(this should be not mixed up with a “spectrogram”)
This waterfall diagram gives normally in the x-axis the frequency, in the z-axis the level and in the y-axis the time and shows the “decay” of the sound.
It is e.g. calculated by a sweep like produced by the excitation of the DFFT and can be used in rooms in the far field to detect modes and resonances. In measuring loudspeakers (near field) it can be used to detect resonances.
(please see to enclosed example)