# Interpreting First Graphs



## ddgtr (Nov 1, 2009)

Hi,

I need a bit of help interpreting these graphs. I don't quite understand what they mean as far as room acoustics is concerned. First graph has the 1/3 octave smoothing applied, second is the raw one and third is the waterfall.

Room is rectangular, 15.5 feet x 19.5 feet x 9.5 feet high. I have 12 absorption panels, 4 feet x 2 feet x 2 inches, 4 bass traps and 14 poly diffusers.

With REW speaker signal.

In the "Filter Tasks" under "Find Peaks" it gave me three "Peaks Found" : first at 11.6db, 60.2Hz, second at 5.2db, 160.7Hz and third at 2.9db, 87.0Hz.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Please read this post. 

Waterfall graphs are only useful up to about 300 Hz.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Thanks Wayne,

Can you elaborate a bit please. I did read the thread you referenced above and I know I need to change the waterfall graph to 300hz max.

How about the other graph? Do I need to show L and R speakers? I did try to read the help files and it's a lot of specialized information, a bit overwhelming. Do I unhook one speaker at a time and measure?

I am really sorry for the dumb questions...

Cheers


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## Wayne A. Pflughaupt (Apr 13, 2006)

Yeah - I know it’s overwhelming. Wish I could help with that, but unfortunately, that’s just the nature of the beast. 

The main problem with your first two graphs is the vertical scaling. Change them from -20 – 120 dB to 45 – 105 dB. The link posted in my last post has directions on how to do that. If you want to look at bass response specifically, limit the horizontal axis to 200 Hz.

Yes, for the main speakers, it’s best to measure them one at a time. Unplug all the rest. 

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Great! Thanks for the pointers, this was exactly what I needed! I'll be making more graphs tonight! Funny thing is, no matter how frustrating I love doing this, it was the missing link in my setup. I've spent a lot of time and energy into building my system and when it'll be rewarding when I can finally figure out the REW part. Notice I say "when", not "if" !!:T


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## Wayne A. Pflughaupt (Apr 13, 2006)

> I'll be making more graphs tonight!


Did you save your previous measurements? If so you don’t have to take new ones. You can simply open them into REW and change the graph scales.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Didn't think of that, with all the excitement... I've just taken new ones...

Here they are!!

5 graphs:

1. Both speakers combined, with sub crossed at 70hz, 1/6 octave smoothing

2. Same as above, waterfall

3. and 4. Left and Right speakers, no sub, 1/6 octave smoothing

5. Averaging the above.

Hopefully I got it right this time...


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## Wayne A. Pflughaupt (Apr 13, 2006)

Wayne A. Pflughaupt said:


> The main problem with your first two graphs is the vertical scaling. Change them from -20 – 120 dB to 45 – 105 dB. The link posted in my last post has directions on how to do that.


Aside from that, it looks like your sub could benefit from some equalization. 










A waterfall graph isn’t of much use with a 60 dB floor. Lower it to at least 45 dB. 




> 3. and 4. Left and Right speakers, no sub, 1/6 octave smoothing


A graph of the main speakers limited to 200 Hz and below will only show us if your crossover is working properly and rolling out the bass. It appears to be. 

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Hi Wayne, sorry about you having to repeat the same thing twice, the "Please Read: Getting Graphs Ready to Post" thread you linked was very clear on the formats. My mistake... 

Here they are, the individual speaker graphs are taken with no sub and all have 1/6 octave smoothing.


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## Wayne A. Pflughaupt (Apr 13, 2006)

> Hi Wayne, sorry about you having to repeat the same thing twice, the "Please Read: Getting Graphs Ready to Post" thread you linked was very clear on the formats.


