What a REW waterfall plot is telling you
A waterfall is not a fancier frequency response. It answers a question the frequency response cannot: how long does each frequency keep going after the music stops.
A waterfall plot is a stack of frequency responses taken at successive moments after the signal stops, drawn one behind the other. Frequency runs left to right, level runs up and down, and time runs back into the screen. A ridge extending backwards is a frequency still sounding after its neighbours have gone, which is a resonance. It answers how long, where a frequency response only answers how loud.
6 min read · published 2026-07-31 · updated 2026-07-31
What the three axes are
The plot below is generated from the same modal model the calculator uses, for a 4.5 by 3.4 m room. Every slice is one frequency response at one instant, and the settings are fixed, which is the only condition under which two waterfalls can be compared at all.
- 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.
What to actually look for
- 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.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.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.
Waterfall versus spectrogram versus RT60
All three describe decay and they suit different questions. The waterfall is the hardest to read accurately and the easiest to be impressed by.
| View | Best for | Limitation |
|---|---|---|
| Waterfall | Spotting one specific frequency ringing on | Hard to read quantitatively: the near ridges hide the far ones |
| Spectrogram | Diagnosis. The same data as a heat map from above, with nothing hidden behind anything else | Less dramatic, so it gets posted less often |
| RT60 by band | Sizing absorption. The only one of the three that gives a number you can put in a calculation | One line per band, so a single ringing mode can hide inside its band average |
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.
Apply this to your own measurement
The analyser runs every check in this guide against your REW export and your room dimensions, and tells you which findings EQ can help with.
Analyse a measurement