In Central Mining Institute 6 cycles of velocity tests of elastic waves in roofs of worked out longwall workings have been carried out by profiles parallel to longwall front moving away to goaves as the extraction advances. It is stated that as they move away from the longwall front towards the worked out area the velocity of longitudinal waves decreases. This drop is different for three selected cohesion groups of roof rocks. Namely for correct behaving roofs when mining with caving the velocity falls from 4000–5000 m/s at the longwall front to about 1200 m / s at the goaves, for intermediate roofs this drop occurs in the intervall from 3000 to 1500 m/s, while for difficult roof sections with rock fall hazards constant velocity of about 1000 to 1500 m / s has been found on the whole width of excavation and even the local increases of the velocity from the front to goaves. The velocity of about 1300 m/s has been defined as a limit of critical loosening of roof over the excavation.

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