Index of content:
Volume 117, Issue 3, March 2005
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
117(2005); http://dx.doi.org/10.1121/1.1848174View Description Hide Description
An extension of Fikioris and Waterman’s formalism is developed in order to describe both the reflection and transmission from a slab-like fluid region in which elastic cylindrical scatterers are randomly placed. The dispersion equation of the coherent wave inside the slab must be solved numerically. For solid cylinders, there is only one solution corresponding to a mean free path of the coherent wave larger than one wavelength. In that case, the slab region may be described as an effective dissipative fluid medium, and its reflection and transmission coefficients may be formally written as those of a fluid plate. For thin hollow shells, a second solution of the dispersion equation is found, at concentrations large enough for the shells to be coupled via the radiation of a circumferential Scholte–Stoneley Awave on each shell. This occurs at a few resonance frequencies of the shells. At those frequencies, then, two different coherent waves propagate in the slab, and it can no longer be considered a dissipating fluid slab.
117(2005); http://dx.doi.org/10.1121/1.1841631View Description Hide Description
In the food industry and other industries, rheological measurements and determination of particle sizes in suspensions and emulsions is of great importance for process and quality control. Current test cell based ultrasonic methods exist but are often inconvenient. An attractive alternative could be to insert a simple measurement “dipstick” into the fluid; this paper presents an initial study of the feasibility of using measurements of the velocity and attenuation of the quasi-Scholte mode on a plate to obtain the longitudinal velocity and attenuation of an embedding medium. The attenuation of the quasi-Scholte mode is caused by two mechanisms: shear leakage and attenuation due to the bulk longitudinal attenuation of the embedding material. In a calibration test the bulk longitudinal velocity and viscosity of glycerol were determined experimentally. Measurements agreed well with results from conventional methods and literature data. Quantitative results and an independent validation for honey, a very viscous fluid, are also presented. For Newtonian liquids like glycerol and honey, the shear leakage and longitudinal bulk attenuation are both related to viscosity. To demonstrate the sensitivity to nonviscous attenuation mechanisms, qualitative measurement results on fluids that mainly exhibit attenuation due to scattering are presented.
Finite elements methods for modeling the guided waves propagation in structures with weak interfaces117(2005); http://dx.doi.org/10.1121/1.1841731View Description Hide Description
This paper describes two methods using a finite element(FE) code for modeling the effects of weak interfaces on the propagation of low-order Lamb modes. The variable properties of the interfaces are modeled by either a thin layer or a uniform repartition of compression and shear springs that insure the continuity of the stresses and impose a discontinuity in the displacement field. The method is tested by comparison with measurements that were presented in a previous paper [J. Acoust. Soc. Am. 113(6) 3161–3170 (2003)]. The interface was the contact between a rough elastomer with high internal damping loaded against one surface of a glass plate. Both normal and shear stiffnesses of the interface were quantified from the attenuation of and Lamb waves caused by leakage of energy from the plate into the elastomer and measured at each step of a compressive loading. The FE model is made in the frequency domain, thus allowing the viscoelastic properties of the elastomer to be modeled by using complex moduli as input data. By introducing the interface stiffnesses in the code, the predicted guided wavesattenuations are compared to the experimental results to validate the numerical FE methods.
117(2005); http://dx.doi.org/10.1121/1.1857527View Description Hide Description
Polyurethane (PU) and other plastic foams are widely used as passive acoustic absorbers. For optimal design, it is often necessary to know the viscoelasticproperties of these materials in the frequency range relevant to their application. A nonresonance technique (dynamic stiffness) based on a forced vibrations procedure is used to investigate the frequency dependent complex shear modulus of a PU foam. This modulus is first measured, in a quasistatic configuration, in the frequency range (0.016–16 Hz) at different temperatures between 0 and 20 °C. It is afterwards predicted over a wide frequency range (0.01–3000 Hz) using the frequency-temperature superposition principle. The validation of this principle is discussed through quasistatic experiments. Under the assumption that Poisson’s ratio of polymericfoams is real and frequency independent on the frequency range used, the frequency dependence of the complex shear modulus obtained is used to predict the complex stiffness of the acoustic foam on a wide frequency range.
117(2005); http://dx.doi.org/10.1121/1.1858171View Description Hide Description
The application of a slightly curved reflector to increase the amplitude of an ultrasonic standing wave in a semi-infinite rectangular channel was explored. Air was assumed to be the acoustic medium in the channel. Excitation of the standing wave was assumed to be provided by a square transducer flush-mounted to one wall of the channel. A slight curvature was placed in the reflecting wall of the channel. A finite element analysis was used to predict the amplitude of the standing wave that would be excited in the channel. A perfectly matched layer was used to model the semi-infinite channel geometry. At frequencies near 50 kHz, for source ranging from 6.6 to 26.6, and channel depths necessary to excite standing waves at one-half and one wavelength resonance, the computations predicted that an increase in acoustic pressure amplitude from 2 to 11 dB could be achieved with a reflector whose depth of curvature was 16% of the channel depth. Much of this increase could be obtained with curvatures of smaller depth. Experiments with a channel and reflector of representative geometry gave a measured increase in acoustic pressure amplitude of 4.86 dB.