What a REW waterfall plot is telling you

• Left to right: frequency, exactly as on the normal SPL graph.
• Up and down: level in decibels.
• Back into the screen: time. The front slice is the response at the moment the signal stops, and each slice behind it is the response a few milliseconds later.

So a ridge running back into the screen is a frequency that is still sounding long after everything around it has decayed. That is a resonance. A peak that is tall at the front but disappears immediately behind is loud, not resonant, and is a different problem.

1. 1.Ridges below 200 Hz. These are room modes ringing. They correspond to the peaks on your frequency response, and they are what makes bass sound slow and one-note.
2. 2.How far back the ridges extend relative to their neighbours. A frequency still audible 300 ms after everything else has gone is a real problem. Uniform decay across the bass, even if it is slow, is a much smaller one.
3. 3.The overall slope of the back edge. It should tilt so that high frequencies decay fastest. A flat or reversed slope means the room's top end is decaying too slowly relative to its bass, which is unusual and normally means very hard surfaces.

All three describe decay and they suit different questions. The waterfall is the hardest to read accurately and the easiest to be impressed by.

• Waterfall: good for spotting one specific frequency ringing on. Hard to read quantitatively because the near ridges hide the far ones.
• Spectrogram: the same data drawn as a heat map from above. Less dramatic, far easier to read, and nothing is hidden behind anything else. Prefer it for diagnosis.
• RT60 by frequency band: a single line per band, and the only one of the three that gives you a number you can put into an absorption calculation. This is the export that lets a tool tell you how many panels to build.

If you are trying to decide what to buy, export the RT60 data. If you are trying to work out why one bass note sounds wrong, look at the spectrogram. The waterfall is for showing someone else what you fixed.