May 14, 2011

REW: Understanding decay and waterfall plots

In order to get the most out of REW, you need to understand how to read decay and waterfall plots. They are the most useful to consider in using REW for bass integration.

Here is a nearfield subwoofer measurement which excludes the impact of the room.


You can see in both plots the kind of result you would see in an anechoic chamber, a nice smooth response with very even and quick decay. You might say this is an ideal waterfall plot. You can also see some mains hum that comes from connecting the PC to the sound system. It it easy to pick because it occurs at 50 Hz and it is continuous, showing no sign of decay.

They decay plot shows a 2D version where the response is plotted at time intervals, in this case out to 150 ms.  Now let's look at a waterfall measured with the room response included. Unlike the previous measurements, this includes subs and mains together.


We can now see the influence of room modes, most of the peaks marked with the red dots. One in particular is marked. You can see it starts as a tiny peak that would hardly be noticed in a response plot. Over time it decays at a slower rate - this is referred to as modal ringing. Now look at all the points at the bottom of the chart where these peaks stand out. This is the main thing we are looking for in decay and waterfall plots. 

The room shown above might be considered a good room. It has light construction with plasterboard/drywall and a timber floor. Moderate EQ has been used to tame some of the peaks. Many rooms will in fact be worse than this, however this room really does need acoustic treatment as well to control the modal ringing. 

This is a far better measurement and it includes 3 large bass traps. The decay is now more even and modal ringing has been damped. There is still some room for improvement on paper, but this level of performance would be considered excellent. Subjectively the result is very tight and fast. 

Tips
  • When generating the measurement signal, limit the bandwidth to 15 - 400 Hz. This will result in a smaller file size (REW files are large). 
  • Set the time window to 300 ms
  • Pay attention to the chart limits as they can affect how the chart is perceived. Aim to compare plots with the same limits.
  • Waterfalls can be misleading. Notice how the lighter green waterfall appears to have worse ringing below 25 Hz. In reality the improvement is minor, but the dark green waterfall has early roll-off. This makes it appear as if the decay were better. This is where the decay plot proves useful as you can more easily see the relative decay of each. 

3 comments:

  1. Nice article...I've heard that it's REALLY hard to tame freq below 60hz with bass trapping or that u just need A LOT. I wish I had that dark green room lol.

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  2. Depends on what you call "a lot."

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  3. Spatial computer has a sub device called "Black Hole" to be put into the back of the room to actively damp standing waves and bass build up at the back of the room. They don't tell how but it has a 12 inch driver, a microphone, and a 350 watt amplifier. Probably sampling through the microphone and generating an out of phase mirror image to cancel out bass actively.

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