No problem Danny – I know it’s a lot for a first-timer to take in. Actually, the only reason I dropped in the quote was because it was easier than typing it over again – heh heh! :hide:

Now that we got the graphs lined out, we can get back to your original questions:



> I need a bit of help interpreting these graphs. I don't quite understand what they mean as far as room acoustics is concerned.
> 
> Room is rectangular, 15.5 feet x 19.5 feet x 9.5 feet high. I have 12 absorption panels, 4 feet x 2 feet x 2 inches, 4 bass traps and 14 poly diffusers.


Waterfall and RT60 graphs tell you the most about acoustics. You might see subtle changes in _smoothed_ frequency response graphs (like the ones you posted) due to treatments, but they have more of an effect on signal decay times than frequency response. When using treatments, it’s best to take “before” and “after” readings to gauge the improvement you’re getting from them.

However, you can learn a lot about your acoustics with an _unsmoothed_ frequency response graph, which can tell you how reflective your room is in the mid-to-upper frequencies. Multiple reflections will be picked up by the mic, and they will show up on the graph as “raggedness,” for lack of a better term. 

A couple of pictures will better show what I’m talking about. The first one is a fairly “live” room with hard floors and walls, etc. The second one is a well-dampened room with treatments. The difference in reflections is obvious.















​

Regards,
Wayne


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## lsiberian (Mar 24, 2009)

You could certainly benefit from EQ not only in the subwoofer range but also in that area from 500 to 800hz.


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## ddgtr (Nov 1, 2009)

Ok, I think I'm beginning to understand a bit more. The graphs you posted above have helped.

My RT60 values before room treatments were made with a web calculator: reverb time: 1.33 sec recommended: 0.6 to 1.2 sec (with the empty room...)
At 125: 0.4sec, 260: 0.9sec, 500: 1.3 sec, 1000: 2.2sec, 2000: 1.7sec and 4000: 1.5 sec. 

Below it's the latest REW RT60 graph and text.

I also repositioned the sub checked levels and recalibrated everything (I might have made a mistake before). So what I'm looking for in a waterfall are smoother, "rolling hill" like curves as opposed to sharper, more abrupt ones, correct? 

Does the same apply to the mids and highs?

Thank you!


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## ddgtr (Nov 1, 2009)

lsiberian said:


> You could certainly benefit from EQ not only in the subwoofer range but also in that area from 500 to 800hz.


I see. Do you mean the "bump" between the 500 and 800Hz? So what I need is for it to be more or less in line with the rest of the frequencies past 800?

This was one of my next questions. My system consists of a two channel system with a sub. So naturally I do not have a processor to handle the eq.

I am running foobar2000, a music program with the Electri-Q parametric equalizer. I don't think REW can talk to it, right? 

How do I eq with what I currently have? Because I think I cannot run sweeps through the eq the way I have it set up now. Perhaps there is a workaround? (wishful thinking, right?). 

Do I need to get the BFD? I would like to avoid too much processing not because I don't trust the eq but I don't know how much BFD would add/subtract to or distort my current sound. I read a couple of reviews on it but I don't know what to believe.

Thank you so much!!


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## Wayne A. Pflughaupt (Apr 13, 2006)

> So what I'm looking for in a waterfall are smoother, "rolling hill" like curves as opposed to sharper, more abrupt ones, correct?


Essentially, yes. The waterfalls also will identify room modes. They’re typically a high peak with an exceedingly long decay time, like you have at ~30 Hz.




> Does the same apply to the mids and highs?


No, waterfalls are only useful up to about 300 Hz. 

Regards,
Wayne


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## Wayne A. Pflughaupt (Apr 13, 2006)

ddgtr said:


> lsiberian said:
> 
> 
> > You could certainly benefit from EQ not only in the subwoofer range but also in that area from 500 to 800hz.
> ...


Correct.




> How do I eq with what I currently have? Because I think I cannot run sweeps through the eq the way I have it set up now. Perhaps there is a workaround? (wishful thinking, right?).


I don’t know anything about foobar, but you might be able to use REW’s RTA feature. That would let you tweak filters in real time, foobar permitting.




