Acoustic tomography on the basis of travel-time measurement

Abstract

Acoustic images of variable parameters of objects can be reconstructed by means of tomographic techniques which utilize the propagation of sound waves in the investigated medium. The technique described here utilizes sound frequencies in the audio range for acoustic imaging. The temperature-dependent sound speed as well as the flow field can be estimated by measuring the travel time of a defined acoustic signal between a sound source and a receiver when the distance between them is known exactly. The properties of the flow field are reconstructed using reciprocal sound rays to separate the direction-independent Laplace sound speed from the effective sound velocity. The temperatures in the flow field are then calculated by a combined inversion of all travel-time information resulting from the Laplace sound speed using an algebraic reconstruction technique. This reconstruction technique provides a cross section of the temperature distribution throughout the investigated area or volume. The tomographic system has been generalized to allow flow phenomena and temperature fields to be investigated with adapted sampling rates. The technique and procedures are exemplified by means of a scalable (from model-sized up to large-scale outdoor tomography) commercially oriented prototype of a tomograph which can utilize the whole audio and near-audio ultrasonic range. The software technology approach forms an inherent part of the realization.

Tomographic reconstruction of atmospheric turbulence with the use of time-dependent stochastic inversion

“…Most of the algorithms used so far employed a partition of a tomographic volume into grid cells where the values of T and Ṽ are constant, and a subsequent solution of a set of algebraic equations. [3][4][5][6][7][8][9]11,12,14,15 Numerical simulations have shown 12 that these algebraic algorithms can give a good reconstruction of T and Ṽ fields if the number of the grid cells is small enough so that a corresponding inverse problem is overdetermined ͑the number of equations is greater than the number of unknowns͒. However, in two-dimensional ͑2D͒ .…”

Line-averaging measurement methods to estimate the gap in the CO<sub>2</sub> balance closure – possibilities, challenges, and uncertainties

“…The acoustic measurements are controlled by an in-housedeveloped software (MATLAB) which comprises generation of sound signals, control of sound transmission and reception, and subsequent real-time signal analysis. The core hardware (analogue-digital conversion) is an acoustic multichannel spectrometer card (Harmonie PCI octav, sample rate: 51.2 kHz; SINUS Messtechnik GmbH, Germany) which offers eight input and four output channels that are synchronized on a common time basis (Holstein et al, 2004;Barth and Raabe, 2011). This, in turn, is a precondition for accurate travel-time measurements.…”

“…Thus, an uncertainty for travel-time estimation of about 2 µs resulted from sampling (Holstein et al, 2004).…”

Recent progress in acoustic travel-time tomography of the atmospheric surface layer

“…Finally note that scientists from the University of Leipzig have also built a scalable, indoor prototype of their outdoor array for acoustic tomography HOLSTEIN et al, 2004). The horizontal size of the indoor array varied from 1 m to 20 m. The transmitted signal was a pseudo-random noise.…”