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Newbie question on levels for waterfall graphs
Old 2 weeks ago
  #1
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Newbie question on levels for waterfall graphs

Hi,

I get the general principle of looking at waterfall graphs to see the decay time in the lower frequencies. It seems that if my measurements are at a higher SPL, then there's a longer overall decay time. If the measurements are at a lower SPL, there's a shorter overall decay time. How do I compare waterfall charts that were made at different times with different volume levels? Is there some way to adjust the decay time for peak or average SPL? My goal is to compare decay times for graphs that were made at different loudnesses.

Thanks
Old 2 weeks ago
  #2
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Jens Eklund's Avatar
Decay time does not depend on level, but low recording level (especially if short sweep length and only a single sweep) will result in higher noise floor that might prevent you from seeing the full decay.
Old 2 weeks ago
  #3
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Here's what I mean. I made two tests. The only change was increase the volume on my amp. I'm using a calibrated umik mike. In the first one the sound level on the REW test is -53.7. In the second it's -46.4 (which was 77db when I tested in rew with pink noise). So I turned the volume up and the decay times moved out past 1,000 ms. The bass is interesting: I have a subwoofer cut off at about 80hz according to the dial on the back, and I have the sub volume pretty low. Deep bass notes match the perceived volume of higher notes where the speakers take over. But I don't know how to compare these two charts since it would appear that the decay time increases with volume. I tried measurements at a higher level, and it made something in my subwoofer "pop" :(
Thanks for your patience in helping me understand this!
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Newbie question on levels for waterfall graphs-ovp488.png   Newbie question on levels for waterfall graphs-qnffro.png  
Old 2 weeks ago
  #4
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Jens Eklund's Avatar
Before comparing, match the levels of the measurements (add an offset to one to match the other) and you'll see what I mean. Also ply around with different settings to get a feel for how the waterfall graph works and how to read it properly.
Old 2 weeks ago
  #5
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"match the levels of the measurements (add an offset to one to match the other)" I guess this is what I'm trying to figure out. I've gone to Set Target Level in EQ and I've also typed in target levels there, but the graphs still look the same. After numerous googles for REW offset, matching, etc., I haven't found how to set this. Any help is appreciated. I think I'm looking for what do you actually click to make it happen. Maybe I need to find a REW thread? Or if anyone can share the solution, I appreciate it.
Old 2 weeks ago
  #6
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Jens Eklund's Avatar
Check section "Graph Controls":
https://www.roomeqwizard.com/help/he.../overlays.html
Old 2 weeks ago
  #7
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SPL, Controls, Offset seems to fix the problem. When both graphs top out at the same levels, the waterfalls look like "apples to apples" in that the difference in treatments now corresponds with the measurements. Thanks!
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