Ultrasonic velocities were measured on a block composed of lucite plates with roughened surfaces pressed together with a static normal stress to simulate a fractured medium. The measurements, normal, parallel, and oblique to the fractures, show that for wavelengths much larger than the thickness of an individual plate, the block can be modeled as a particular type of transversely isotropic (TI) medium that depends on four parameters. This TI medium behavior is the same as that of an isotropic solid in which are embedded a set of parallel linear slip interfaces, specified by (1) the excess compliance tangential to the interfaces and (2) the excess compliance normal to the interfaces. At all static stress levels, we inverted the data for the background isotropic medium parameters and the excess compliances. The background parameters obtained were basically independent of stress level and agreed well with the bulk properties of the lucite. The excess compliances decreased with increasing static closing stress, implying that increasing static stress forces asperities on either side of a fracture into greater contact, gradually eliminating the excess compliance that gives rise to the anisotropy. A medium with such planes of excess compliance has been shown, theoretically, to describe the behavior of a medium with long parallel joints, as well as a medium with embedded parallel microcracks.

We develop the concept of frequency dependent effective mass,M (ω), of jammed granular materials which occupy a rigid cavity to a filling fraction of 48%, the remaining volume being air of normal room condition or controlled humidity. The dominant features ofM (ω) provide signatures of the dissipation of acoustic modes, elasticity and aging effects in the granular medium. We perform humidity controlled experiments and interpret the data in terms of a continuum model and a "trap" model of thermally activated capillary bridges at the contact points. The results suggest that attenuation in the granular materials studied here can be influenced significantly by the kinetics of capillary condensation between the asperities at the contacts. 81.05.Rm A distinct feature of jammed or loosely packed granular materials made of a variety of different materials such as sand, steel, polymer or glass is the ability to dissipate acoustic energy through the network of interparticle contacts or viscous dissipation through the surrounding medium. Indeed, loose grains damp acoustic modes very efficiently [1][2][3][4][5] and they are routinely used as an effective method to optimize the damping of unwanted structure-borne acoustic signals [1]. Despite its fundamental importance and practical applications, the microscopic origins of the mechanisms of dissipation in jammed granular materials are still unknown.In this Letter, we pursue the concept of a frequency dependent effective mass,M (ω), of a loose granular aggregate contained within a rigid cavity [5]. The effective massM (ω) is complex valued; its real part reflects the inertial and elastic properties while its imaginary part reflects the dissipative properties of the granular medium. We demonstrate how the features ofM (ω) allow the study of some of the mechanisms of damping of acoustic modes, aging and elasticity in granular matter.Generally speaking, the real part ofM (ω) exhibits a sharp resonance which we interpret in terms of an effective sound speed. The imaginary part ofM (ω) shows a broad resonance peak which quantifies the attenuation of acoustic waves in the system. We observe significant changes in the stiffness and attenuation as a function of humidity. By monitoring the effective mass in time, we find a logarithmic aging effect in the resonance frequency as well as an increase of the damping upon humidification. These effects can be modeled as capillary condensation occurring between the asperities at the contact surfaces between the grains during humidity-dry cycles. We interpret this phenomenon in the context of a "trap model" of thermally activated liquid bridges. Our results suggest that, in the granular materials investigated in the present study, dissipation of acoustic energy is dominated by the asperities at the interparticle contact surfaces. In addition, humidity drastically affects the attenuation of the material through the capillary condensation of liquid bridges.Experiments.-A cylindrical cavity (of diameter D=25.4 mm) excavated in a rigid aluminu...

The quantitative determination of the shape of liquid drops levitated in an ultrasonic standing wave has provided experimental data on the radiation pressure-induced deformations of freely suspended liquids. Within the limits of small deviations from the spherical shape and small drop diameter relative to the acoustic wavelength, an existing approximate theory yields a good agreement with experimental evidence. The data were obtained for millimeter and submillimeter drops levitated in air under 1 g, where g is the sea level gravitational acceleration.

We report a theoretical and experimental investigation into the fundamental physics of why loose granular media are effective deadeners of structure-borne sound. Here we demonstrate that a measurement of the effective mass,M (ω), of the granular medium is a sensitive and direct way to answer the question: What is the specific mechanism whereby acoustic energy is transformed into heat? Specifically, we apply this understanding to the case of the flexural resonances of a rectangular bar with a grain-filled cavity within it. The pore space in the granular medium is air of varying humidity. The dominant features ofM (ω) are a sharp resonance and a broad background, which we analyze within the context of simple models. We find that: a) On a fundamental level, dampening of acoustic modes is dominated by adsorbed films of water at grain-grain contacts, not by global viscous dampening or by attenuation within the grains. b) These systems may be understood, qualitatively, in terms of a height-dependent and diameter-dependent effective sound speed [∼ 100 − 300 (m · s −1 )] and an effective viscosity [∼ 5 × 10 4 Poise]. c) There is an acoustic Janssen effect in the sense that, at any frequency, and depending on the method of sample preparation, approximately one-half of the effective mass is borne by the side walls of the cavity and one-half by the bottom. d) There is a monotonically increasing effect of humidity on the dampening of the fundamental resonance within the granular medium which translates to a non-monotonic, but predictable, variation of dampening within the grain-loaded bar.

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