A waterfall plot (also called cumulative spectral decay or CSD) shows: X-axis = frequency, Y-axis = amplitude, and Z-axis (into the page) = time. The 'front ridge' is the initial frequency response. The 'tails' extending backward show how long each frequency rings after the initial signal stops.
Waterfalls reveal problems invisible in a simple frequency response: a speaker that measures flat but rings at 3 kHz for 10ms, a room mode at 80 Hz that persists for 500ms, or a tweeter resonance at 15 kHz. In room acoustics, waterfalls are used to evaluate the effectiveness of bass traps and absorption panels.