In June 2003 a series of acoustic propagation experiments were conducted off the coast of Panama City, Florida. The experiments were designed to measure and provide an understand of signal phase and amplitude fluctuations, and signal spatial and temporal coherence over several large horizontal and vertical arrays. The propagation measurements were conducted in a water depth of 8.8m and at ranges of 70 m and 150 m. The acoustic measurements cover frequencies from 1 to 140 kHz. The propagation measurements were supported by data obtained by wave rider buoys, CTD's, thermister chains and current meters. Bottom penetration data was also obtained using a buried hydrophone array. The experiments will be outlined and the data sets described.
An extensive series of sound absorption measurements were taken in air over a range of frequencies from 10–2500 Hz, of temperature from 10°–50 °C, and of relative humidity from 0.3%–92%, all at a pressure of 1 atm. The N2 and O2 vibrational absorption components were extracted from a relatively large background absorption by means of a differential technique using a background gas, 89.5% N2–10.5% Ar, which matches the sound speed of air but has no molecular absorption over the range of experimental frequencies. The measurements reveal: (1) The humidity dependence of the relaxation frequency of N2 in air exceeds that in binary N2–H2O mixtures, possibly because the vibrational modes of CO2 provide a competing relaxation path for V–V exchange between N2 and H2O molecules; (2) the temperature dependence of the relaxation frequency of humid N2 is nearly the same in air as in binary mixtures; and (3) at very low humidities the relaxation frequency of O2 appears to approach a limiting value much lower than that determined from a prior study. The present experimental results, together with those of an earlier study at high frequencies, provide a substantial data base leading to revised formulas for standard values of the relaxation frequencies of N2 and O2.
Sound absorption measurements were conducted by the resonant tube technique to study vibrational relaxation in moist N2 at 301°, 343°, and 387 °K. Analysis of the data yields the following results: (1) The improved measurement accuracy at the higher temperatures establishes vibrational–vibrational (V–V) energy transfer as the operative relaxation path for the de-excitation of N2* by H2O beyond the uncertainty of experimental error. (2) The best-fitted temperature dependence of the V–V rate constant is k30 = 5.41×106 exp(−25.3/T1/3 )(atm⋅s)−1, an expression which is consistent with seven independent sets of both acoustical and nonacoustical data. (3) The measured humidity and temperature dependence of the relaxation frequency of N2 in air differs substantially from that specified by the recent ANSI Standard S1.26/ASA23–1978. The N2 contribution causes error, up to a few dB/1000 ft, in the total sound absorption coefficient predicted by both the ANSI Standard and ARP866A.
An extensive set of sound absorption measurements was taken in air over a range of frequency from 20–2500 Hz, of temperature from 20°–50 °C, and of relative humidity from 0.3%–100%. Over the lower portion of this frequency range, where relaxation in N2 is prominent (except in very dry air), prior measurements are scanty. This study yielded the following conclusions: (1) The humidity dependence of the relaxation frequency of N2 differs in air from that in binary N2-H2O gas mixtures (246-Hz/atm. mole % in air vs 184-Hz/atm. mole % in binary mixtures at 20 °C). (2) The temperature dependence of the relaxation frequency of N2 is the same in air as in binary mixtures. (3) At low humidities (∼0.01% mole ratio), where relaxation in O2 dominates sound absorption in air, the measured relaxation frequencies of O2 agree with those reported by Harris and Tempest [J. Acoust. Soc. Am. 36, 2390–2394 (1964)] and lie substantially lower than specified by ANSI Standard S1.26-1978.
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