摘要:

Nonlinear vibrations of a string may become unstable when the string is driven with sufficient amplitude near resonance. This paper describes the experimental conditions under which transverse oscillations of a horizontal string, driven in the vertical plane, become unstable. It is found that the planar oscillations become unstable in two different ways. One instability involves a spontaneous growth or decrease in the amplitude of the driven vertical motion. The other instability involves the spontaneous appearance of a transverse horizontal component of vibration perpendicular to the driving force, thus leading to a whirling or nonplanar motion of the string. The paper also describes the response of the nonplanar mode of oscillation, and it is found, for example, that for certain conditions the driven vertical component of oscillation decreases in amplitude when the driving level is increased. The experimental results agree qualitatively with theoretical predictions based in part on the work of Anand [J. Acoust. Soc. Amer.46, 667–677 (1969)].

ISSN:0001-4966    

DOI:10.1121/1.1912945

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

A theoretical formulation of the large deflections of helical springs is given, and coupled nonlinear equations of motion for a typical spring element are derived. Linearized forms of these equations are solved numerically and compared with experimentally obtained streak photographs of an impacted spring. The agreement between theory and experiment is good, as long as adjacent coils of the spring do not touch.

ISSN:0001-4966    

DOI:10.1121/1.1912946

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The response of an infinite Bernoulli‐Euler plate placed on the surface of a randomly vibrating viscoelastic half‐space is calculated, allowing for the presence of shear stresses between the plate and the half‐space. The shear stresses arise from the condition that the relative motion between the plate and the half‐space vanishes at the interface. The three components of displacement of the free surface of the half‐space before the plate is added are assumed to be stationary homogeneous random functions of position and time. From the wavenumber‐frequency spectra of these displacements the frequency spectra of the three components of displacement of the plate half‐space interface are calculated. As an example, the frequency spectrum of the vertical interface displacement is calculated for two assumed forms of the wavenumber spectra of the free‐surface displacements.

ISSN:0001-4966    

DOI:10.1121/1.1912947

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

An analysis is given for a damping addition employing several thin viscoelastic sheets and thin metallic layers. The damping layers are alternated with the constraining layers, which are segmented and are not anchored to the base plate. The effectiveness of such additions in increasing the damping of thin simply supported square plates is evaluated and methods for optimizing the design are considered. An experimental investigation of the effectiveness of such damping additions is also reported. Additions weighing a few percent of the weight of the base plate were found to produce substantial increases in the damping of the system. A nonoptimal design was found to produce less, but still substantial, damping.

ISSN:0001-4966    

DOI:10.1121/1.1912948

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1912949

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

This paper gives a brief survey of the state of knowledge in ocean acoustics circa 1948 and reviews the advances made during the past two decades. Twenty years ago the thermostructure of the ocean was known in moderate detail. while the character of the boundaries could be specified less precisely. A principal deficiency was in the ability to handle a mathematical model of the ocean which would give a prediction of the acoustic field in detail. While much remains unknown today, the past 20 years have seen a considerable accumulation of knowledge concerning the geographic and seasonal variations of the medium. Reasonably good specifications of the ocean boundaries are now available. The acoustic absorption coefficient in the ocean is known imperfectly, and the ability to specify the thermo microstructure is poor. Nonetheless, the intensive efforts of many investigators over the last 20 years have yielded a computational capability which now allows a general specification of propagation in relatively great detail.

ISSN:0001-4966    

DOI:10.1121/1.1912950

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The purposes, problems, and progress of radiated noise, self‐noise, and ambient noise research are reviewed. Purposes are related primarily to national defense, but applications to fishery and to the utilization of other natural resources are also noted. Basic problems, most of which were recognized 20 years or more ago, involve ascertainment of properties of the noise, identification of noise sources and mechanisms of noise generation, and the discovery and definition of noise dependencies on environmental factors. Many radiated and self‐noise sources and mechanisms have been identified. Major problems are those of noise measurement, noise reduction, and prevention. In the field of ambient noise, most measurements have been of sound‐pressure level.Someof the noise sources and environmental factors have been identified, and a capability for qualitative and gross prediction has been achieved. Recommended are further investigations of the variation of ambient noise with receiver depth, directionality of the noise field, statistics of both noise level and instantaneous noise values, additional work at frequencies below 10 Hz, and additional geographic coverage, making full use of current knowledge to fashion models for experimental guidance. Challenging problems exist in procedures and instrumentation for noise studies.

ISSN:0001-4966    

DOI:10.1121/1.1912921

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Development in the art of transducer design and array configuration has been spurred on mainly by two motivating forces: (1) Extensive research in underwater propagation has established the advantages of increasingly lower frequencies for long‐range detection and localization of targets and noise sources. (2) The advent of new piezoelectric and structural materials and of solid‐state electronic devices has led to substantial improvements in transducer performance with regard to sensitivity, power output, and depth of submersion. Theory has kept abreast with these developments, particularly in handling large arrays with their mutual impedance problems, in pointing to digital multibeamforming techniques, and in revealing the significance of waveform design to detection performance. A few new concepts were born and have won acceptance during this period. These include: the generation of sound by hydroacoustic means, the employment of compliant metal tubes for acoustic focusing and reflection, the exploitation of structural flexing modes for sound radiation, and the use of free‐flooded cavity structures as deep submergence sources. As we continue to adapt our sources and receivers to the ocean's transfer functions, noise patterns, and targets—false and true—there will be an increasing need for fundamental research into the acoustic properties of materials and for field test of experimental acoustic detection systems.

ISSN:0001-4966    

DOI:10.1121/1.1912922

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

These 20 years have seen much progress in understanding of underwater sound propagation. Few new topics have appeared, but far more complex problems can now be handled—in considerable part, thanks to computers. Emphasis has shifted to lower frequencies, longer ranges, and wave‐theoretical methods. We are still limited by extreme variability of the medium and its boundaries, and by a gap of understanding between mechanical—chemical and acoustical properties. Efforts to narrow this gap should continue, as should the search for “invariants” of problems and for statistical theories to match averaged data.

ISSN:0001-4966    

DOI:10.1121/1.1912923

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The last 20 years have seen a tremendous amount of work in the area of reverberation and scattering. An effort is made to summarize with graphs and charts the amount and nature of this work. Comparisons are made between the results of measurements at sea, measurements with models, and theoretical calculations. Since theoretical solutions of scattering from randomly rough surfaces involve numerous approximations, these different solutions are classified with the aid of tree diagrams so that one can chart a path through the various approximations. Most of the unclassified work on echo structure has been devoted to intensive analyses for targets of relatively simple geometric shape. For example, the sphere and the cylinder (with and without internal structure) still present interesting problems.

ISSN:0001-4966    

DOI:10.1121/1.1912924

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP