Biomedical subjects

D R Dowling

Publications and source records attributed to D R Dowling.

4 recordsLinked to original sources

PubMed2001

Time-reversing array retrofocusing in noisy environments.

Acoustic time reversal is a robust means of retrofocusing acoustic energy, in both time and space, to the original sound-source location. However, noise may limit the performance of a time-reversing array (TRA) at long source-array ranges, or when the original-source or TRA-element power levels are low. The operation of a TRA requires two steps (reception and transmission) so both TRA-broadcast noise and ambient noise must be taken into account. In this paper, predictions are made for how a simple omnidirectional noise field influences the probability that the signal amplitude from a narrow-band TRA will exceed the noise at the TRA's retrofocus. A general formulation for the probability of TRA retrofocusing, which can be used for TRA design, is developed that includes: the variance of the noise field, the original source strength, the TRA's element output power, the number of TRA elements (N), and the propagation characteristics of the environment. This formulation predicts that a TRA's array gain (in dB) at the retrofocus may be as high as + 10log10(N) to + 20 log10(N) depending on the relative strengths of the original source and the TRA's elements. Monte Carlo simulations in both a free-space environment and a shallow-ocean sound-channel environment compare well to this probability formulation even when simple approximate parametric relationships for the appropriate Green's functions are used. The dominant deviation between theory and simulation in the sound channel is caused by acoustic absorption.

Environment↗

PubMed2001

Computed narrow-band azimuthal time-reversing array retrofocusing in shallow water.

The process of acoustic time reversal sends sound waves back to their point of origin in reciprocal acoustic environments even when the acoustic environment is unknown. The properties of the time-reversed field commonly depend on the frequency of the original signal, the characteristics of the acoustic environment, and the configuration of the time-reversing transducer array (TRA). In particular, vertical TRAs are predicted to produce horizontally confined foci in environments containing random volume refraction. This article validates and extends this prediction to shallow water environments via monochromatic Monte Carlo propagation simulations (based on parabolic equation computations using RAM). The computational results determine the azimuthal extent of a TRA's retrofocus in shallow-water sound channels either having random bottom roughness or containing random internal-wave-induced sound speed fluctuations. In both cases, randomness in the environment may reduce the predicted azimuthal angular width of the vertical TRA retrofocus to as little as several degrees (compared to 360 degrees for uniform environments) for source-array ranges from 5 to 20 km at frequencies from 500 Hz to 2 kHz. For both types of randomness, power law scalings are found to collapse the calculated azimuthal retrofocus widths for shallow sources over a variety of acoustic frequencies, source-array ranges, water column depths, and random fluctuation amplitudes and correlation scales. Comparisons are made between retrofocusing on shallow and deep sources, and in strongly and mildly absorbing environments.

Journal Article↗

PubMed2000

Computed narrow-band time-reversing array retrofocusing in a dynamic shallow ocean.

A time-reversing array (TRA) can retrofocus acoustic energy, in both time and space, to the original sound-source location without any environmental information. This unique capability may be degraded in time-dependent or noisy acoustic environments, or when propagation losses are prevalent. In this paper, monochromatic propagation simulations (based on the parabolic equation code, RAM) are used to predict TRA retrofocusing performance in shallow-water sound channels having characteristics similar to those measured during the recent SWARM (shallow-water acoustics in a random medium) experiment. Results for the influence of source-array range, source depth, acoustic frequency, bottom absorption, internal wave strength, and round-trip time delay are presented. For a fixed channel geometry, higher frequencies, deeper sources, and lower bottom absorption improve TRA performance and allow retrofocusing at longer ranges. In a dynamic shallow-water channel containing a random superposition of linear internal waves, the size of the retrofocus is slightly decreased and sidelobes are suppressed compared to the static channel results. These improvements last for approximately 1 to 2 min for source-array ranges near 10 km at a frequency of 500 Hz. For longer time delays, the internal waves cause significant TRA retrofocus amplitude decay, and the decay rate increases with increasing internal wave activity and acoustic frequency.

Electronic Data Processing↗

PubMed1999

Photoacoustic detection and localization of small gas leaks.

Leak detection and localization are critical manufacturing quality-control processes. Many industrial and domestic machines use or convey pressurized gases or liquids. Unintended leaks from machine components may be detrimental to consumers, manufacturers, and the environment. This paper describes a leak detection technique based on photoacoustic sounds produced by the interaction of a carbon dioxide (CO2) laser tuned to 10.6 micrometers and a photoactive tracer gas, sulfur hexaflouride (SF6), emitted by calibrated leak sources. Acoustic signals generated by a high-speed scan of the laser beam through the cloud of tracer gas formed near the leak are recorded in a bandwidth from 3 to 52 kHz by multiple microphones. From the recorded signals, the presence or absence of a leak may be deduced by comparison with the background noise level at the signal frequencies, which occur at the harmonics of the scan rate. When a leak is present, its location is determined from a simple model of the acoustic environment and matched field processing (MFP). Current results show that a gas leak of 1 cm3 per day can be detected and localized to within +/- 3 mm in a few seconds using four microphones, placed 0.41 m from the leak location, and an incoherent average of the MFP ambiguity surfaces at eight signal frequencies. Comparisons of the Bartlett and minimum-variance-distortionless matched field processors are also presented.

Acoustics↗