Index of content:
Volume 125, Issue 5, May 2009
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
125(2009); http://dx.doi.org/10.1121/1.3106129View Description Hide Description
An ultrasonic technique for estimating viscoelastic properties of hydrogels, including engineered biological tissues, is being developed. An acoustic radiation force is applied to deform the gel locally while Doppler pulses track the induced movement. The system efficiently couples radiation force to the medium through an embedded scattering sphere. A single-element, spherically-focused, circular piston element transmits a continuous-wave burst to suddenly apply and remove a radiation force to the sphere. Simultaneously, a linear array and spectral Doppler technique are applied to track the position of the sphere over time. The complex shear modulus of the gel was estimated by applying a harmonic oscillator model to measurements of time-varying sphere displacement. Assuming that the stress-strain response of the surrounding gel is linear, this model yields an impulse response function for the gel system that may be used to estimate material properties for other load functions. The method is designed to explore the force-frequency landscape of cell-matrix viscoelasticity. Reported measurements of the shear modulus of gelatin gels at two concentrations are in close agreement with independent rheometer measurements of the same gels. Accurate modulus measurements require that the rate of Doppler-pulse transmission be matched to a priori estimates of gel properties.
125(2009); http://dx.doi.org/10.1121/1.3097767View Description Hide Description
The theory of thermoacoustic mixture separation is extended to include the effect of a nonzero axial temperature gradient. The analysis yields a new term in the second-order mole flux that is proportional to the temperature gradient and to the square of the volumetric velocity and is independent of the phasing of the wave. Because of this new term, thermoacoustic separation stops at a critical temperature gradient and changes direction above that gradient. For a traveling wave, this gradient is somewhat higher than that predicted by a simple four-step model. An experiment tests the theory for temperature gradients from 0 to 416 K/m in 50–50 He–Ar mixtures.
125(2009); http://dx.doi.org/10.1121/1.3106125View Description Hide Description
This research investigates the influence of partial reflection on the measurement of the absolute ultrasonic attenuation coefficient using contact transducers. The partial, frequency-dependent reflection arises from the thin fluid-layer interface formed between the transducer and specimen surface. It is experimentally shown that neglecting this reflection effect leads to a significant overestimation in the measured attenuation coefficient. A systematic measurement procedure is proposed that simultaneously obtains the ultrasonic signals needed to calculate both the reflection coefficient of the interface and the attenuation coefficient, without disturbing the existing coupling conditions. The true attenuation coefficient includes a correction based on the measured reflection coefficient—this is called the reflection correction. It is shown that including the reflection correction also reduces the variation (random error) in the measured attenuation coefficient. The accuracy of the proposed method is demonstrated for a material with a known attenuation coefficient. The proposed method is then used to measure the high attenuation coefficient of a cement-based material.