Frequency Dependences of Sound Attenuation and Phase Velocity in Suspensions Containing Encapsulated Microbubbles

Frequency dependences of the sound attenuation and phase velocity in an encapsulated bubbly liquid, such as ultrasound contrast agent or Levovist suspensions, at three different concentrations are studied over a bandwidth 1.5–4.5 MHz by using an ultrasonic spectroscopy technique. Measurement of acoustic attenuation spectra demonstrates that the resonant frequency of the Levovist suspension is nearly 2.3–2.5 MHz, and the sound attenuation enhances with the increasing concentration. With the measured sound attenuation spectra, the shear modulus and the shear viscosity are estimated to be 80 Mpa and 1.3 Pa s, respectively. The phase velocity exhibits a rapid rise with frequency smaller than 3.0 MHz, then appears to approach a frequency-independent limit above 3.0 MHz, and the change of the phase velocity over the measured frequency range is also proportional to the concentration.