Displacement detection using optical interferometric techniques allows for low minimum detectable displacements which are unmatched by other displacement measurement methods as device sizes are scaled down. The use of diffractive optical elements as beam splitters has proven an effective way to realize miniature and robust optical interferometers. Diffraction gratings commonly used in such applications, however, can generate a zeroth-order reflected beam, which results in reduced sensor performance, packaging limitations, and laser instability. A diffraction grating concept has been designed, fabricated, and tested, which has the effect of reducing the zeroth-order component by imparting a half-wavelength phase shift to a portion of the reflected light. The design criteria for zeroth-order beam elimination are illustrated using a simple model based on phasor arithmetic. The microfabrication process used to prototype gratings is presented, and experimental measurements collected from the prototype are reported. The minimum detectable displacement achievable in sensor applications is found to be 3.6 fm/√Hz, which is comparable to sensors built using more conventional gratings. Finally, comparisons are made between the test results and the simple model predictions.
VAI Resort, situated across the street from the site of Super Bowl LVII, presents a number of acoustical challenges, not the least of which is a world-class, 360-degree outdoor performance venue. Performers will blast an 8-storey hotel located 250 ft away with music of all types, which audiences will enjoy from the balconies of their hotel rooms. The challenge of modeling such a venue comes in simplifying it down to manageable pieces. Does a balcony with a hotel room behind it behave like a Helmholtz resonator? How do the acoustics change if 90% of the balconies are open to the hotel rooms beyond? What if 50% are open? 25% are open? Such a variable acoustic space presents a significant modeling challenge. A methodology for tackling such a large-scale project is presented, with modeling results, auralizations, and comparisons to real-world measurements. Findings include a swing in RT60 of more than 25% when all balcony doors are opened versus closed.
Under windy conditions, vortex shedding can cause perforated metal skins to emit undesirable noise. This phenomenon was observed, measured, and replicated. The acoustic spectrum was analyzed and compared to the modes of a vibrating plate. An extensional-damping layer was added to the plates for vibration isolation. The damping layer was applied as a high-loss painted coating to the surface of the plates. The acoustic spectrum was then analyzed again and compared to the spectrum without treatment. This study describes the process used to investigate and solve this noise control problem.
Ground reflections have a significant impact on the propagation of sound from a horizontal rocket firing. The impedance of the ground relies strongly on effective flow resistivity of the surface and determines the frequencies at which interference nulls occur. For a given location, a softer ground, with lower effective flow resistivity, shifts the location of interference nulls to lower frequencies than expected for a harder ground. The difference in the spectral shapes from two horizontal firings of GEM-60 rocket motors, over snowy ground, clearly shows this effect and has been modeled. Because of the extended nature of high energy launch vehicles, the exhaust plume is modeled as a partially correlated line source, with distribution parameters chosen to match the recorded data sets as best as possible. Different flow resistivity values yield reasonable comparisons to the results of horizontal GEM-60 test firings.
It is common to have noise complaints from outdoor event and entertainment venues in urban and non-urban environments due to the loudness of the reinforcement system, rhythmic nature of the music, acoustical spectrum, and the geometric spreading of said system. Complaints about noise from events, bars, and restaurants are on the rise. Increasing traffic, construction, and commercial activities tend to increase the ambient noise level in urban environments, creating noise creep. The pandemic restrictions helped reduce the ambient levels in the urban and non-urban areas. However, after the reopening, more and more noise complaints started to be issued since residents were more sensitive to noise pollution. This paper presents a practical application of the source directivity for a loudspeaker system. The loudspeaker canopy is designed to produce a radiation pattern directing the main beam lobe to the front of the array and reduce the noise propagation to other directions. This device can be implemented in entertainment and outdoor events (such as wedding parties) to reduce the noise impact on sensitive receivers and meet the jurisdictional noise regulations. Real-world acoustic intensity measurements were made to obtain the system's sound power and assess the noise impact at distances up to 35 feet away from the source. The viability of using the system for outdoor activities within locations restricted by the City's ordinances is discussed.
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