Wunderli J.

A measurement concept is presented which is based on the combination of a pressure microphone with one or two velocity sensors. As the sensitivity of the velocity sensors depends on the cosine of the angle of sound incidence they can be used in the far field as directional microphones. This allows a direct focusing on distinct regions of complex sound sources respectively a suppression of unwanted contributions. Alternatively sound source identification and separation can be done by establishing a system of sound propagation equations for each measurement signal. It is shown that this system of equations can be solved for up to six sound sources. With a suitable optimization strategy during signal analysis not only sound power but also the positions of the sources can be derived for up to three sound sources. The method has been developed and tested for measuring train pass-bys. Measurements have so far been performed at travelling speeds of 50 to 220 km/h. Further tests are planned at speeds around 300 km/h. Velocity sensors based on pressure gradients as well as recently developed sensors of the type Microflown have been used. The later are based on the measurement principle of a hot wire anemometer. As a consequence of the high miniaturisation a pressure sensor as well as three velocity sensors can be combined in a single probe. The possibilities and limitations of the measurement concept are discussed based on theoretical considerations as well as measurement results.