the dissipation in longitudinal waves is less than that on bending waves, longitudinal waves can provide an efficient means of power flow between bending waves generated at one location and bending waves that are a source of acoustic radiation at another location. In order to design and locate effective treatments, knowledge of the power flow in longitudinal as well as bending waves is required. The measurement of power in both bending and longitudinal waves when both waves are present is demonstrated. Measurements conducted on a straight beam and a T-beam are compared to predictions obtained using finite element methods. The effect of coupling between waves at the junction in a T-beam is illustrated using results from measurements of power flow.

An investigation of induction motor zeroth-order magnetic stresses, vibration, and sound radiation

A computational approach is undertaken to better understand what can be one of the primary contributors to sound radiation from induction motors-the n=0 magnetic stresses. An electromagnetic finite-element model, a structural dynamic finite-element model, and an acoustic boundary-element model of an induction motor are used to illustrate some sources of n=0 electromagnetic stresses, and how their resultant vibration field on the stator can be enhanced by boundary conditions, such as stator supports, to result in peaks in the radiated noise spectrum.

Advancements toward a high-power, carbon nanotube, thin-film loudspeaker

The carbon nanotube (CNT) thermophone has been explored as a novel loudspeaker. Potential advantages of this technology in the audio industry include ultra-lightweight, low production cost, compact size, and independence from rare-earth materials. In this paper, progress toward a practical CNT loudspeaker is presented. Large, high quality CNT thin-film assemblies are designed and built. Design guidance for these types of assemblies is provided. Maximum sound output level, total harmonic distortion, and power efficiency tests are performed. A maximum source level of 111 dBA at 1 m is achieved at 2 kHz with the new sources. The main hurdle to this technology remains power efficiency. Several paths forward are discussed as this technology continues to advance to a position where it may be able to compete with current state-of-the-art, moving-coil loudspeakers.

A two‐channel laser Doppler vibrometer technique for characterizing the dynamic properties of elastomeric materials

An optical technique is described for obtaining the dynamic loss factor and Young’s modulus of elastomeric materials. The technique involves a two‐channel laser Doppler vibrometer for measuring the relative amplitude and phase at two points on a longitudinally excited material sample. Replacing the two accelerometers ordinarily used in this materials characterization technique with nonintrusive laser probes offers numerous advantages. The primary advantage is the simplification of the mathematical procedure for obtaining the material properties, since end mass (accelerometer) effects can be discarded. Results from a test conducted to compare optical and accelerometer data show very good agreement except at frequencies where the sample and/or test apparatus exhibit a high degree of flexural motion. The lack of agreement in these cases is explained. In such cases, it is recommended that the optics be rotated in order to detect these flexural modes so that they can be avoided, if warranted, when evaluating t...

Structural-acoustic tailoring of metal structures by laser free-forming

Laser Free-Forming (LFF) is a manufacturing process that facilitates the mixture of different metal alloys and provides new opportunities in structural-acoustic tailoring. In this paper, a brief introduction is given to the LFF manufacturing process and the potential benefits, in terms of process cost and strength and reliability, are discussed. A hydrofoil for an underwater vehicle is used as an example problem to explore strategies for structural-acoustic design optimization and noise mitigation. It is shown through numerical studies that the sound radiation of a hydrofoil can be appreciably reduced by spatially varying its material properties.

LONG GAGE LENGTH INTERFEROMETRIC FIBER OPTIC SENSOR FOR STRUCTURAL DAMAGE DETECTION

Abstract Two areas of research are brought together in the present study to develop a nondestructive damage detection system and methodology: long gage length interferometric fiber optic sensors and modal analysis. Using modal analysis techniques, the physical state of a concrete beam and steel beam is assessed by monitoring its vibratory signature, i.e. resonant frequencies, mode shapes, or damping. Traditionally, modal analysis has been performed using data from point sensors such as accelerometers, seismometers, LVDTs, or electric strain gages with limited success. A long gage length optical sensor has been developed in the present study for modal analysis on civil structures. Because modal analysis involves monitoring global attributes, a long gage length sensor is used to allow a larger portion of a structure to affect the sensor output. The present study uses a long gage length interferometric fiber optic sensor on a steel beam and a concrete beam to monitor induced physical changes, or damage, of t...

