Now What? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 10-08-08, 08:38 PM Thread Starter
Shackster
elee532

Join Date: Aug 2008
Posts: 90
Now What?

Can anyone point me to an amateur guide for actually reading the results of REW?

Here is my first graph. I'm going to be putting some bass traps in my room and I want to see what kind of difference they make. What changes should I be looking for in this graph? Am I even looking at the right graph?

Thanks!
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Last edited by elee532; 10-08-08 at 10:20 PM. Reason: Graph updated to reflect suggested axis values.
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Old 10-09-08, 07:07 AM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Now What?

Quote:
What changes should I be looking for in this graph? Am I even looking at the right graph?
No, effectiveness of traps will be seen using Waterfall plots..

brucek
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Old 10-09-08, 09:57 AM Thread Starter
Shackster
elee532

Join Date: Aug 2008
Posts: 90
Re: Now What?

Thanks brucek! Any suggestions on where an amateur might learn how to read a waterfall plot?
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Old 10-09-08, 10:30 AM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Now What?

Not much to it really.

Use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 200Hz) using the Graph Limits button in the top right corner of REW.

Set the mode to LOG rather than LIN using the Freq Axis button in the top right corner of REW.

Best to set the time range to 600msec and leave the Window at 300msec.

Set the X=1, Y=50, Z=150.

Select the measurement in the REW left pane you want to look at, and Generate the waterfall.

Now you are looking at a response measurement with the addition of the time domain. It will be in 30 slices of time of 20msec (=600ms/30) each after the sweep.

Slide the Slice over to 0 and you're looking at your frequency response graph. Now move to slice 1, and you're looking at the sound after 20msec, and so on to 30 slices at 600msec. If you see the sound still persisting (ringing) at 600msec (over a half a second) with any level that you might actually hear (> 45dB), then you can see that you would like that ringing to be reduced with a trap.

Give it a try and you'll see what I mean.

brucek
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Old 10-09-08, 07:43 PM Thread Starter
Shackster
elee532

Join Date: Aug 2008
Posts: 90
Re: Now What?

Thanks brucek. Here is my first shot at a waterfall plot following your suggestions. Unfortunately, I can't make much of it. Can someone get me started on understanding what this means, and what I should expect to see change as I add some bass traps to my room? Thanks
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Old 10-09-08, 09:04 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Now What?

Change the Freq Axis from LIN (that your graph is using) to LOG.

Measure to 200Hz, not 80Hz.

Set a target level of 75dB.

brucek
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Old 10-09-08, 10:37 PM Thread Starter
Shackster
elee532

Join Date: Aug 2008
Posts: 90
Re: Now What?

Thanks again brucek. Hope you don't mind a few more follow-ups...

How do I know whether the Freq Axis is set for LIN or LOG? I understand how to toggle back and forth, I just can't seem to find an indicator of which setting is which.

When I toggle between LIN and LOG, the measure seems to change from 80hz to 200Hz. Why is this?

I'm running into an issue seetting target level to 75db? I do Settings > Check Levels. All windows volumes are set to 100. Sweep is -12. My AVR volume is set so that SPL meter reads 75. When I try to run a measurement, I get a message along the lines of "level too low." So, I increase AVR volume until SPL memter reads 85. What am I doing wrong here?

Thanks!
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Old 10-09-08, 10:41 PM
Senior Shackster
Chris

Join Date: May 2007
Location: Lincoln, NE, USA
Posts: 393
Re: Now What?

I can't answer the rest of your questions, but the difference between LOG and LIN on the horizontal axis is quite obvious if you look at the divisions between the frequencies. LIN has the divisions equally spaced, with a division every 20Hz. The LOG axis numbering is logarithmic, and divisions will not be equally spaced. Look at the first graph you posted in this thread.
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Old 10-10-08, 08:30 AM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Now What?

Quote:
How do I know whether the Freq Axis is set for LIN or LOG?
The graph changes from Logarithmic scale to Linear scale.
Surely you played with log paper in school at some point and recognize what it looks like?
Also the Freq Axis button changes from a pic of LOG and a pic of LIN.

Quote:
When I toggle between LIN and LOG, the measure seems to change from 80hz to 200Hz. Why is this?
Known bug in the software.

Quote:
I get a message along the lines of "level too low."
The Check Levels routine in the Measurement screen and the Check Levels routine in the Settings screen use different values to generate the pink noise level setup test signal.

The Check Levels routine in the the Settings screen uses the fact that you have the Check/Set levels with Subwoofer pull-down selected to generate its test noise (this a band limited signal with a low and high cut of 30Hz - 80Hz).

REW double checks and verifys that the end frequency of the sweep that you have selected in the Measurement panel roughly matches the test signal setting (for sanity). Its Check Level routine uses the end frequency as the test pink noise hi-limit cutoff. So if you had 20KHz (as an example) as the end frequency to measure for your sweep, you can see how the pink noise energy would be different (between the two Check Levels) and as such the low level signal you're experiencing. Be sure to set the start and end frequency for subs to 0Hz-200Hz.

brucek
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Old 10-10-08, 08:41 PM Thread Starter
Shackster
elee532

Join Date: Aug 2008
Posts: 90
Re: Now What?

Quote:
brucek wrote: View Post
Surely you played with log paper in school at some point and recognize what it looks like?

brucek
Um, should I be embarassed that I had 15 credits of upper level college math, including 3 semesters of calculus?

Quote:
brucek wrote: View Post
The Check Levels routine in the Measurement screen and the Check Levels routine in the Settings screen use different values to generate the pink noise level setup test signal.
Can you clarify, from your earlier post, which sound level you were suggesting needs to be set at 75dB?

Quote:
brucek wrote: View Post
Be sure to set the start and end frequency for subs to 0Hz-200Hz.
I'm actually taking readings right now with full-range mains but no subwoofer yet. Is this OK?

Thanks!!
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