> Do I need to get the BFD? I would like to avoid too much processing not because I don't trust the eq but I don't know how much BFD would add/subtract to or distort my current sound. I read a couple of reviews on it but I don't know what to believe.


You certainly wouldn’t want to put the BFD in your full-range signal chain, but it’s fine for subwoofers. Assuming your system would allow you to insert it in the subwoofer signal chain.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Great!

A couple of things, just to make sure I'm understanding this correctly:

1. How would I eq the 500 to 800 range above if I am to only use the BFD for the subwoofer only?

2. Unrelated to eq; is there a way to tell if the room is too "dead" by looking at the graphs I posted? I assume it would be the RT60 graph, so what would I be looking for there? Does the plot need to be above say 0.3 sec or is there another way to determine if the room is too "dead"?

Thank you!!


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## Wayne A. Pflughaupt (Apr 13, 2006)

> 1. How would I eq the 500 to 800 range above


With foobar. If that’s not possible, it would require an outboard EQ.




> 2. Unrelated to eq; is there a way to tell if the room is too "dead" by looking at the graphs I posted? I assume it would be the RT60 graph, so what would I be looking for there? Does the plot need to be above say 0.3 sec or is there another way to determine if the room is too "dead"?


I suggest posting your RT60 graph in our Acoustics Forum. The folks there are more familiar with them.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Got it!! Many thanks Wayne!!


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## lsiberian (Mar 24, 2009)

For full range work you would need a DCX2496. I'd not suggest it until after you get the sub worked out. Doing eq on a sub is more basic than doing full-range work. One step at a time as they say.


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## ddgtr (Nov 1, 2009)

Thanks, lsiberian!

I will do that. I have a dbx223xl active crossover set at 70hz going to my front speakers and to the sub. I would insert the BFD between it and the sub... It's my next step, especiall since BFD is pretty reasonable in price.

I am trying to play a sweep tone with my player (foobar) to graph it in REW. THis way, I can play a bit with it since I have a good parametric EQ in foobar. Is there a place where I can download the same sweep tone that REW is using, or are there too many factors involved in order to get a close to identical sweep tone playing?


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## lsiberian (Mar 24, 2009)

ddgtr said:


> Thanks, lsiberian!
> 
> I will do that. I have a dbx223xl active crossover set at 70hz going to my front speakers and to the sub. I would insert the BFD between it and the sub... It's my next step, especiall since BFD is pretty reasonable in price.
> 
> I am trying to play a sweep tone with my player (foobar) to graph it in REW. THis way, I can play a bit with it since I have a good parametric EQ in foobar. Is there a place where I can download the same sweep tone that REW is using, or are there too many factors involved in order to get a close to identical sweep tone playing?


In that case you could certainly eq the L-R channels, but only do that big humps not every little variance. 

It never hurts to try. I'd insert the BFD in between the receiver and the crossover. If it doesn't workout you can always change it.:T


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## ddgtr (Nov 1, 2009)

Awesome, thank you very much, lsiberian and Wayne, you have really helped me a lot!!!


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## JohnM (Apr 11, 2006)

ddgtr said:


> I am trying to play a sweep tone with my player (foobar) to graph it in REW. THis way, I can play a bit with it since I have a good parametric EQ in foobar. Is there a place where I can download the same sweep tone that REW is using, or are there too many factors involved in order to get a close to identical sweep tone playing?


As Wayne said, best is to use REW's RTA feature. Use the REW signal generator on the Pink PN setting, which you can save to a WAV file.


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## ddgtr (Nov 1, 2009)

John, thank you for stopping by and for writing this great piece of software!!

I will go ahead and save that as a WAV file and go from there...

Going back to setting up the mic/preamp combo, I just want to make sure I have everything right in terms of the knob settings on the preamp (Xenyx 502). I am attaching a picture.

Also, from searching through the threads here I figured out I must have some sort of noise in my sound card because even with nothing playing, the SPL in REW shows about 60db. I used Wayne's instructions here, post #2 
http://www.hometheatershack.com/forums/rew-forum/33223-rew-tutorial-please-help.html
to set up the spl.

I don't think the measurements are vastly affected by it, but I will try to use another computer to see if I can lower that...


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## JohnM (Apr 11, 2006)

60dB would be unusually high, but if there is some mains hum being picked up that might contribute to it. Try disconnecting the lead from the 502 to the soundcard at the soundcard input to see whether the signal is being picked up in the 502 or in the soundcard. Assuming it is coming from the 502, try reducing the 502 mic gain to see if the signal goes down with the mic gain, which would suggest it is noise being picked up by the mic.


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## ddgtr (Nov 1, 2009)

It's in the soundcard. I disconnected the lead at the soundcard input and the REW SPL was still showing it. I'll try to setup another pc... 

Thanks!!


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## ddgtr (Nov 1, 2009)

About the REW RTA feature and being able to graph a peq'd file from an outside player (foobar):

- I got REW to generate filters for the waterfall graph and input them into the PEQ in foobar.

- Generated a Pink PN wav file and saved it, then opened it in foobar and applied the filters
- Proceeded to play the EQ'd file in foobar, outputting sound through the mains and sub.

- Under the "Spectrum" tab in REW, enabled the red RTA button. The EQ'd file currently playing in foobar showed on the graph at the bottom.
- While file was playing, I clicked the "Save" button next to the red RTA button. That captured an instance of the file playing, over the specified 20 Hz to 20k freq. range.

- I went ahead and saved the graph to my measurements folder and verified the file was there. I believe it had a .mdat extension.
- I then opened the file and it showed fine under the "Filter Adjust" tab. I got to compare it to the raw measurement.

Now here is where I had a problem, and I do not know if this is normal: I tried to plot a Waterfall graph to better see what the foobar PEQ did in terms of adjusting the problem frequencies below 200 hz. For some reason, everything was grayed out, including the "Generate Waterfall" button. I verified the the graph was loaded and showing under "Filter Adjust" but still no luck, all the options were greyed out.

I am sure I'm missing something obvious, but I cannot figure it out... 

Thank you much!!


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## laser188139 (Sep 19, 2009)

That makes sense. You are playing a pink noise spectrum, modified by the calculated filters. For REW to generate a waterfall, it has to generate a sweep itself, because it needs to be measuring an impulse. After all, a waterfall is a measure of how sound decays through time, and to have decay, it has to have a well defined stop (and start), i.e., be an impulse. 

Bill


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## ddgtr (Nov 1, 2009)

_I am trying to play a sweep tone with my player (foobar) to graph it in REW. THis way, I can play a bit with it since I have a good parametric EQ in foobar. Is there a place where I can download the same sweep tone that REW is using, or are there too many factors involved in order to get a close to identical sweep tone playing? _



JohnM said:


> As Wayne said, best is to use REW's RTA feature. Use the REW signal generator on the Pink PN setting, which you can save to a WAV file.





laser188139 said:


> That makes sense. You are playing a pink noise spectrum, modified by the calculated filters. For REW to generate a waterfall, it has to generate a sweep itself, because it needs to be measuring an impulse. After all, a waterfall is a measure of how sound decays through time, and to have decay, it has to have a well defined stop (and start), i.e., be an impulse.
> 
> Bill


I understand, I was thinking that if I could "capture" one time interval say 3 secs and since the pink noise covers the needed frequency range then a waterfall plot could be generated. I also thought the decays were present when the pink noise is played since it's a sound released into a space, and thus I could get away with an easy workaround...

So given my situation, would it be possible to somehow import a sweep tone into my player, run it and capture it into REW so I can see the effects of the eq filters?

Thank you.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Can the sweep be saved as a WAV file?