Beam steering for a long range Acoustic Hail and Warning Device for maritime applications

A new means for steering the beam of a phased line array, electromechanical steering, is described and applied to a long range Acoustic Hail and Warning Device (AHWD) developed for maritime applications. In electromechanical steering, the array elements pivot individually resulting in elements being off-set and rotated in the desired steered beam/sound projection direction. A digital signal processor is used to align the phase fronts of the elements due to the mechanical offsets. Electromechanical steering has several advantages over the purely electronic steering of the array. The electromechanically steered beams are not subject to the individual element aperture function attenuation that reduces the sound pressure level (SPL) that commonly occurs for electronically only steered beams at large steering angles. Also, the primary lobe distortion and (lower frequency) onset of grating lobes that occur when a beam is steered electronically is absent with electromechanical steering. In short, with electromechanical steering, the SPL and directivity from the AHWD is entirely equivalent to that obtained by rotating the AHWD to the desired angle with a conventional pan and tilt mount but requires much less space and provides for more flexible mounting options including improved system stealth. In addition, a novel type of line array phase shading is presented which allows the AHWD to be electronically switched from directional mode (typical beam steering highly focused mode) to non-directional wide field of regard for applications that require simultaneous ensonification of targets over large lateral angles. ©2011 Institute of Noise Control Engineering

Atmospheric effects on voice command intelligibility from acoustic hail and warning devices

Voice command sound pressure levels (SPLs) were recorded at distances up to 1500 m. Received SPLs were related to the meteorological condition during sound propagation and compared with the outdoor sound propagation standard ISO 9613-2. Intelligibility of received signals was calculated using ANSI S3.5. Intelligibility results for the present voice command indicate that meteorological condition imposes little to no effect on intelligibility when the signal-to-noise ratio (SNR) is low (<-9 dB) or high (>0 dB). In these two cases the signal is firmly unintelligible or intelligible, respectively. However, at moderate SNRs, variations in received SPL can cause a fully intelligible voice command to become unintelligible, depending on the meteorological condition along the sound propagation path. These changes in voice command intelligibility often occur on time scales as short as minutes during upward refracting conditions, typically found above ground during the day or upwind of a sound source. Reliably predicting the intelligibility of a voice command in a moderate SNR environment can be challenging due to the inherent variability imposed by sound propagation through the atmosphere.

Development of a long range acoustic hail and warning device for maritime applications

An Acoustic Hail and Warning Device (AHWD), intended to project intelligible voice warning messages to listeners at 500 yards (457 m) where at least 80% of the words in an unknown message are intelligible, was designed, fabricated and tested. Design was facilitated by a methodology developed to help ensure long range intelligibility in different listener environments. Two intelligibility goals, i.e. listener environments, were evaluated: I) an outdoor listener at a distance of 457 m over water and in the presence of 89 dBA background noise levels, and II) a listener inside a closed watercraft enclosure at 457 m and in the presence of 63 dBA background levels. The 457 m distance corresponds to the radius of the protection zone surrounding U.S. Naval vessels and the background noise levels are representative of those encountered in maritime scenarios. Component testing and analyses indicated that a uniformly phased horizontal line array of 8 horn-loaded compression drivers would satisfy both goals I and II. The array was then fabricated and its voice warning message spectrum was measured. The measured spectrum and subsequent prediction showed that the 8 horn/driver array satisfies both goals I and II with 89% and 94% of the words in an unknown voice warning message being intelligible in those respective scenarios assuming homogeneous, quiescent atmospheric conditions. ©2009 Institute of Noise Control Engineering

Phased array for acoustic hail and warning.

Background is given for the need of a high output acoustic hailing device (AHD) for military use. Those detrimental effects, which require high output power of an AHD, such as refraction, absorption, and insertion loss are briefly reviewed along with associated experimental data. The background for intelligibility is touched to address the importance of frequency response. A prototype phased array AHD, THOR, is then described and characterized via experimental results. [Work supported by the DOD.]