Manuela Barth

Acoustic tomography on the basis of travel-time measurement

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.

Acoustic tomographic imaging of temperature and flow fields in air

Acoustic travel-time tomography is a remote sensing technique that uses the dependence of sound speed in air on temperature and wind speed along the sound propagation path. Travel-time measurements of acoustic signals between several sound sources and receivers travelling along different paths through a measuring area give information on the spatial distribution of temperature and flow fields within the area. After a separation of the two influences, distributions of temperature and flow can be reconstructed using inverse algorithms. As a remote sensing method, one advantage of acoustic travel-time tomography is its ability to measure temperature and flow field quantities without disturbing the area under investigation due to insertion of sensors. Furthermore, the two quantities—temperature and flow velocity—can be recorded simultaneously with this measurement method. In this paper, an acoustic tomographic measurement system is introduced which is capable of resolving three-dimensional distributions of temperature and flow fields in air within a certain volume (1.3 m × 1.0 m × 1.2 m) using 16 acoustic transmitter–receiver pairs. First, algorithms for the 3D reconstruction of distributions from line-integrated measurements are presented. Moreover, a measuring apparatus is introduced which is suited for educational purposes, for demonstration of the method as well as for indoor investigations. Example measurements within a low-speed wind tunnel with different incident flow situations (e.g. behind bluff bodies) using this system are shown. Visualizations of the flow illustrate the plausibility of the tomographically reconstructed flow structures. Furthermore, alternative individual measurement methods for temperature and flow speed provide comparable results.

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

Acoustic travel-time tomography allows one to reconstruct temperature and wind velocity fields in the atmosphere. In a recently published paper [S. Vecherin et al., J. Acoust. Soc. Am. 119, 2579 (2006)], a time-dependent stochastic inversion (TDSI) was developed for the reconstruction of these fields from travel times of sound propagation between sources and receivers in a tomography array. TDSI accounts for the correlation of temperature and wind velocity fluctuations both in space and time and therefore yields more accurate reconstruction of these fields in comparison with algebraic techniques and regular stochastic inversion. To use TDSI, one needs to estimate spatial-temporal covariance functions of temperature and wind velocity fluctuations. In this paper, these spatial-temporal covariance functions are derived for locally frozen turbulence which is a more general concept than a widely used hypothesis of frozen turbulence. The developed theory is applied to reconstruction of temperature and wind velocity fields in the acoustic tomography experiment carried out by University of Leipzig, Germany. The reconstructed temperature and velocity fields are presented and errors in reconstruction of these fields are studied.

Flow field detection using acoustic travel time tomography

Acoustic travel time tomography uses the dependence of sound speed on temperature and flow properties along the propagation path to measure these parameters. An algorithm is introduced that is capable of resolving the two-dimensional flow field within a specified measuring area. Different flow fields have been simulated to explore the reconstruction properties of the algorithm. Furthermore, an experiment within the Barrel of Ilmenau has been carried out, which demonstrates that the acoustic tomographic method is able to detect flow fields in a closed convection chamber. The simulations reveal that the vector tomographic algorithm can resolve the flow field characteristics speed and direction. Furthermore it is demonstrated that the distribution of sound sources and receivers has to be homogeneous around the measuring site. For the reconstruction of flow distributions, this requirement is even more important compared to temperature reconstructions, since information along inclined sound ray paths within each grid cell are needed to reconstruct the flow vector properly.

Acoustic Tomography of the Atmosphere: Comparison of Different Reconstruction Algorithms

Acoustic travel-time tomography in the atmosphere is based on travel-time measurements of sound signals propagating along different known ray paths through a medium. Because the speed of sound mainly depends on temperature and flow properties, an inversion of these travel times allows an estimation of temperature and wind velocity fields. The main reconstruction techniques for solving such inverse problems are least-squares methods and stochastic inversion algorithms. In this study, five representatives belonging to these types of inverse approaches are evaluated by reconstructions of two-dimensional temperature distributions from synthetically generated and experimental data. The comparison of the reconstruction results reveals several differences between the algorithms concerning spatial resolution of the reconstructed image, accuracy, and computational efficiency. The stochastic approach provides accurate reconstructions of spatially highly resolved temperature fields when the turbulence characteristic is chosen carefully. Nevertheless, the choice of suitable turbulence parameters, the determination of measurement errors prior to an experiment as well as comparatively high memory requirements of this method are unfavorable for real-time analysis of measured data although possible. In contrast, fast and simple on-site interpretations of temperature fields with acceptable accuracy are feasible with least-squares methods.

