Andrés Larraza

Examination of time-reversal acoustics in shallow water and applications to noncoherent underwater communications

The shallow water acoustic communication channel is characterized by strong signal degradation caused by multipath propagation and high spatial and temporal variability of the channel conditions. At the receiver, multipath propagation causes intersymbol interference and is considered the most important of the channel distortions. This paper examines the application of time-reversal acoustic (TRA) arrays, i.e., phase-conjugated arrays (PCAs), that generate a spatio-temporal focus of acoustic energy at the receiver location, eliminating distortions introduced by channel propagation. This technique is self-adaptive and automatically compensates for environmental effects and array imperfections without the need to explicitly characterize the environment. An attempt is made to characterize the influences of a PCA design on its focusing properties with particular attention given to applications in noncoherent underwater acoustic communication systems. Due to the PCA spatial diversity focusing properties, PC arrays may have an important role in an acoustic local area network. Each array is able to simultaneously transmit different messages that will focus only at the destination receiver node.

Experimental studies of applications of time-reversal acoustics to noncoherent underwater communications

The most difficult problem in shallow underwater acoustic communications is considered to be the time-varying multipath propagation because it impacts negatively on data rates. At high data rates the intersymbol interference requires adaptive algorithms on the receiver side that lead to computationally intensive and complex signal processing. A novel technique called time-reversal acoustics (TRA) can environmentally adapt the acoustic propagation effects of a complex medium in order to focus energy at a particular target range and depth. Using TRA, the multipath structure is reduced because all the propagation paths add coherently at the intended target location. This property of time-reversal acoustics suggests a potential application in the field of noncoherent acoustic communications. This work presents results of a tank scale experiment using an algorithm for rapid transmission of binary data in a complex underwater environment with the TRA approach. A simple 15-symbol code provides an example of the simplicity and feasibility of the approach. Covert coding due to the inherent scrambling induced by the environment at points other than the intended receiver is also investigated. The experiments described suggest a high potential in data rate for the time-reversal approach in underwater acoustic communications while keeping the computational complexity low.

Acoustic communications in an enclosure using single-channel time-reversal acoustics

This letter presents experimental results of a scheme for encrypted acoustic communication in the reverberant environment of an enclosure using single-channel time-reversal acoustics. Because time-reversal acoustics in an enclosure can focus a signal within a half wavelength of sound, it possesses natural encryption for points other than the intended receiver. In this way, distinct multiple messages can be sent simultaneously from one source to different locations in the enclosure. The communication scheme is based on multiple broadband signals with identical bandwidth and windowed source spectra.

The force between two parallel rigid plates due to the radiation pressure of broadband noise: An acoustic Casimir effect

Theoretical and experimental results are presented for the force law between two rigid, parallel plates due to the radiation pressure of broadband acoustic noise. The noise is in the band of 5–15 kHz and has an intensity of 133 dB (re: 10−12 W/m2). Excellent quantitative agreement is shown between theory and experiment. These results constitute an acoustic analog for the Casimir effect, which is the force experienced by two closely spaced uncharged conducting plates due to the quantum electromagnetic zero-point field. In contrast to this case, however, band-limited noise can cause the force to be attractive or repulsive as a function of separation between the plates.

Examination of time‐reversal acoustics and applications to underwater communications

Recent experimental work [Roux et al., Appl. Phys. Lett. 70, 1811–1813 (1997); Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)] has shown that the novel technique of time‐reversal acoustics (TRA) can be applied in shallow‐water environments to focus energy back at a source location from a transmitted signal received at a distant receiver array. This refocusing produces a local intensification of the field as well as a convergence of the signal in time. These two aspects of TRA techniques suggest it may be a viable approach for use in underwater communication systems. Because TRA techniques rely on the reconvergence of multipath information into focused arrivals at the original source locations, the influence of multipath spreading is removed. The energy transmitted is then enhanced by the focusing, thereby increasing the SNR. Specifically of interest in this study will be an analysis of how well the signal can be refocused in typical shallow‐water environments and what type of data transfer rates c...

