A new Type of Line-Array for Aeroacoustic and Acoustic Source Localization at Drive-by Tests of Trains

Kurzfassung

The application of microphone arrays for the allocation of sound sources at objects moving along a known trajectory is of special interest in case of the investigation of noise-sources at trains. Especially in case of high-speed trains it is a challenging task, since the angular velocity of the sound sources is high if the recording takes place at close distances of around 10 m from the track. Different techniques exist for the localization and quantification of sound sources. Examples are elliptical acoustic mirrors and phased microphone arrays. In case of microphone-arrays the focussing process is carried out by means of summing up signals from different sensors with defined phase delay. A short overview of the history of microphone array measurements for train investigations is provided by Michel 2006. In order to detect a flat or even spatial distribution of sound sources with a directional microphone with a concave acoustic mirror, the concave mirror has to be moved with the microphone at its focal point for scanning the volume of interest. This implies a fixed position of the sound sources of interest. When using a microphone-array, the focus point can be varied as desired in the post-processing of the recorded signals, leading to much smaller acquisition times compared to measurements with acoustic mirrors. However, a high spatial resolution requires a large number of microphones distributed on a 2-dimensional mesh. In the following a new type of microphone-array combining a line array with a two-dimensional elliptical acoustic mirror is described. The combination of both techniques allows a three-dimensional reconstruction of the source distribution. The design was successfully tested on a high speed train track in Germany, which is used by different type of Inter City Express and other passenger trains. It was possible to improve the quality of the results signifficantly and individual wheel sets and aeroacoustic noise caused by roof structures can be identified in the source maps.