摘要:

The propagation of audible sound through air‐water fogs is studied theoretically using a generalized Burgers’ equation applicable to media exhibiting relaxational as well as classical thermoviscous absorption. The effects of vibrational and rotational relaxation of the gas molecules, and of polydisperse water droplet relaxation due to mass, momentum, and heat exchange with the gas are included in the formulation. Analytic expressions showing the dependence of dispersion and attenuation parameters on gas and droplet properties and on sound frequency are derived. Agreement between theoretical predictions and available experimental measurements is good. The results clarify those conditions under which sound propagation in fogs is significantly different from propagation through humid air containing no suspended droplets.

ISSN:0001-4966    

DOI:10.1121/1.381567

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

We analyze the high‐frequency scattering of a plane‐wave incident on a submerged air‐filled cylindrical steel shell coated with a viscoelastic layer. The scattered pressure field consists of a geometrical acoustic contribution and another part due to creeping waves. We analyze the resulting generalized reflection coefficient as a function of azimuthal angle, for two shell sizes and various viscosity levels of the coating. We also obtain the locations of roots of the 10×10 characteristic determinant in the complex ν‐plane for the two shell sizes of interest. These zeroes are compared to the zeroes of rigid and soft cylinders and to the roots of a simpler 3×3 determinant corresponding to the case of a solid sound‐absorbing cylinder of the same size. The results show that the region in the ν‐plane where many of the roots are usually concentrated for bare structures, contain no roots after the absorbing layer is bonded to the steel shell. All the above information is then used to compare the normal‐mode solution for the sonar cross section of the coated cylindrical shell found in an earlier study of ours (and extended here to higherk1cvalues), and the corresponding expression obtained to lowest order from an application of the Watson–Sommerfeld transformation. This comparison, as exhibited in various graphs, is found to be very favorable. It verifies that the lowest‐order term in the Watson–Sommerfeld transformed solution is capable of accurately reproducing the exact scattering solution at high frequencies for the cylindrical coated target considered here.

ISSN:0001-4966    

DOI:10.1121/1.381568

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The reflection of an acoustic plane wave by an elastic cylinder of infinite length, which may absorb energy, is calculated and measured. Computations of reflection by an infinitely long elastic cylinder as a function of frequency and angle, are compared with experimental data taken using finite cylinders immersed in water. Good agreement is obtained.

ISSN:0001-4966    

DOI:10.1121/1.381569

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

When the received level of a transmitted acoustic wave reaches an upper limit that cannot be exceeded regardless of how much acoustic power is radiated by the source, the wave is said to have saturated. Experiments on saturation of plane waves in air are reported in this paper. The measured quantity is the pressure amplitudep1of the fundamental. To obtain a theoretical prediction that is valid close to and including saturation, we assume the decay rate of the fundamental is the sum of the sawtooth and ordinary absorption decay rates (Rudnick’s assumption). Solution of the assumed equation givesp1=2p10e−αx/[1+(1−e−αx)/α?], wherep10is the source amplitude, α is the ordinary (small‐signal) attenuation coefficient, and ? (proportional to 1/p10) is the shock formation distance. The saturation amplitude is found by lettingp10→∞. The experiments were done in a plane‐wave tube over a frequency range of 0.5 to 4 kHz at source levels up to 163 dB (re0.0002 μbar). Qualitatively, saturation is evident in the waveforms and the amplitude response (input–output) curves. Quantitatively, the data confirm the theoretical prediction. Dispersion, which is caused by tube‐wall boundary layer effects, causes asymmetry of the waveforms but apparently has little effect on the fundamental amplitude.

ISSN:0001-4966    

DOI:10.1121/1.381570

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A study of the influence of echo carrier frequencies and antenna dimensions on the performance of acoustic‐radar antennas is reported with features of practical interest. For a steady‐state plane‐wave propagation, appropriate expressions are given for the acoustic impedance, the homogeneous boundary conditions analogous to the electromagnetic type of problems, and the time‐averaged acoustic energy. Diffraction pattern integral equations for echosonde antennas are evaluated in closed forms using the Zernike polynomials and the generalized‐hypergeometric functions, and physical interpretations are given where appropriate. Accurate computer simulations can contribute toward: (1) Better understanding of acoustic remote sensing of atomspheric structure/properties and (2) time‐saving/avoidance of hidden problems in field work. Antenna‐pattern simulations are examined using modifications of antenna design which improve antenna‐system performance. By proper choices of echo carrier frequencies and antenna dimensions, quasiuniform phase distributions pertinent to modest phase shifts of experimental measurements (in striking contrasts with rapidly changing phases of previous results), are obtained. Severe side lobes are major detriments in acoustic remote sensing. Results presented include computer simulations of an antenna employed in probing the marine atmosphere remotely from a moving ship during a cruise in the Pacific Ocean/Caribbean Sea, and simulations of an antenna employed over dry‐land; ground‐level side lobes versus echo carrier frequencies; and 3‐dB beamwidth variations with echo carrier frequencies and with antenna dimensions. For 1–5‐kHz carrier‐frequency range, acceptable antenna dimensions are 1.22, 1.8, and 1.8 m for the illuminating‐ transmitting‐aperture diameters, and height of the absorbing cuffs, respectively. Half‐power beamwidths within 7°–13.5° are obtained in the 1–2‐kHz frequency range; and selection of antenna flare angles between 15° and 18° tends to give optimum side‐lobe attenuations. Comparisons between theory and measurements show overall good agreements. Maximum relative side‐lobe rejections of about −56.5 dB (in good accordance with measurements) in the 20° region near the ground, are obtained at 2‐kHz carrier frequency.