A water wave analog of the Casimir effect

Two rigid plates are vertically suspended by thread such that they are parallel to and opposite each other. The plates are partially submerged in a dish of liquid that is attached to the top of a vertical shake table. When the shake table is driven with noise in a frequency band, random surface waves are parametrically excited, and the plates move toward each other. The reason for this attraction is that the waves carry momentum, and the wave motion between the plates is visibly reduced. The behavior is analogous to the Casimir effect, in which two conducting uncharged parallel plates attract each other due to the zero-point spectrum of electromagnetic radiation. The water wave analog can be readily demonstrated and offers a visual demonstration of a Casimir-type effect. Measurements of the force agree with the water wave theory even at large wave amplitudes, where the theory is expected to break down. The water wave analog applies to side-by-side ships in a rough sea and is distinct from the significant ...

Solitons, pulse splitting, and AM–FM conversion in cylindrical ducts

For propagation above cutoff in an acoustic duct, the dispersion is positive and weakly nonlinear effects are of two kinds. When the wavelength decreases with amplitude, a localized envelope propagates without spreading or distortion and the resultant final state is a soliton. Sloshing modes in a cylindrical duct of finite eccentricity belong to this class. When the wavelength increases with amplitude, a localized envelope splits into two disturbances moving with two different velocities of propagation. As a consequence, if a signal is amplitude modulated at the source, spatial beating between the two disturbances will occur, and at periodic positions in space the signal will become frequency modulated. Axially symmetric cross modes in a cylindrical duct of circular cross section belong to this class. A quantitative analysis of the results of the theory is presented along with its experimental consequences.

Laboratory experiments of time reversed acoustics applied to sonar in shallow water waveguides

Active sonar research at the Naval Postgraduate School Advanced Acoustic Research Laboratory (AARL) has focused on real time adaptation of the active sonar transmit signal. Recent work with Time Reversal Acoustics (TRA) has demonstrated the ability of this approach to adapt in real time to complex environments with little knowledge of the environmental parameters. The present work applies TRA techniques to shallow water active sonar to improve target echo SNR. The approach is to conduct scaled laboratory tank experiments of the TRA sonar and compare results to standard matched filter active sonar processing techniques. Results of laboratory TRA sonar experiments will be presented, to include the high SNR case, the effects of increasing noise levels, and the TRA sonar approach as applied to an active barrier concept.

An acoustic Casimir effect

Theoretical and experimental results are presented for the force law between two rigid parallel plates due to the radiation pressure of broadband acoustic noise. The noise is in the band of 5 to 15 kHz and has an intensity of 133 dB (re: 10−12 W/m2). Good quantitative agreement is shown between theory and experiment. These results constitute an acoustic analog for the Casimir effect, which is the force experienced by two closely spaced uncharged conducting plates due to the quantum electromagnetic zero point field. In contrast to this case, however, band limited noise can cause the force to be attractive or repulsive as a function of the distance of separation of the plates. [Work sponsored by the Office of Naval Research.]

Analysis of a thermoacoustics prime mover above onset of self‐oscillation

Nonlinear effects that lead to amplitude saturation in a thermoacoustic prime mover, are described. The evolution of the acoustic amplitude is described by a homogeneous Ginzburg equation of the form dA/dt=αA−βA2. The linear term represents the contribution from the power output due to the temperature gradient and viscous and thermal losses. The coefficient α is positive above onset. The cubic term is a consequence of the nonlinear induced vorticity at the boundary layer that originates from irreversible terms. In the approximation considered, the coefficient β is positive and a steady state results from the balance between the linear growth and the nonlinear saturation. The steady‐state amplitudes are in qualitative agreement with observations made by Wheatley [Frontiers in Physical Acoustics, Varena (1986)] and Hofler et al. [see abstract in this session]. Observations made by Swift [J. Acoust. Soc. Am. 92, 1551 (1992)] on the dependence of acoustic pressure versus heater power are also in qualitative a...