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.

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 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 ͑2006͔͒, a time-dependent stochastic inversion ͑TDSI͒ 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.

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.

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