摘要:

A set of two coupled differential equations with square law nonlinearities is shown to have dominant and stable subharmonic solutions. Mathematical expressions characterizing the subharmonic solutions and their regions of stability are obtained. It is further shown that under appropriate choice of system parameters a resulting system described through these coupled differential equations possesses an exact stable subharmonic of any real order. From this, a design theory is obtained for a system which yields an arbitrary dominant subharmonic. The theory is also directly applicable to the creation of arbitrary stable harmonics.

ISSN:0001-4966    

DOI:10.1121/1.383911

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Nonlinear interaction of two collinear, spherically spreading sound beams is considered and applied to a discussion of parametric acoustic arrays. A general solution is given in the quasilinear approximation in terms of spherical harmonics and used to discuss the asymptotic behavior of the generated sound. Other expansions are also discussed. The main results, however, are obtained by introducing perturbation methods and deriving simplified equations governing the generated sound field, taking into account diffraction, absorption, and nonlinearity. From these equations, solutions are obtained and matched to give a solution valid in the whole space of propagation. Analytical and numerical results are presented and compared with available observations by different authors.

ISSN:0001-4966    

DOI:10.1121/1.383912

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The magnetic field that is generated by the sound field of a transient acoustic source immersed in electrically conducting fluid in the presence of ambient magnetism is calculated from a recent theory of such ’’sonomagnetic’’ phenomena. A general solution for the sonomagnetic pulse emitted by an arbitrary transient source is derived in terms of the acoustic pressure field directly (as opposed to a less certain source description). For a model pressure pulse, representative of underwater explosion/implosion pulses, the solution is reduced to tabulated functions. The result consists of a sonomagnetic shock plus precursor and relaxation waves, whose properties are discussed analytically and illustrated numerically.

ISSN:0001-4966    

DOI:10.1121/1.383913

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A simple numerical simulation model is presented which includes the effects of oceanic finestructure together with previously developed theoretical treatments of the effects of internal waves on acoustic transmission. The wave equation solution to particular layer geometries developed by Brekhovskikh [WavesinLayeredMedia(Academic, New York, 1960)] is combined with the theoretical model of acoustic scattering based on internal waves developed by Desaubies [J. Acoust. Soc. Am. 64, 1460–1469 (1978)]. In this treatment it is assumed that a single finestructure layer is passively advected by internal waves. The layer modulates the effects of internal waves. Experiments carried out at Cobb Seamount over a fixed, horizontal, wholly refracted path indicate that the power spectrum of the phase fluctuations is fit almost exactly by models based on internal waves only, while the power spectrum of the log amplitude fluctuations is not. In the present treatment the predictions of the phase fluctuations remain unaffected by the presence of finestructure, and the predicted power spectrum of the log amplitude is in considerably better agreement with observation.

ISSN:0001-4966    

DOI:10.1121/1.383914

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A general model for averaging the acoustic target strength functions of fish is stated in calculable form. It accounts for the influences of the distribution of generally coupled spatial and orientation states of fish, geometric perspective, and beam patterns on observations of target strength. The model is developed and applied to observation of fish by directional, downward‐looking sonars. A particular example is considered in which the sonar is represented by an ideal circular piston, the spatial distribution of fish is homogeneous, and the orientation distribution is spatially homogeneous and characterized by a uniformily distributed azimuthal variable and an independent, essentially normally distributed tilt angle variable. Averaged and averaged‐squared backscattering cross sections are computed from high quality gadoid target strength functions measured at two ultrasonic frequencies. Results for a sonar half‐beamwidth of 2.5 deg for three different realizations of the tilt angle distribution are expressed in the logarithmic domain and regressed linearly on fish length. The significance of species, frequency, and orientation distribution differences among the regressions is noted. Estimates of the mean ratio of averaged‐squared backscattering cross section and squared‐averaged backscattering cross section are presented.

ISSN:0001-4966    

DOI:10.1121/1.383915

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A model is developed for the acoustic source strength of blade rate line energy produced by single‐screw merchant vessels. These source strengths are based on observed cavitation time histories on merchant vessels and on limitations imposed by considerations of propeller design procedures and ship vibration criteria. Relationships are presented for the expected value of the blade rate source strength for ships of different lengths, expressed both as a monopole source strength located at a known depth below a free surface and as a dipole source strength that describes the pressure radiated to the farfield. These relationships are based on a small sample of merchant ship characteristics and are exercised for the estimated population of ships at sea. This calculation yields a statistical description of the distribution of source level and frequency of propeller blade rate acoustic energy for the fleet of single‐screw merchant vessels.

