We present a model applicable to ultrasound contrast agent bubbles that takes into account the physical properties of a lipid monolayer coating on a gas microbubble. Three parameters describe the properties of the shell: a buckling radius, the compressibility of the shell, and a break-up shell tension. The model presents an original non-linear behavior at large amplitude oscillations, termed compression-only, induced by the buckling of the lipid monolayer. This prediction is validated by experimental recordings with the high-speed camera Brandaris 128, operated at several millions of frames per second. The effect of aging, or the resultant of repeated acoustic pressure pulses on bubbles, is predicted by the model. It corrects a flaw in the shell elasticity term previously used in the dynamical equation for coated bubbles. The break-up is modeled by a critical shell tension above which gas is directly exposed to water.

Chair's Introduction-7:30 Invited Papers 7:355aAA1. Why we should design for optimum reverberation times in rooms for speech communication. J. S. Bradley ͑Inst. for Res. in Construction, Natl. Res. Council, Montreal Rd., Ottawa, Canada K1A 0R6͒Various texts list recommendations for optimum reverberation times as ideal goals in rooms for speech communication. However, some newer documents, such as ANSI S12.60, talk about maximum recommended reverberation times in rooms for speech communication. The source of the changed approach can be traced to interpretations of experimental results that do not consider the full implications for speech communication in real rooms, and also to the fact that reverberation time is not an ideal predictor of the quality of speech communication. In the extreme, minimizing reverberation times would lead to near anechoic rooms for speech and inadequate signal-to-noise ratios. The need for optimum reverberation times can be explained as a simple need to first achieve adequate signal-to-noise ratios. However, a more complete understanding is obtained by examining the benefits of early-arriving reflections of speech sounds on the intelligibility of the speech to listeners. Attempts to determine optimum reverberation times for normal hearing listeners, which are based on a balance between avoiding excessive reverberation and maintaining adequate signalto-noise ratios, lead to a range of acceptable values that can vary with the ambient noise level. This discussion will be supported with examples from room acoustics measurements in classrooms and meeting rooms. 7:555aAA2. The relation between speech transmission index, clarity, and reverberation time and listening difficulty in the impulse response database of AIJ. Hiroshi Sato ͑Natl. Inst. of Adv. Industrial Sci. and Technol. ͑AIST͒, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan͒, Yoshio Nishikawa ͑Konoike Co., Ltd., Tsukuba 305-0003, Japan͒, Hayato Sato, and Masayuki Morimoto ͑Kobe Univ., Nada, Kobe 657-8501, Japan͒The Speech Communication Research Working Group of AIJ ͑Architectural Institute of Japan͒ is collecting information on rooms including digitized impulse responses ͑IRs͒ to establish a database for evaluating and designing the speech transmission quality of rooms. This database consists of 966 measured IRs. This study presents the relationships between speech transmission index ͑STI͒, clarity ͑Cx͒ and reverberation time ͑T͒ as measures to consider for the design and evaluation of the speech transmission performance of rooms. The data show a wide range of STI and Cx values for a given T, and the minimum STI at each reverberation time can be obtained by diffused field theory. STI and Cx are seen to be better indicators than T for the design of rooms for speech and that T is not as good. Relationships between STI, Cx, and listening difficulty ratings from previous studies ͓Proc. of RADS ͑2004͒, Proc. of Forum Acusticum, pp. 1713-1718 ͑2005͔͒ found both STI and Cx can be used as predictors of listening difficulty ratings. Finally, ...

An ultrasonically driven air bubble can become shape-unstable through a parametric instability. Here, we report time-resolved optical observations of shape oscillations (mode n=2 to 6) of micron-sized single air bubbles for a range of acoustic pressures. The observed mode number n was found to be linearly related to the resting radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which as expected is minimum at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small, nonspherical linear perturbation into a Rayleigh-Plesset-type equation model which includes a physical thermal damping mechanism describing heat and mass transport in the thin boundary layer at the bubble-to-water interface. Indeed, a parametric instability is responsible for the shape oscillations, and the Rayleigh-Plesset-type equation captures the experimental observations in great detail.

We investigate an excitation approach for contrast agents based on chirps. This technique, named chirp reversal, consists in transmitting an up sweep frequency chirp (UPF) followed by a down sweep frequency chirp (DNF). Simulations using a modified Rayleigh-Plesset equation were carried out. Chirps with center frequencies from 1.4 MHz to 2 MHz, pressures from 50 kPa to 200 kPa and frequency bandwidths from 30% to 65% were considered. High speed optical observations and acoustical measurements were performed using individual contrast bubbles of radii from 1 μm to 5 μm and a diluted solution of contrast agent respectively. Simulations showed differences between bubbles' oscillations following UPF and DNF chirps in terms of amplitude and duration. Maximal differences occurred for bubbles that were around 80% and 140% of the resonance size. Bubbles at resonance or far away from resonance provided identical responses to UPF and DNF chirps. Larger bandwidths and higher acoustic pressures accentuate further the difference between the UPF and DNF responses. These findings were confirmed through optical data and acoustical measurements. The results reveal the potential of chirp reversal for contrast agent detection.

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