Volume 105, Issue 3, March 1999
Index of content:
- BIOACOUSTICS 
105(1999); http://dx.doi.org/10.1121/1.426732View Description Hide Description
Encapsulated types of contrast agents possess a specific acoustical signature. When the applied acoustic pressure exceeds a specific threshold, the scattering level increases abruptly for a short time. A “dualistic” character of the encapsulated gas bubbles explains this signature, observed for Quantison™ (air bubbles encapsulated by a shell of human albumin). For acoustic pressures below a threshold, the bubbles act as encapsulated gas bubbles and are stable linear or nonlinear scatterers, depending on the applied acoustic pressure. For acoustic pressures above the threshold, the bubbles rupture and release the contained gas, subsequently acting as free-gas bubbles. The effect is transient and lasts until the released free-gas bubbles are dissolved in the surrounding liquid. This explanation was investigated experimentally and evaluated by theoretical models. A 15–20-dB increase in scattering, the appearance of higher harmonics, and a finite duration of the effect could be measured and agreed with corresponding theory. Therefore, ultrasound in combination with this dualistic character suggests that encapsulated gas bubbles can be construed as a robust vehicle for localized delivery of free-gas bubbles, the ultimate ultrasound contrast agent.
Shock wave–inertial microbubble interaction: Methodology, physical characterization, and bioeffect study105(1999); http://dx.doi.org/10.1121/1.426733View Description Hide Description
A method of generating in situ shock wave–inertial microbubbleinteraction by a modified electrohydraulic shock wave lithotripter is proposed and tested in vitro. An annular brass ellipsoidal reflector that can be mounted on the aperture rim of a Dornier XL-1 lithotripter was designed and fabricated. This ring reflector shares the same foci with the XL-1 reflector, but is 15 mm short in major axis. Thus, a small portion of the spherical shock wave, generated by a spark discharge at the first focus of the reflector, is reflected and diffracted by the ring reflector, producing a weak shock wave approximately 8.5 μs in front of the lithotripter pulse. Based on the configuration of the ring reflector (different combinations of six identical segments), the peak negative pressure of the preceding weak shock wave at the second focus can be adjusted from −0.96 to −1.91 MPa, at an output voltage of 25 kV. The preceding shock wave induces inertial microbubbles, most of which expand to a maximum size of 100–200 μm, with a few expanding up to 400 μm before being collapsed in situ by the ensuing lithotripter pulse. Physical characterizations utilizing polyvinylidene difluoride (PVDF) membranehydrophone, high-speed shadowgraph imaging, and passive cavitation detection have shown strong secondary shock wave emission immediately following the propagating lithotripter shock front, and microjet formation along the wave propagation direction. Using the modified reflector, injury to mouse lymphoid cells is significantly increased at high exposure (up to 50% with shock number >100). With optimal pulse combination, the maximum efficiency of shock wave-induced membrane permeabilization can be enhanced substantially (up to 91%), achieved at a low exposure of 50 shocks. These results suggest that shock wave–inertial microbubbleinteraction may be used selectively to either enhance the efficiency of shock wave-mediated macromolecule delivery at low exposure or tissue destruction at high exposure.
105(1999); http://dx.doi.org/10.1121/1.426734View Description Hide Description
Budgerigars are small Australian parrots that learn new vocalizations throughout adulthood. Earlier work has shown that an external acoustic model and auditory feedback are necessary for the development of normal contact calls in this species. Here, the role of auditory feedback in the maintenance of species-typical contact calls and warble song in adult budgerigars is documented. Deafened adult birds (five male, one female) vocalized less frequently and showed both suprasegmental and segmental changes in their contact calls and warble song. Contact calls of all adult-deafened budgerigars showed abnormalities in acoustic structure within days to a few weeks following surgery. Within 6 months of surgery, nearly all contact calls produced by deafened birds were strikingly abnormal, showing highly variable patterns of frequency modulation and duration. The warble song of deafened male budgerigars also differed significantly from that of normal budgerigars on several acoustic measures. These results show that auditory feedback is necessary for the maintenance of a normal, species-typical vocal repertoire in budgerigars.
105(1999); http://dx.doi.org/10.1121/1.426735View Description Hide Description
Although the mammalian larynx exhibits little structural variation compared to sound-producingorgans in other taxa (birds or insects), there are some morphological features which could lead to significant differences in acoustic functioning, such as air sacs and vocal membranes. The vocal membrane (or “vocal lip”) is a thin upward extension of the vocal fold that is present in many bat and primate species. The vocal membrane was modeled as an additional geometrical element in a two-mass model of the larynx. It was found that vocal membranes of an optimal angle and length can substantially lower the subglottal pressure at which phonation is supported, thus increasing vocal efficiency, and that this effect is most pronounced at high frequencies. The implications of this finding are discussed for animals such as bats and primates which are able to produce loud, high-pitched calls. Modeling efforts such as this provide guidance for future empirical investigations of vocal membrane structure and function, can provide insight into the mechanisms of animal communication, and could potentially lead to better understanding of human clinical disorders such as sulcus vocalis.
Detection of modulation in spectral envelopes and linear-rippled noises by budgerigars (Melopsittacus undulatus)105(1999); http://dx.doi.org/10.1121/1.426736View Description Hide Description
Budgerigars were trained to discriminate complex sounds with two different types of spectral profiles from flat-spectrum, wideband noise. In one case, complex sounds with a sinusoidal ripple in (log) amplitude across (log) frequency bandwidth were generated by combining 201 logarithmically spaced tones covering the frequency region from 500 Hz to 10 kHz. A second type of rippled stimulus was generated by delaying broadband noise and adding it to the original noise in an iterative fashion. In each case, thresholds for modulation depth (i.e., peak-to-valley in dB) were measured at several different ripple frequencies (i.e., cycles/octave for logarithmic profiles) or different repetition pitches (i.e., delay for ripple noises). Budgerigars were similar to humans in detecting ripple at low spatial frequencies, but were considerably more sensitive than humans in detecting ripples in log ripple spectra at high spatial frequencies. Budgerigars were also similar to humans in detecting linear ripple in broadband noise over a wide range of repetition pitches. Taken together, these data show that the avian auditory system is at least as good, if not better, than the human auditory system at detectingspectral ripples in noise despite gross anatomical differences in both the peripheral and central auditory nervous systems.