ISSN:0001-4966    

DOI:10.1121/1.383916

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Results are presented of an experimental investigation of the use of thin metal parabolic reflectors to form underwater quasiplane wave regions in the nearfield of the reflector. This quasiplane wave volume is then used to calibrate underwater sound transducers in terms of directivity and sensitivity. Such reflectors, mass produced for use with electromagnetic wave devices, are readily available, inexpensive, and rugged. Experiments were conducted with reflectors ranging from 45 to 122 cm in diameter at frequencies from 30 to 300 kHz. It might be noted that at the higher frequencies the acoustic wavelength is approximately an order of magnitude shorter than that for which the reflectors were designed. Test transducers, typically half the size of the reflectors, were calibrated both in the nearfield of the reflector and in the farfield (using conventional techniques) and the results were compared. It was found that for the major lobe there was excellent agreement and that the levels of the minor lobes were in acceptable agreement but that the fine detail of the minor lobe structure differed for the two types of measurements. Sensitivity measurements differed by about ±1.0 dB. For low‐frequency operation the metal reflector needs to be backed by air or other pressure release material in order to be an efficient reflector. It is concluded that the parabolic reflector technique has useful accuracy for many applications.

ISSN:0001-4966    

DOI:10.1121/1.383917

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Both a deterministic dipole model and the law of conjugate depths are applied to study the depth dependence of the low‐frequency ambient noise in the main sound channel and, when used together, a reasonable agreement with experiment is obtained. Scattering or diffusion models for the region below critical depth agree well with experiment. Complete area and line source models for the ambient level are derived. Three approximate predictions of the low‐frequency attenuation come from the deep ambient depth dependence, ambient differences with site position, and ambient differences with site and depth, respectively. All three are consistent with conventional measurements and with the scattering diffusion and bottom loss mechanism.

ISSN:0001-4966    

DOI:10.1121/1.383918

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In order to establish a method of estimating the engine structure surface responses, a series of experiments was made on a four‐cylinder (bore: 86 mm, stroke: 84 mm), in‐line, water‐cooled high‐speed diesel engine. The results obtained in static conditions are summarized with the necessary background. The propagation behavior of excitation forces in the engine structure, the vibration behavior of the vibration transmission paths, and the transfer functions of the transmission paths were examined in static conditions. The distribution of the damping in the engine structure, the influence of cooling water, the lubrication oil, the oil pressure, and the crank position on the transfer functions were also examined. After the experiments, we found that (a) a different excitation force induces different responses on the engine structure surfaces because of its different transmission paths and their transmission behavior; (b) the forces applied on the cylinder head can be transmitted along the cylinder head bolts, and the engine structure can be excited by this force just as strongly as by the identical force applied on the piston top; and (c) most of the damping in the engine structure is induced by the oil film existing around the main moving parts such as the pistons, the connecting rods, and the crankshaft.

ISSN:0001-4966    

DOI:10.1121/1.383919

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In order to gain a better understanding of the transmission behavior of the excitation forces in the engine structure, such as the gas forces, the inertia forces, the piston slap forces, and the valve seating impact forces, a series of motoring tests was carried out on a four‐cylinder (bore: 86 mm, stroke: 84 mm), in‐line, water‐cooled high‐speed diesel engine. The motoring tests were carried out by changing the combinations of the excitation forces, and at different engine speeds. Under the motoring tests, it could be seen that even the cylinder block bottom sides could be steadily excited by the valve seating impact forces. The transmission behavior of each of the excitation forces can be identified distinctly by the particular frequency components in the power spectral density diagrams of the engine structure responses. Finally, new types of transfer functions were proposed in order to identify the relationship between the kinetc energy of the impulsive excitation forces (such as the piston slap forces and the valve seating impact forces) and the induced engine structure responses. These new transfer functions could be determined under motoring conditions and can greatly overcome the difficulties in estimating the engine structure responses.

ISSN:0001-4966    

DOI:10.1121/1.383920

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP