This is essentially the same as the frequency response plot. The new axis that adds a third dimension is time in milliseconds, over a 312 millisecond range. When the sound system tries to play a note that suddenly stops, the sound does not suddenly stop. The loudspeaker enclosure walls, driver cone, etc. also continue to vibrate for a fraction of a second. The waterfall plot shows the relative frequency response of this lingering sound, as it decays.
Room resonances persist the longest.
The plot clearly shows room resonances at the lower frequencies. There is a particularly large ridge at 90 Hz, and another prominent one at 50 Hz. The 90 Hz peak is very evident in the engineering department frequency response curve as well. These resonances are discussed in the section on room acoustics . A more rapid decay at higher frequencies may be preferable for good transient response.