Regards,
Wayne


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## JohnM (Apr 11, 2006)

ddgtr said:


> So given my situation, would it be possible to somehow import a sweep tone into my player, run it and capture it into REW so I can see the effects of the eq filters?


Afraid not, sorry. Allowing REW to work with external signal sources is on the list for future implementation, but it will not be soon. However, the RTA works well for live filter adjustment, adjustments that make the response flatter also improve the decay times. 

For detailed filter adjustment with live waterfall update you need to make a sweep measurement with REW V5 and use the EQ window filter adjustments to see the filter effects; the Generic setting may correspond to the foobar EQ operation, or one of the other built-in EQ settings may be a good match - try measuring the shape of a foobar filter setting directly on the PC output using the RTA and save that, then compare it with the shapes of equivalent filter settings using the REW equaliser models to try and find a good match.


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## ddgtr (Nov 1, 2009)

Thanks John, as always this is some great advice! I will download V5 and try it. I am also excited (and grateful) for the future implementations!

I've tried setting a few filters on the Electri-Q PEQ plugin for foobar and after I exported the Pink Noise wav into REW it did show a smoother curve in the LF area.

The Electri-Q allows me to enter the filters manually, with fields for frequency, gain adjustments and bandwidth. What I did is found a nice Q to bandwidth calculator online and so I can fill in those values. 


On a different note: 

I switched to another PC because of the noisy card (over 60db), now it's only around 30db. But on this machine, the Input Volume is greyed out so I have to use the Level on the mic preamp and the card's mixer volume controls.

I did get both output and input boxes to show the correct devices, also the "speaker" and "line in" were correctly displayed.

With a calibrated ECM8000 - when "checking levels", before calibrating the SPL - turn up the processor's volume until the spl shows about 75db then adjust the "input volume" so the input level on the VU meter shows about -18db.

Once I turn up the processor's volume to read 75 on REW spl, when I try to get the input level down to -18 the REW spl also decreases, so I was at a bit of disadvantage trying to figure out which was the correct reading because when I turned it up it stared to clip and to climb into the -10 and up range... I had a Radio Shack meter laying around and I kind of used that to get a feeling if I was in the neighborhood of 75 when done. 

So I used a combination of mic preamp level volume and OS mixer volume controls but my thing was the 75db value I set with the processor's volume kept increasing/decreasing when either the mic preamp or mixer volumes were used so I couldn't figure out if something was off. That's why I used the RS meter to kind of get some approx guidance.

Am I missing something or is it normal for the REW spl to fluctuate while input volume is used? Because if it fluctuates, how does one know wheather the 75 db value is accurate?

Thanks, and sorry, I don't mean to ask so many questions...

Danny


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## JohnM (Apr 11, 2006)

ddgtr said:


> when I try to get the input level down to -18 the REW spl also decreases, so I was at a bit of disadvantage trying to figure out which was the correct reading because when I turned it up it stared to clip and to climb into the -10 and up range... I had a Radio Shack meter laying around and I kind of used that to get a feeling if I was in the neighborhood of 75 when done.


That would indicate that the line in is being mixed back into the output, i.e. line in is not muted in the playback mixer, so you have a feedback loop.


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## ddgtr (Nov 1, 2009)

JohnM said:


> That would indicate that the line in is being mixed back into the output, i.e. line in is not muted in the playback mixer, so you have a feedback loop.


I've followed the help files to the letter in addition to reading some of the excellent info provided in the local threads here.

Here is what I'm dealing with, and I don't know wheather it's normal or not. I am attaching screenshots of my soundcard settings, playback and recording in addition to the soundcard calibration graph and the loopback measure graph. 

After I calibrate the soundcard, I try to do the Check Level routine. I turn up the processor's volume for the REW SPL gauge to show 75. At this point, the signal already resides somewhere around -16db.

Then I move to adjust the signal to approx. -18db. I can do that via either my mic preamp's level or the soundcard's settings (rec line in).