ISSN:0001-4966    

DOI:10.1121/1.381571

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The parabolic‐equation technique has received considerable attention since its introduction to the underwater‐sound community by Tappert and Hardin three years ago. The technique relies upon removing the primary radial dependence of the acoustic field from the elliptic wave equation, and approximating the resulting equation with a parabolic (or Schroedinger) equation which may then be rapidly solved on digital computers. The parabolic approximation has been shown for layered media to result in an error in the phase velocity of the normal modes. In underwater‐sound propagation the phase‐velocity error can cause substantial shifts in the range of convergence zones. A significant reduction in this error appears achievable by a simple modification to the refractive‐index profilen(z) which transforms the (n,z) pairs of points into a new set of points (?,?). Within the accuracy of the WKB approximation, the normal modes for this new environment have the same phase velocities as the equivalent wave‐equation modes in the original environment. Thezto ? mapping indicates the correspondence between field points in the old and new environments. This mapping is designed to approximately map turning point into turning point. Comparisons with normal‐mode solutions of the full elliptic wave equation are presented to illustrate the improvement in accuracy of the parabolic results under this transformation. A ray comparison is also given. Problems associated with extending the technique to a range‐dependent environment are discussed and in limited testing have been found to be minimal.

ISSN:0001-4966    

DOI:10.1121/1.381572

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The speeds of sound of liquid deuterium oxide relative to normal water were measured at 1 atm over the temperature range of 4°–60°C and at pressures up to 1000 bars over the temperature range of 5°–50°C. Three velocimeters were used to measure the sound speeds of deuterium oxide at 1 atm relative to normal water. These three sets of 1‐atm data were fitted to a polynomial equation of temperature with a standard deviation of ±0.09 m/sec. The results of the high‐pressure measurements were fitted to an equation of the form (with a standard deviation of ±0.07 m/sec) (UH2O−UD2O)−(U0H2O−U0D2O) =AP+BP2+CP3+DP4, whereUH2OandUD2Oare, respectively, the speeds of sound in normal water and deuterium oxide (superscript zero denotes 1 atm).A,B,C, andDare temperature‐dependent parameters which were determined by the least‐squares method. Although the values ofU0D2Oobtained in this study are on the average 0.6 m/sec lower than the data of Wilson [J. Acoust. Soc. Am. 33, 314–316 (1961)], the pressure effect on the relative sound speeds from the above equation [(UH2O−UD2O) −(U0H2O−U0D2O)] agree with the work of Wilson to an average of 0.26 m/sec. One atmosphere adiabatic and isothermal compressibilities for deuterium oxide which are reliable to ±0.005×10−6bar−1, were calculated from the sound‐speed data. The results were compared with the data of various workers. The values of isothermal compressibility obtained in this study are systematically higher than the work of Fine and Millero [J. Chem. Phys. 63, 89–95 (1975)] (average 0.05×106bar−1); however, they agree with the data of Millero and Lepple [J. Chem. Phys. 54, 946–949 (1971)] and Emmet and Millero [J. Chem. Eng. Data 20, 351–356 (1975)]to within ±0.02×10−6bar−1.

ISSN:0001-4966    

DOI:10.1121/1.381573

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

An equation is presented for sound absorption in sea water as a function of frequency, temperature, and pressure based on laboratory data. The equation includes contributions to absorption due to boric acid, magnesium sulfate, and water. The effect of pressure on sound absorption due to magnesium sulfate and water has been treated differently than in the Schulkin and Marsh equation. At 4°C our results for absorption at frequencies from 10–400 kHz and pressures up to 500 atm are substantially lower than those calculated from the Schulkin and Marsh equation.

ISSN:0001-4966    

DOI:10.1121/1.381574

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The NRL computer program SHIP is used to compute the diffraction constants for finite‐length cylindrical radiators and circular pistons in the end of finite‐length cylindrical baffles. Radiation impedances are calculated. The diffraction constants can be used to calculate the sensitivity of piezoceramic tube elements above the omnidirectional range.

ISSN:0001-4966    

DOI:10.1121/1.381575

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The absorption of sound in air at frequencies from 4 to 100 kHz in 1/12 octave intervals, for temperatures from 255.4° K (0° F) to 310.9° K (100° F) in 5.5° K (10° F) intervals, and at 10% relative‐humidity increments between 0% and saturation has been measured. The values of free‐field absorption have been analyzed to determine the relaxation frequency of oxygen for each of the 92 combinations of temperature and relative humidity studied and the results are compared to an empirical expression. The relaxation frequencies of oxygen have been analyzed to determine the microscopic energy‐transfer rates.

ISSN:0001-4966    

DOI:10.1121/1.381576

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP