The very young discipline of time reversal acoustics is concerned with generation of locally restricted ultrasonic pulses. Here the undisturbed interference of ultrasonic waves is used, which constructively results in local maxima or destructively in local minima of the sound pressure. This is done by emitting an initial sound pulse, recording it, and then chronologically reversing and emitting the received signal again back to the initial point.
Every change in the acoustic system during the time reversal process results in an audible amplification of noise in comparison to the actual signal. This effect is similar to the very sensitive “Schlieren-effect” from optics and can be used in many ultrasonic applicatio
In chemistry, food-processing or biotechnology liquid levels have to be measured without accessing the container physically. As a solution, the Measurement Engineering Group has extended the known ultrasonic puls-echo-method, which will also work when built-in components such as heating pipes block the direct path between sensor and the liquid's surface. Based on the time reversal principle a virtual sound source is created at the certain point in the liquid.
The method can be divided into two phases. In the first phase a calibration is done by sending a short acoustic signal from the wanted filling level, down towards the receiver at the bottom of the tank to determine the transfer function. Due to interactions with the tank’s boundaries the receiver records a series of pulses. In the second phase the receiver is used as transmitter and sends the time reversed received signal back into the vessel. This way the actual sequence is produced on or at least close to the surface. The resulting answers are then correlated with the recorded signal sequences. Extracting the correlation maximum the time of flight can be identified and the actual filling level is calculated. FEM-simulation approves the viability of the introduced process, even though the tank geometries or the exact position of built-in devices have a significant impact on the quality of the refocusing. Methods for optimizing the region of interest are being used to adjust the tank's structure, resulting in increased robustness and high measurement accuracy.
An alternative variant, which does not use the liquid, but the tank itself as an acoustic medium, can be used to get additional information about liquid-gas-phases and is currently being reviewed in simulations and experiments.
Selective acoustic irradiation
In different audio applications, the aim is to offer information only to selected areas. This selection can be reached by conventional array techniques when using the acoustically non-linear properties of air or with the use of additional time-reversed acoustics.
In doing so a pulse-like acoustic event (cracking a balloon) is emitted and received by a microphone array. Many reflections on walls and obstacles expand the initial pulse on its way to the receivers. The chronological reversion of the received signal and the consecutive transmission from the same place as the microphone-array is placed, using a loudspeaker-array, results in a spatial and temporal limited sound event at the actual point of source. Other experiments show that the focusing is enhanced with increasing amount microphone-speaker-pairs.
The information on the ambient conditions known from the time signal can be used to generate speech or music at a particular place. The aim is to set up acoustic markers or direction signs in museums, without creating noise pollution in the whole room. First trials showed that an implementation in a room and in HIFI quality would need about 30.000 speakers, which is not workable at the moment. This is why the Measurement Engineering Group is trying to develop methods and systems to be able to offer the same quality with less effort.