Single coherent bubble contributions to the incoherent underwater noise of spilling breakers have been studied in an anechoic laboratory facility. The waves are generated by a plunger, they propagate 17 m along a 1.2×1.2-m water waveguide, and ‘‘spill’’ and create bubbles at the surface of a 3×3×3-m anechoic cube of water. Several species of bubbles have been identified. In general, they act as transient dipoles of duration from 2 to several milliseconds, with peak axial source strength of the order of tenths of pascals, at 1 m. The noise is emitted when the bubble is within hundreds of micrometers or a few millimeters of the surface. Bubbles were observed in the 2 decades of frequency from 500 to 50 000 Hz. The average of the individual bubble events yielded a spectrum that slopes at about 5 dB/oct from 1 to 20 kHz, the same as the Knudsen wind noise spectra at sea. The magnitude of the laboratory breaker noise during continual wave-breaking events was approximately 80 dB re: 1 μ Pa2/Hz at 1 kHz, which is essentially the same as observed during the continual bubble production that occurs with very high winds at sea. The reasons for this agreement are discussed.
Pressure broadening and line shift parameters for the 1←0 and 2←1 rotational transitions of carbon monoxide in collision with helium have been measured between 1 and 600 K. Measurements below 30 K were made using the collisional cooling technique, while measurements at higher temperatures were made in an equilibrium cell. The experimental apparatus and techniques employed in the study are described. Pressure broadening and line shift cross sections are compared with theoretical predictions based on the best available potential energy surface, and differences between theoretical predictions and the experimental results are discussed. Potential sources of systematic experimental error are examined as a possible origin of these differences.
Square and triangular lattice two-dimensional (2-D) photonic crystals (PC's) composed of lossy dielectric rods in air were constructed with a microwave bandgap between 4-8 GHz. Fabry-Perot resonators of varying length were constructed from two of these PC's of adjustable thickness and reflectivity. The quality factor of cavity modes supported in the resonators was found to increase with increasing PC mirror thickness, but only to a point dictated by the lossiness of the dielectric rods. A 2-D periodic Green's function simulation was found to model the data accurately and quickly using physical parameters obtained in separate measurements. Simple rules are developed for designing optimal resonators in the presence of dielectric loss.
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