Volume 126, Issue 2, August 2009
Index of content:
- BIOACOUSTICS 
Numerical and experimental simulation of the effect of long bone fracture healing stages on ultrasound transmission across an idealized fracture126(2009); http://dx.doi.org/10.1121/1.3158938View Description Hide Description
The effect of various stages of fracture healing on the amplitude of 200 kHz ultrasonic waves propagating along cortical bone plates and across an idealized fracture has been modeled numerically and experimentally. A simple, water-filled, transverse fracture was used to simulate the inflammatory stage. Next, a symmetric external callus was added to represent the repair stage, while a callus of reducing size was used to simulate the remodeling stage. The variation in the first arrival signal amplitude across the fracture site was calculated and compared with data for an intact plate in order to calculate the fracture transmission loss (FTL) in decibels. The inclusion of the callus reduced the fracture loss. The most significant changes were calculated to occur from the initial inflammatory phase to the formation of a callus (with the FTL reducing from 6.3 to between 5.5 and 3.5 dB, depending on the properties of the callus) and in the remodeling phase where, after a 50% reduction in the size of the callus, the FTL reduced to between 2.0 and 1.3 dB. Qualitatively, the experimental results follow the model predictions. The change in signal amplitude with callus geometry and elastic properties could potentially be used to monitor the healing process.
126(2009); http://dx.doi.org/10.1121/1.3158924View Description Hide Description
Male frogs and toads call in dense choruses to attract females. Determining the vocal interactions and spatial distribution of the callers is important for understanding acoustic communication in such assemblies. It has so far proved difficult to simultaneously locate and recover the vocalizations of individual callers. Here a microphone-array technique is developed for blindly locating callers using arrival-time delays at the microphones, estimating their steering-vectors, and recovering the calls with a frequency-domain adaptive beamformer. The technique exploits the time-frequency sparseness of the signal space to recover sources even when there are more sources than sensors. The method is tested with data collected from a natural chorus of Gulf Coast toads (Bufo valliceps) and Northern cricket frogs (Acris crepitans). A spatial map of locations accurate to within a few centimeters is constructed, and the individual call waveforms are recovered for nine individual animals within a . These methods work well in low reverberation when there are no reflectors other than the ground. They will require modifications to incorporate multi-path propagation, particularly for the estimation of time-delays.
The effect of altered auditory feedback on control of vocal production in budgerigars (Melopsittacus undulatus)126(2009); http://dx.doi.org/10.1121/1.3158928View Description Hide Description
Budgerigars learn their vocalizations by reference to auditory information and they retain the ability to learn new vocalizations throughout life. Auditory feedback of these vocalizations was manipulated in three experiments by training birds to produce vocalizations while wearing small earphones. Experiments 1 and 2 examined the effect of background noise level (Lombard effect) and the effect of manipulating feedback level from self-produced vocalizations (Fletcher effect), respectively. Results show that birds exhibit both a Lombard effect and a Fletcher effect. Further analysis showed that changes in vocal intensity were accompanied by changes in call fundamental frequency and duration. Experiment 3 tested the effect of delaying or altering auditory feedback during vocal production. Results showed subsequent production of incomplete and distorted calls in both feedback conditions. These distortions included changes in the peak fundamental frequency, amplitude, duration, and spectrotemporal structure of calls. Delayed auditory feedback was most disruptive to subsequent calls when the delay was 25 ms. Longer delays resulted in fewer errors.