Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 431 - AVS Forum
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post #12901 of 12917 Old 12-23-2014, 04:08 AM
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Originally Posted by Audionut11 View Post
I'm not jumping to anything markus. I've sparked discussion.
That's good but I've been through this discussion multiple times so please forgive me if I don't sound as enthusiastic as you might expect. Rest assured that I know all those references down to the single papers.

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"In science, contrary evidence causes one to question a theory. In religion, contrary evidence causes one to question the evidence." - Floyd Toole
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post #12902 of 12917 Old 12-23-2014, 04:24 AM
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That's good. Because being over enthusiastic leads to mistakes.

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post #12903 of 12917 Old Yesterday, 08:09 AM
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I have a new sub so have been taking some measurements recently. I noticed something interesting that had never occurred to me before, not sure if this is stating the obvious to everyone else but thought I'd comment anyway

This is the raw response of my sub in my completely untreated room, I have grown to know and love that 40Hz spike



the 60Hz null goes if you move the mic a few cm so ignore that for now

I was just doing some quick compression testing so 1 PEQ filter & a few sweeps later produces



I stopped at this point as my door frame was creaking! distortion for the top sweep



Notice the substantial elevation in distortion around 20Hz in the 2nd harmonic and ~13.3Hz in the 3rd harmonic & note how it falls either side of that frequency. If the sub was being overdriven (I know it's not, the drivers are not even working that hard) then you'd normally see distortion continue to escalate as frequency drops. My thinking is this is an artefact of using EQ to tame a mode, i.e. the EQ targets the fundamental frequency only but the mode still exists and still serves to amplify the harmonic distortion from lower frequencies. Hence you see this "artificial" (but still real) boost in distortion. This sounds plausible to me, any thoughts welcome though.

This made me think about building a tuned bass trap to tackle the 40Hz mode instead, not sure whether it will be practical in my room but worth investigating anyway. Does anyone have any experience of building/using such a thing?
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post #12904 of 12917 Old Yesterday, 10:50 AM
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3ll3d00d,

I am not sure how meaningful measuring the sub by itself is. When listening, your hearing everything going at once. Are there other subs or just the one?

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post #12905 of 12917 Old Yesterday, 12:25 PM
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3ll3d00d,

I am not sure how meaningful measuring the sub by itself is. When listening, your hearing everything going at once. Are there other subs or just the one?
It is a single sub that has 2 18" drivers. I was really looking for clean output capability here BTW.
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post #12906 of 12917 Old Yesterday, 12:29 PM
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It is a single sub that has 2 18" drivers. I was really looking for clean output capability here BTW.
(2) subs with (1) 18" in each would have offered more versatility.

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post #12907 of 12917 Old Yesterday, 12:36 PM
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For sure however the positions I have in my room, for a 2nd sub, provide no appreciable benefit with respect to smoothness of response.
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post #12908 of 12917 Old Yesterday, 01:17 PM
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Originally Posted by 3ll3d00d View Post
My thinking is this is an artefact of using EQ to tame a mode, i.e. the EQ targets the fundamental frequency only but the mode still exists and still serves to amplify the harmonic distortion from lower frequencies.
At those frequencies (and especially at that volume! ), the Rockwell figurines in my family room start to dance around. I'm assuming that that causes most of the distortion I get on my graphs.
You say your room is "untreated"; is there "stuff" around that could be vibrating/rattling/moving?

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Did you really need to quote that entire post in your reply?
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post #12909 of 12917 Old Yesterday, 01:50 PM
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Quote:
Originally Posted by 3ll3d00d View Post
This made me think about building a tuned bass trap to tackle the 40Hz mode instead, not sure whether it will be practical in my room but worth investigating anyway. Does anyone have any experience of building/using such a thing?
I did an evaluation of GIK Acoustics Scopus tuned traps a while back. the report is here:


Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs
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post #12910 of 12917 Old Yesterday, 03:45 PM
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ok guys I bought new UMM mic with calibration file of 90 degrees. I was following the guide to copy the sensitivity factor. The first line that needs to be copied, is it the whole line with quotes, just the number, .... ??? Assuming the line is
"Sens Factor =-12.444dB, SERNO: 123456"

exactly what part needs to by copied over (please include double quotes as well if that needs to be copied)?
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post #12911 of 12917 Old Yesterday, 04:38 PM
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Originally Posted by SherazNJ View Post
ok guys I bought new UMM mic with calibration file of 90 degrees. I was following the guide to copy the sensitivity factor. The first line that needs to be copied, is it the whole line with quotes, just the number, .... ??? Assuming the line is
"Sens Factor =-12.444dB, SERNO: 123456"

exactly what part needs to by copied over (please include double quotes as well if that needs to be copied)?
The whole line. Page 53 of the guide actually has an example.
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post #12912 of 12917 Old Yesterday, 10:40 PM
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The whole line. Page 53 of the guide actually has an example.
Thx. Yes the guide has it but it was confusing. I shows a highlighted part that doesn't highlight whole line. Plus it says
"Copy this sensitivity factor".

Therefore I got confused since the line has two parts Sens Factor and Sensor:----

But now its clear. Thx a bunch.
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post #12913 of 12917 Old Today, 10:13 AM
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guides shows two ways to measure floor noise. One using REW SPL Meter and other using RTA. There is a difference of 10db in my case in b/w the two. SPL is coming around 45 and RTA around 55. When generating water fall graph, which one should be used as lower limit?
thx
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post #12914 of 12917 Old Today, 10:20 AM
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Originally Posted by SherazNJ View Post
guides shows two ways to measure floor noise. One using REW SPL Meter and other using RTA. There is a difference of 10db in my case in b/w the two. SPL is coming around 45 and RTA around 55. When generating water fall graph, which one should be used as lower limit?
thx
Set the lower limit so you see approximately 40dB from the peak level of your measurement, assuming your measurement is in the 85-95dB range. What is really important when interpreting the waterfall is the first 20dB from the top, but to get a good picture, you should have about 40dB range.
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post #12915 of 12917 Old Today, 10:50 AM
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Originally Posted by AustinJerry View Post
Set the lower limit so you see approximately 40dB from the peak level of your measurement, assuming your measurement is in the 85-95dB range. What is really important when interpreting the waterfall is the first 20dB from the top, but to get a good picture, you should have about 40dB range.
Jerry,
Can you explain this a bit more please. I attached the graph of water fall.
1 - "assuming your measurement is in the 85-95db range": Does it mean that the top of graph is somewhere in b/w 85/95 range?
2 - "What is really important when interpreting the waterfall is the first 20dB from the top, but to get a good picture, you should have about 40dB range: What are we looking for in this first 40db:" I understand that it should be decaying. As I understand, its bad if mountains are coming at you. Does that mean that if there are no mountains coming at you within 40db from top of graph, we are good?

The attach graph shows mountains coming out below 50hz and I have been trying to get rid of them. If I only need to worry about first 40db from top, then I have small ringing in b/w 25-37Hz. Is my interpretation correct?
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post #12916 of 12917 Old Today, 11:44 AM
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To give you some perspective, here are examples of my waterfall and spectrogram:





You might try looking at your spectrogram, which offers a slightly different perspective of resonance. Place the cursor at 450ms, and look for any "plumes" that extend above the 450ms line. These plumes would be what you would want to address. Anything below 40Hz is ridiculously difficult to address, but anything above 40Hz can be improved by the judicial placement of the right kind of bass traps.

HST, I don't think your waterfall is that bad. Perhaps the 40-50 Hz range could be improved by traps, but that depends on what you think of how the system sounds.
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post #12917 of 12917 Old Today, 12:41 PM
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Quote:
Originally Posted by AustinJerry View Post
To give you some perspective, here are examples of my waterfall and spectrogram:





You might try looking at your spectrogram, which offers a slightly different perspective of resonance. Place the cursor at 450ms, and look for any "plumes" that extend above the 450ms line. These plumes would be what you would want to address. Anything below 40Hz is ridiculously difficult to address, but anything above 40Hz can be improved by the judicial placement of the right kind of bass traps.

HST, I don't think your waterfall is that bad. Perhaps the 40-50 Hz range could be improved by traps, but that depends on what you think of how the system sounds.
Thx a million Jerry. That helps a lot and also explains 450ms. Knowing the fact that Spectrogram is a top view of waterfall makes it very clear as well. I think Spectrogram is much simpler to read since it shows top view and its very easy to see how frequencies are decaying with time. Just with that, I have realized that I reduced ringing by having my subs facting MLP instead of facing side walls :-).
Here is the graph. Looks much better. Just a little ringing in b/w 25-30. But as you said, below 40 its very veyr hard to get rid of.
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