摘要:

Experiments are reported which pertain to the phenomenon of enhanced sonic booms or superbooms generated as a result of atmospheric refraction in threshold‐Mach‐number flights. Such ’’refraction superbooms’’ are created in a ballistic range by firing projectiles at low supersonic speeds into stratified medium of varying sound speed consisting of an inhomogeneous mixture of CO2and air. The pressure signature recorded by a fast‐response transducer inserted in the flow field is studied in conjunction with sets of dual‐schlieren pictures obtained simultaneously during each run. The flow field observations indicate that the generated shocks are reflected near the sonic cutoff elevation, where the local sound speed equals the projectile speed, provided that such an elevation exists. The incident shock and its reflected portion join to form a Y‐shaped configuration near the cutoff region. Enhanced shock overpressures are detected near the ’’critical altitude’’ where the flow speed behind the shock becomes sonic. A maximum focus factor of 1.7 has been measured for the leading shock which, with some theoretical assumptions, leads to an estimated value of 1.3 for the constant which appears in the Guiraud–Thery scaling law. The same value is also deduced from field data taken during aircraft threshold Mach‐number flight by Haglund and Kane. The substantial agreement between the two experimentally derived values of the constant and the theoretical estimate of 1.4 given by Thery, Lecomte, and Reggiani seems to suggest the validity of the above‐mentioned scaling law. However, the discrepancy of these numbers with the value of 2.6 derived by Gill and Seebass indicates that further investigation is still needed.Subject Classification: [43]25.20, [43]25.30; [43]28.55; [43]50.55.

ISSN:0001-4966    

DOI:10.1121/1.380907

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Ballistic range experiments for the study of the propagation of converging shocks are described and the similarity between the observed phenomenon and that expected for superbooms created by accelerating supersonic aircraft is discussed. For weak shocks (shock Mach numbers of about 1.03), a structure resembling that of a folded shock predicted by geometrical acoustics theory is observed while for stronger shocks, a concave front with enhanced overpressure is recorded. Other results are in general accord with the basic concepts of shock propagation and in conjunction with some theoretical scaling laws indicate that the peak magnification of sonic booms due to aircraft flight acceleration in the real atmosphere should be in the range of 6 to 13.Subject Classification: [43]25.20, [43]25.30; [43]28.55; [43]50.55.

ISSN:0001-4966    

DOI:10.1121/1.380908

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

In a previous paper, the author examined the bearing response pattern of a line array to a plane wave in a random medium. It was found that random effects are strongest when the source is broadside to the array. In this paper, the corresponding situation for a cylindrical wave is examined. A comparison is made with the plane‐wave case.Subject Classification: [43]30.20, [43]30.40; [43]45.10; [43]20.15; [43]30.82.

ISSN:0001-4966    

DOI:10.1121/1.380909

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Recently published data on the attenuation of compressional (sound) waves in marine sediments and sedimentary rocks as a function of frequency and sediment porosity have been added to previously published figures. The revised figure illustrating attenuation as a function of frequency (5 Hz–1 MHz), and attendant discussion, continue to support the conclusion that attenuation is approximately related to the first power of frequency in sands, muds, and sedimentary rocks. The revised figure illustrating sediment porosity versus attenuation affirms that sediment porosity is an important index property which can be used to predict sound attenuation in surficial sediments. Data were collected and illustrated on sound attenuation as a function of depth in the sea floor. It is concluded that attenuation decreases with about the −1/6 power of depth in sands. As a silt–clay sediment (mud), or turbidite, is placed under increasing overburden pressure, there may be a progressive increase in attenuation due to reduction in sediment porosity, and a progressive decrease in attenuation due to increasing pressure on the sediment mineral frame. At about 200‐m depth a null point may be reached. Thereafter, pressure is the dominant effect, and attenuation decreases smoothly with depth and overburden pressure. The figure can be used to aid prediction of sound attenuation in sediment and rock layers in the sea floor.Subject Classification: [43]30.20.

