Volume 6, Issue 4, October 2005
6(2005); http://dx.doi.org/10.1121/1.1989785View Description Hide Description
The minimum variance (MV) criterion is widely used for the weight vector estimation of the adaptive microphone array. The drawback of this criterion is the cancellation of the desired speech signal and its degradation in multipath wave propagation environment. Applying the maximum signal to interference ratio (MSIR) instead of the MV criterion has two benefits. The first one is the high suppression of the interferences and the second one the desired speech enhancement. The proposed MSIR criterion is applied to the new generalized eigenvalue based beamformer (GEVBF). Its superiority is experimentally proved by simulating a room with reverberation.
Big brown bats and June beetles: Multiple pursuit strategies in a seasonal acoustic predator–prey system6(2005); http://dx.doi.org/10.1121/1.1985957View Description Hide Description
Infrared video and ultrasonic recordings were made of echolocating big brown bats hunting for June beetles in the short May–June “beetle season.” Adult beetles emerge from the ground at twilight, beat their wings to warm up, and fly into nearby trees, where they swarm actively for ∼1 h. Beetles are attacked by bats while buzzing in the grass, while flying in the open during their brief ascent, and while still active among the leaves of trees. Bats routinely used different capture strategies during the course of a few minutes and may hear the beetles’ buzzing to guide attacks in vegetation.
6(2005); http://dx.doi.org/10.1121/1.1985956View Description Hide Description
Repeated measures of low-frequency underwater hearing sensitivity in individuals of three pinniped species tested over 4–7 years are presented. Despite changes in the experience of the subjects and certain testing parameters (e.g., equipment and research personnel), measured underwater hearing thresholds within subjects over relatively long periods of time were quite similar at the frequencies tested (0.2–6.4 kHz). These data address the reliability of acoustic signal detectionmeasurements over time in pinnipeds using psychophysical techniques. They are also relevant in considering the cumulative effects of aging, experience, and noise exposure on pinniped hearing in certain frequency bands.
Measurement of the Doppler shift in forward-scattered waves caused by moderate sea surface motion in shallow waters6(2005); http://dx.doi.org/10.1121/1.1989747View Description Hide Description
A study of the impulse response of the acoustic channel in shallow waters is presented with respect to space, time, and frequency shift over a time window of two hours. A broadband chirp (42–54 kHz) and a narrow band sine wave (58 kHz) are transmitted from a static source located at 51 and 166 m from a vertical line receiver array. In 20 m of water with 0.4 m of wave height, an average Doppler shift of 20 Hz is measured at 51 m range, and 10 Hz at 166 m range, due to the sea-surface motion.
6(2005); http://dx.doi.org/10.1121/1.2046567View Description Hide Description
Many species of bats use frequency-modulated (FM) “chirps” to sense their environment by sonar. A method was developed for locating the chirps of individual bats while simultaneously tracking the positions of multiple bats in infrared videos. Three microphones and one infrared camera recorded multiple bats flying above a small open pool. Azimuth and elevation was computed for all chirps in a given 15-s interval and matched to position coordinates given by the infrared camera. By grouping spatially continuous sets of chirps, each individual bat's chirp pattern can be obtained. This combination of visual and acousticinformation will provide new insight into the behavior and sonar capabilities of FM ranging bats.
Fluctuations in sound transmission through sound speed profiles that are periodic in range and geotime6(2005); http://dx.doi.org/10.1121/1.2033087View Description Hide Description
Acoustic propagation through sound speed profiles (SSPs) that are periodic in range and geotime is studied using the parabolic equationmodel and the WKB/adiabatic model. Fluctuations are shown to be significantly reduced when acoustic propagation is through a periodic range-dependent SSP, compared to a range-independent SSP. For a fixed source-receiver range, the number of tidal wavelengths contained within the range is an important factor that determines the fluctuations. The relationship of this factor to the direction of transmission relative to the tidal direction is presented. A useful criterion for predicting the extent of fluctuations under different environments has been developed by examining the range integral of the horizontal wave number for each mode.
6(2005); http://dx.doi.org/10.1121/1.2033088View Description Hide Description
Plane waves and the Pochhammer-Chree solutions are both used to describe longitudinal axially symmetric wave propagation in solid cylinders. For interpreting experiments, plane waves provide a physical explanation for signals with multiple distinct arrivals. For signals without distinct arrivals the Pochhammer-Chree solutions are often used for interpretation, but the complexity of the solutions makes accurate interpretations difficult. This paper discusses some previous misinterpretations and shows how the Pochhammer-Chree solutions relate to signals with and without distinct arrivals by considering more than just the dispersion curves from the solutions.
6(2005); http://dx.doi.org/10.1121/1.2006247View Description Hide Description
Estimates of range and depth resolution are required in order to determine computational grid spacings for matched field processing (MFP). In this paper it is shown how previous results for range-independent waveguides can be extended to account for range dependence by using standard adiabatic formulations. Significant differences in MFP resolution are observed for up-slope versus down-slope propagation due to changes in the path-integrated attenuation loss. MFP resolution is set by multipath interference lengths, so resolution estimates also relate to expected spatial scales for transmission loss fading.
6(2005); http://dx.doi.org/10.1121/1.2127115View Description Hide Description
Measurements using a fiber-optic probe hydrophone, high-speed camera, and B-mode ultrasound showed attenuation of the trailing negative-pressure phase of a lithotripter shock pulse under conditions that favor generation of cavitation bubbles, such as in water with a high content of dissolved gas or at high pulse repetition rate where more cavitation nuclei persisted between pulses. This cavitation-mediated attenuation of the acoustic pulse was also observed to increase with increasing amplitude of source discharge potential, such that the negative-pressure phase of the pulse can remain fixed in amplitude even with increasing source discharge potential.
6(2005); http://dx.doi.org/10.1121/1.2127116View Description Hide Description
A shallow-water ocean acoustics experiment is proposed from which it is possible to determine in principle the scattering data necessary to recover the sound-speed profile in the ocean bottom. These data are a “reflection coefficient,” which is not the usual one. The reflection coefficient is amplitude of the outgoing wave at infinite depth due to a unit amplitude plane wave incident from infinite depth. The unusual fact is that this reflection coefficient can be recovered from a measurement in the ocean. The essential assumptions are that the sound-speed profile is known in the ocean but unknown in the bottom, and that the measurements are made in the ocean layer.