What signifies a bad roof when testing?

A hollow sound when struck signifies a bad roof because it indicates possible voids or weaknesses in the structure. When a miner tests the roof by striking it, a hollow sound suggests that the material has deteriorated or that there may be air pockets, indicating instability. This condition can point to the potential for a roof collapse, as it shows that the integrity of the roof is compromised. Miners need to be aware of such signs to ensure their safety while working underground.

While a high-pitched sound, no sound at all, or a loud echo might also appear under different conditions, these do not typically convey the same alarming indication of structural weakness that a hollow sound does. Instead, those sounds may not directly correlate with immediate safety risks in the same manner as a hollow resonance would.