Non-invasive measurement of sound velocity profiles

In ultrasonics, the time of flight to the object interfaces is often the only information that is considered. From this information, one can either determine distances or sound velocities if the other value is known. A combined determination of the sound velocity and the distance of the scattering particles to the transducer makes it possible to measure sound velocity profiles. To determine distance and sound velocity simultaneously, we use the focus position as a second piece of information beside the ultrasonic time of flight. The focus position can be determined, because the echo becomes strongest when the scattering particle is located within the maximum of the sound field. Thus the position of the averaged echo signal amplitude indicates the time of flight to the focus. In previous measurements with a strongly focusing transducer with lens an accuracy of 99.9% was achieved with this method. The current presentation deals with first steps toward the measurement of sound velocity profiles. To measure a velocity profile, the focus position, i. e. the distance of the sound field maximum from the transducer, is varied by beam forming with an annular array. By using adequate calibration curves, the sound velocity on different points along the acoustic axis can be determined in dependence of the chosen set of time lags used for focusing and the time of flight corresponding to the maximum of the averaged echo signal amplitude. The measurement of the locally resolved sound velocity in media with constant sound velocity yields information about the accuracy that can be reached for varying focus distances.