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Ok, I see, so the surface of the tile is sliding against it but
sometimes 'grabs' an edge to pulling it only to be released as its the
force of me moving the pipe exceeds the friction holding the pipe in
place. I presume that its then slightly muted by the fact that the
pipe is rubbed against the tile while vibrating as it touches the tile
after 'excitation' (as you put it).

Can you explain what you mean by Broadband excitation?