Remote sensing of temperature and wind using acoustic travel-time measurements

A remote sensing technique to detect area-averaged temperature and flow properties within an area under investigation, utilizing acoustic travel-time measurements, is introduced. This technique uses the dependency of the speed of acoustic signals on the meteorological parameters temperature and wind along the propagation path. The method itself is scalable: It is applicable for investigation areas with an extent of some hundred square metres as well as for small-scale areas in the range of one square metre. Moreover, an arrangement of the acoustic transducers at several height levels makes it possible to determine profiles and gradients of the meteorological quantities. With the help of two examples the potential of this remote sensing technique for simultaneously measuring averaged temperature and flow fields is demonstrated. A comparison of time histories of temperature and wind values derived from acoustic travel-time measurements with point measurements shows a qualitative agreement whereas calculated root-mean-square errors differ for the two example applications. They amount to 1.4 K and 0.3 m/s for transducer distances of 60 m and 0.4 K and 0.2 m/s for transducer distances in the range of one metre.

Tomographic reconstruction of indoor spatial temperature distributions using room impulse responses

Temperature can be estimated by acoustic travel time measurements along known sound paths. By using a multitude of known sound paths in combination with a tomographic reconstruction technique a spatial and temporal resolution of the temperature field can be achieved. Based on it, this article focuses on an experimental method in order to determine the spatially differentiated development of room temperature with only one loudspeaker and one microphone. The theory of geometrical room acoustics is being used to identify sound paths under consideration of reflections. The travel time along a specific sound path is derived from the room impulse response. Temporal variances in room impulse response can be attributed primarily to a change in air temperature and airflow. It is shown that in the absence of airflow a 3D acoustic monitoring of the room temperature can be realized with a fairly limited use of hardware.

Acoustic travel‐time tomography of temperature and wind velocity fields in the atmosphere

Acoustic tomography of the atmosphere allows one to reconstruct (estimate) temperature and wind velocity fields in the atmosphere and to monitor the evolution of these turbulent fields in time. In the present paper, we report on a progress in construction of a state‐of‐the‐art array for acoustic tomography of the atmosphere, which will allow us to measure travel times of sound propagation between different pairs of sources and receivers within a few meters above the ground. Using these travel times, the turbulent fields will then be reconstructed using different inversion algorithms. The array is being built at the Boulder Atmospheric Observatory, CO, in a collaborative effort between several organizations in the U.S. Furthermore, we discuss several inverse algorithms for estimation of the turbulent fields, including recently developed time‐dependent stochastic inversion. Finally, some of these algorithms were used to reconstruct temperature and wind velocity fields in indoor and outdoor tomography experi...

AN EDUCATIONAL MODEL AND TOOLBOX FOR ACOUSTIC TOMOGRAPHIC IMAGING

A model apparatus is presented which has been especially designed for educational purposes and experimental testing of tomographic algorithms. The system is similar to an acoustic tomographic system which has been developed to observe local differences in the sound speed in order to recalculate images of physical parameters which influence the sound speed fields under natural conditions. The system has been successfully used for investigations of atmospheric non-homogeneities (horizontal temperature and wind velocity fields, surface effects) by means of travel-time measurements of acoustical signals. This technique has been extended to a more general environment. The basis is an acoustic multi-channel spectrometer operating in the audio range. The model spectrometer completely controls the acoustical experiment (emission and reception). The modular approach is implemented over several levels including the real-time DSPsoftware, the driver for the acoustic spectrometer and the data-processing concept. The controlling software and the algorithmic part have been developed within the framework of a general toolbox concept for acoustic spectrometers. Fundamental aspects of the signal processing in the tomographic experiment form part of the educational approach. For instance, the influences of various acoustic signatures such as MLS (maximum length sequences) can be demonstrated. The extension of the transportable model enables frequency- and travel-time scaled experiments within the audio and nearultrasonic range.

Energetische Probleme und akustische Verfahren

Im Beitrag werden einige Moglichkeiten vorgestellt, die akustische Verfahren fur energetische Fragestellungen im Bereich denkmalgeschutzter Gebaude, Raume, Wande usw. eroffnen. Im Mittelpunkt stehen akustische Verfahren, die fur Anwendungen im baulichen Bereich geeignet sind – typischerweise sind das Dimensionen mit Ausmasen wie bei Fenstern und Turen sowie von Raumen oder Hallen.