For the characterisation of granular matter, several experimental methods are available. For dense packings an established method is the transport of sound waves. It is well known that for higher pressure on the granulate packing the speed of sound increases. So for high pressures, granular matter behaves similar to liquids: The speed of sound varies with the pressure, but not with the amplitude of the sound wave. For very low pressure, however, one is close to a transition where the overall granular packing loses its mechanical stability and the individual particles begin to lose the contacts among each other. Simultaneously, additional contacts can be established in this regime by sound waves. In this parameter range, the speed of sound varies with the wave amplitude.
This unusual behavior of sound in granular matter was probed during the 28th DLR parabolic flight campaign in March 2016. To this end, the different arrival times of a sound wave along the probe cell was determined.