The waterfall is a 3D representation of the speakers' response to a wide-bandwidth signal that stops instantaneously. The Y-axis is level in dB, the X-axis is frequency from 200Hz to 20kHz, and time runs from back to front on the Z-axis (measurement constraints limit the length of time window available as frequency falls). The waterfall plots therefore illustrate how good the speakers are at switching off, and the Z-axis plot for a notional 'perfect speaker' would be empty. Any signals within the plot occurring after zero time represent the decay of mechanical or acoustic resonances. This is output that the speaker adds to the intended signal, colouring the sound and effectively degrading the signal-to-noise ratio.

Phil Ward