In building there is a common test for masonsry structures that involves striking the structure with a hammer and listening to the resulting sound. If the sound is ringing the structure is fine but if the sound is 'drummy' (maybe like a dull thud) then there is an issue with the structure.
I'm presuming this somehow relates to vibrations in the structure. But what actually is the difference between the ringing sound and the 'drummy' sound in terms of the vibrations within the structure?
in the ringing sound, the structure is vibrating as if it were a solid, seamless metal bar like in a xylophone or a wind chime, which indicates that all the individual bricks in it are tightly and firmly cemented together into a single mass, which is a good thing.
if the sound instead is dull and "drummy", it means there are gaps and breaks or "debonds" between the individual bricks which prevent the entire structure from responding as a single big chunk of stuff. instead it responds like a big pile of smaller chunks that are poorly-connected together- which is a very bad thing for a masonry structure.
