The high-speed challenge to reduce noise

High-speed trains are one of the eco-friendliest ways to travel thanks to their extremely high capacity and efficiency, but their big drawback is that they are very loud. CAF Research and Development Engineer, Isaac Gutiérrez explained: “Aerodynamic noise is the most important noise contributor to the overall pass-by noise level and is the most challenging to measure and understand. We were looking for the most accurate way to do this in order to make relevant adjustments to our train design and meet the European TSI requirements.”

CAF contacted sound and vibration specialist, Brüel & Kjær, to discuss possible solutions and following a consultation the company’s acoustic beamforming technology was identified as the best tool for CAF. Using a portable array of microphones, this technology locates noise sources in high definition, even on moving objects. It displays sound as colour-coded contours overlaid on an image of the test object.

As the acoustic dynamic range of a high-speed train passing by is very wide (it goes from almost complete silence to extremely high noise levels in just a few seconds) the beamforming technology’s wide dynamic range is ideal for recording both low and high noise levels at an immense sampling rate.

Along with assistance from Brüel & Kjær, CAF’s team set up a beamforming system with pass-by sites across the Spanish countryside. The project resulted in noise maps indicating sound pressure, pressure contribution density and sound intensity for the train as a whole and for individual parts of the train.

Isaac commented: “From studying the noise maps, we have determined that we, for example, could modify the shape of the cars to change the direction of the air passing between the cars. By making this modification, we were able to reduce the exterior noise by three to five dB(A). We are confident that no other noise measurement system could have helped us achieve this.”