Now What?

[elee532]

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!

[brucek]

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

[elee532]

Thanks brucek! Any suggestions on where an amateur might learn how to read a waterfall plot?

[brucek]

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

[elee532]

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

[brucek]

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

[elee532]

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!

[tenzip]

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.

[brucek]

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.

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

Known bug in the software.

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

[elee532]

brucek wrote:

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?

brucek wrote:

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?

brucek wrote:

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!!

[brucek]

Can you clarify, from your earlier post, which sound level you were suggesting needs to be set at 75dB?

Initially, the SPL level is set at the listening position to 75dBSPL with the SPL meter. Then the Input level of REW is set. Then the Calibrate routine is run, and the level set to 75dB to sync the REW internal SPL meter to the actual 75dB set at the listening position.

taking readings right now with full-range mains but no subwoofer yet. Is this OK?

Sure, as long as you use the Main speaker pink noise to set the Check Levels, and as long as you set the end frequency to the appropriate end frequency you require for the mains, and that you have an SPL meter that can measure full range (such as a Galaxy CM-140, as a Radio Shack meter is not capable past ~3KHz).

brucek

[Wayne A. Pflughaupt]

elee532 wrote:

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?

Bass traps are supposed to reduce low frequency ringing (i.e., extended signal decay times). So "before and after" waterfalls will be able to show if the bass traps have made a difference. Supposedly the more traps you have, the more ringing is truncated.

In order to show that a decrease in ringing has been accomplished, a waterfall graph will indicate that an improved rate of decay has been initiated. Compared to a "before" graph, an improved rate of decay would be seen as increased spacing between the slices, indicating that the signal level is attenuating faster. This is what improved ringing (faster decay) looks like:

Note the significantly faster rate of decay above 140 Hz with the lower graph, which added a number bass traps to a room: At about 15 slices, the signal has dropped as much as 50 dB at some frequencies, in less than 200 ms, compared to the no-traps graph, which shows decay times at twice that rate or more.

Naturally, depending on the number of bass traps you add, your improvement in ringing might not be this dramatic. But if there is any improvement in ringing, the waterfall graph should show it

Regarding LOG vs. LIN modes, I find both to be useful. LOG is good for analyzing below-40 Hz ringing , but it compresses everything above that point. LIN spreads out the graph above 40 Hz, which makes it easier to analyze ringing above that point (hit F11 to see both graphs fully on-screen).

LOG (Top) vs. LIN (Bottom) Waterfall

Regards,
Wayne

[elee532]

brucek wrote:

Initially, the SPL level is set at the listening position to 75dBSPL with the SPL meter.

Is this a routine that is run using REW or would I use, for example, my pre-pro's speaker test signal to set the pre-pro volume to 75db?

brucek wrote:

Then the Input level of REW is set.

Is this what you mean: Settings > Soundcard tab > Check Levels?

When running the Check Levels test, do I use the reading at the top of the meter (peak level) or the reading at the bottom of the meter?

brucek wrote:

Then the Calibrate routine is run, and the level set to 75dB to sync the REW internal SPL meter to the actual 75dB set at the listening position.

Is this what you mean: Settings > Mic/meter tab > Calibrate SPL?

I'm sorry, but I am just totally confused for some reason. All of my windows/soudcard settings are set to max/100. Sweep is set to -12. I run Settings/Check Levels and it reads about -30 (-18 peak). I run calibrate so that SPL and REW read 75db. Then, when I run Measure/Check Levels, it reports that "Level is too low." What am I doing wrong?

Thanks again for your help!!

[brucek]

Is this a routine that is run using REW or would I use

The Check Levels routine produces a signal (adjusted at the soundcard by the sweep level) and adjusted by the AVR to produce 75dBSPL at the listening position.

Is this what you mean: Settings > Soundcard tab > Check Levels?

Yes.

When running the Check Levels test, do I use the reading at the top of the meter (peak level) or the reading at the bottom of the meter?

You read the RMS signal level at the bottom of the VU meter. The red line is the peak and should remain below 0dB.

Is this what you mean: Settings > Mic/meter tab > Calibrate SPL?

Yes.

I run Settings/Check Levels and it reads about -30 (-18 peak)

The input level should be set to -12dB RMS with peak below 0dB

brucek

[elee532]

OK. I think (hope) I might have it now...