ISSN:0001-4966    

DOI:10.1121/1.380910

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A major purpose of this paper is to investigate the influence of mean sound‐speed structures on sound transmission in the presence of a single‐frequency internal wave. A cw signal is transmitted through a shallow ocean over a range that is small compared to the wavelength of the internal wave. A constant value of the Brunt–Vaisala frequency is assumed, and this value is taken as a parameter of the model. The total field associated with refracted/bottom‐reflected rays is studied, and the effect of the internal wave on total‐field phase and transmission loss is examined. Then, the maximum variation of the phase is investigated for different mean sound‐speed structures and internal‐wave amplitudes. This variation shows a general downward trend, as bottom sound speed increases, and an oscillatory behavior possibly due to rapid changes in the rate of change of phase with respect to mid‐depth sound speed. A simple mathematical model is constructed to explain the dominant decreasing trend. Phase variation is shown to vary linearly with bottom sound speed and to be proportional to wave amplitude, range, and acoustic frequency. It is inversely proportional to ocean depth.Subject Classification: [43]30.20, [43]30.25.

ISSN:0001-4966    

DOI:10.1121/1.380911

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Calculations are made of the pulse shapes for sound waves reflected from a plane transducer. These show the effect of reflections from the two parallel surfaces and their relation to pulse width and transducer thickness. Illustrations are given for several representative combinations of samples and backing media, together with photographs of pulse shapes obtained experimentally.Subject Classification: [43]35.80; [43]85.40; [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.380912

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The isothermal response of a bounded, three‐dimensional viscoelastic body, subject to arbitrary body forces, initial conditions, and boundary conditions, is considered within the linear theory of viscoelasticity. The viscoelastic properties of the body are assumed to be isotropic and homogeneous. The boundary surface areas over which the boundary conditions are specified are assumed to remain constant with time. The response is first found formally in terms of a causal Green’s function. It is then shown that when Poisson’s ratio is constant, the causal Green’s function can be expanded in a series of spatial eigenfunctions of an associated elastic eigenvalue problem. The resulting solution for the general problem is an eigenfunction series with Laplace transformed time‐dependent coefficients. The solution is inverted for two special cases, when the body forces and boundary conditions are separable in space and time, and when the relaxation function in simple shear has the form of a series of decaying exponentials.Subject Classification: [43]40.20; [43]35.50.

ISSN:0001-4966    

DOI:10.1121/1.380898

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Longitudinal pulse propagation in slightly curved elastic rods is discussed in terms of an elementary theory due to Hsieh and Lee for which rotary inertia and radial shear are neglected. Moreover, the wavelength of motion is sufficiently long for practically all bending effects to be neglected. Three different approaches are presented. In the first, reduction of the Fourier transform of the governing equation to Bessel’s equation is sought. In the second, generalized Baecklund‐type transformations are utilized to reduce the characterizing equation to the conventional wave equation for certain radius of curvature variations. Finally, the formal Karal–Keller technique is used to solve certain impact problems. Numerical results are presented in graphical form.Subject Classification: [43]40.22; [43]20.15, [43]20.20, [43]20.45.

ISSN:0001-4966    

DOI:10.1121/1.380899

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

In a study of ways to reduce the sound power produced by propeller fans, the performance of various porous blades was compared with that of solid blades. It was found that a porous‐bladed fan could produce flow with characteristics similar to that produced with a solid‐bladed fan, with a reduction of about 5 dB in the sound at the cost of a slight reduction in static efficiency.Subject Classification: [43]50.30, [43]50.40, [43]50.45.

ISSN:0001-4966    

DOI:10.1121/1.380900

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The diffuse sound field as used in standard measurements in reverberant enclosures has been analyzed in terms of two different models, the eigenmode and the free‐wave models. Recently the question has been raised as to the extent to which these models are consistent. To investigate these matters, the energy density as a function of position is calculated for axial, tangential, and oblique modes in a rectangular enclosure. The results show that, for a sound field consisting of several overlapping oblique modes, excited by a narrow band of noise, increases of mean sound pressure levels at the walls, edges, and corners of the enclosure are predicted that are in agreement with the results of the free‐wave analysis and experiment. It is concluded that the two models are consistent. The inplications of the models, regarding the spatial uniformity of energy, are discussed.Subject Classification: [43]55.20; [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.380901

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP