A CSD plot (waterfall) shows time on one axis, frequency on another, and amplitude as color/height. The 'ridge' at the front shows the initial frequency response; the 'tails' extending back in time show how long each frequency rings. Long ridges at specific frequencies indicate resonances — from the room, speaker cabinet, or driver.
CSD is more revealing than a simple frequency response graph because it shows TIME behavior. A speaker might measure flat in frequency response but ring at 2 kHz for 10ms — masking detail and blurring imaging. This is invisible on a standard frequency graph. CSD is used in speaker design and room acoustic analysis.