I set all Windows/soundcard levels to 100%. I ran REW Settings/Check Levels - leaving sweep level at -12 - and adjusted my pre-pro volume until my SPL meter read 75db. At this point, the right input meter in REW read about -19.5 (peak was about -7). Next, I ran Settings/Mic/Calibrate SPL. I set the SPL calibration windows to read 75db to match with my SPL meter. Next, I ran Measure/Check Levels. This time, it reported "Level OK, -19db." From here, I took a measurement. Please tell me I got it right this time. :-)

I included a measurement that I am hoping I can use a baseline to assess what impact acoustical treatments make as I add them to my room?

Also, can you tell me whether Settings/Mic/Calibrate SPL is the same thing as the Calibrate button on the toolbar?

[brucek]

Please tell me I got it right this time

Yes, except your graph scaling is incorrect your response graph.

For subwoofers alone, always 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.

For full range, use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - to upper limits you desire, i.e. 20KHZ - certainly no higher than your soundcard can extend).

For full range, enable smoothing to eliminate the comb filtering. Use a 1/3 octave smoothing.

Remember that a Radio Shack meter is unsuitable for full range measurements. It's only accurate to about 3KHz. which is fine, since treatments are targeting bandwidths lower than that..

can you tell me whether Settings/Mic/Calibrate SPL is the same thing as the Calibrate button on the toolbar?

Yes.

brucek

[elee532]

brucek, thanks for helping me get a handle on this calibration thing!!

Wayne, thanks for the cash course in reading the waterfall graph!! (not that I really understand what I am looking at I did just order a new book - "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" - with the hope that I can get an understanding of this stuff.)

I took these first two readings. Blue is before any treatments. Green is after I added my first two bass traps. From what I can tell, there is an increased spacing between the slices (i.e. an improved rate of decay), which indicates some positive benefit from these first two bass traps.

Is my admittedly rudimentary understanding correct?

Thanks!

[Wayne A. Pflughaupt]

I took these first two readings. Blue is before any treatments. Green is after I added my first two bass traps. From what I can tell, there is an increased spacing between the slices (i.e. an improved rate of decay), which indicates some positive benefit from these first two bass traps.

Is my admittedly rudimentary understanding correct?

Yes, that's correct, although it's hard to tell what's happening with the overlay graph (for me at least - some people prefer them).

Regards,
Wayne

[elee532]

Here they are as two seperate graphs. Am I accurately seeing some improvment?

The green graph is after adding two 4' superchunk bass traps in the front left corner using Roxul Rockboard 60. My plan is to add another two in the front right corner, two in the back right corner, two along the top wall ceiling corner, and maybe two more along this soffit towards the back end of the room.

[brucek]

There is such a large improvement, I'm suspect that the two measurements aren't identically taken somehow. Treatment usually will not have such a dramatic effect down that low. It's almost impossible to affect the 15Hz-30Hz range so much. Look at 15Hz how slice one (the main response) is down from about 65dB to about 58dB? When checking effectiveness of treatment, you need to be careful to do measurements without touching anything but the addition of treatment... is that the case here?

You also need to be sure some background noise didn't stop or start between measurements, such as a furnace...

brucek

[Wayne A. Pflughaupt]

What brucek said - you can ignore any apparent improvement that you're seeing below 40 Hz - traps don't have an effect down that low.

There does appear to be some improvement in the 60-90 Hz range - as you noted, wider spacing between the slices. The dip between 50-60 Hz also appears to be less severe now, another benefit traps can deliver.

It would be easier to see what you have in the 60-90 Hz range by switching to the LIN display. Also, shortening the waterfall window to 300 or even 200 ms would help see any improvement the traps might be making.

Regards,
Wayne

[elee532]

Brucek, Wayne, thanks again!

I can't think of anything substantial that changed between readings. Of course, I took them a day apart, so you never know. Thanks also for the reminder to pay closer attention for any background noises.

I shortered the horizontal axis, switched to LIN view, and changed the time range to 300ms. I still think i see some level of progress being made.

Thanks.

[Wayne A. Pflughaupt]

Well, different graph parameters sure give a different perspective. These two don't really show any notible improvement in signal decay, although the depression at ~110 Hz seems to be a bit narrower.

Not that graphs are necessarily the "be-all." Do you note any audible difference or improvement?

Regards,
Wayne