When I do it via the preamp's level it does decrease it BUT the REW SPL also decreases (the sound level in the room does not, just the gauge). The same thing happens when I use the mixer. I just want to know whether it's normal or not and to make sure my measurements are not adversely affected by this...


Here are the first steps I used:
- Win XP, built-in Realtek card. Muted everything (including Line-in) in Playback settings except for Wave and main output volume. In the Mixer section, muted both Microphone and Stereo Mix. Left only the Line-in active.
- Cleared all previous calibration files, both for the soundcard and mic.
- Proceeded with the graph - came out nice, pretty smooth at both ends. Saved the calibration file.
- With the loopback connection still on the back of the pc, did a measurement (made sure the mic calib file was not loaded)
- Proceeded to the Check Levels...


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## laser188139 (Sep 19, 2009)

ddgtr said:


> ... When I do it via the preamp's level it does decrease it BUT the REW SPL also decreases (the sound level in the room does not, just the gauge). The same thing happens when I use the mixer. I just want to know whether it's normal or not and to make sure my measurements are not adversely affected by this...


Yes, this is normal. You are setting the dB levels on the VU meters to ensure that the output and input levels have about the right signal-to-noise ratio. If you adjust the incoming gain, either at the pre-amp or on the soundcard mic/line input, the reported SPL will change, too. That's all right, because you verify the levels before you do the Calibrate SPL procedure. When you go on to perform Calibrate SPL, at that point you are telling it the current real SPL level, as reported on your external meter, and it sets its internal reporting to match. 

Bill


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## ddgtr (Nov 1, 2009)

Thanks Bill!!

Downloaded V5 and it's great, thanks John!

Question on the Equalizer settings, I'm using a plugin parametric eq for foobar. So am I correct to assume "generic" is the setting I want? I noticed that generic gives me the most amount of filters.

Also, do I select the 24 db octave in the eq's bandpass option to match the REW'S filter settings?

Thanks!


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## ddgtr (Nov 1, 2009)

I've been moving the sub around to see where I can get the best response. I had to take 2min breaks every 5 feet!! I'm now beginning to look like a bodybuilder, that 15" sub is heavy!!

I've managed to find a sweet spot for it, here are 2 graphs, one before and after EQ. This is however the software parametric eq so I would not be able to measure the real changes until John adds that feature in the far future.

I will run a RTA comparison and that should tell me if I'm close.


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## ddgtr (Nov 1, 2009)

Hi everyone,

I've been playing around with REW 5. I have some questions, I think I'm stuck because I do not understand some basic principles. If someone would be so kind as to explain this keeping in mind I know nothing about octaves and such...

1. Smoothing: How do I use it to my advantage? Let's say I take a measurement, and it defaults at 1/48. If I set it to say 1/12 in the controls tab, is that just so that I can better see what's going on with the freq. response??

2. EQ'ing: I am using a software based parametric EQ. Should I check the "generic" tab? I noticed that there are different EQ filters for different EQ types. Shouldn't they be pretty close??


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## Wayne A. Pflughaupt (Apr 13, 2006)

It’s best not to use smoothing on subwoofer graphs. However when you get too the upper frequencies, often the measurement looks really “ragged,” which is due to come filtering. Smoothing takes away some of the “raggedness” and lets you see the underlying trend in response.

Yes, for any EQ not specificially listed in REW, use the Generic setting. However, the filters of different equalizers have different response characteristics. So, if you take a measurement, tweak a filter on screen, apply it to the equalizer and re-measure, you may well find that the results aren’t what was predicted. So, I’ve found it best to use REW’s RTA feature for other equalizers. That way you can tweak filters and see the results in real time.

Regards,
Wayne


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## Joppnl (Nov 2, 2010)

About the “raggedness”: I noticed that when I remove the loop-back cable on the soundcard and tell REW that i did so the “raggedness” has disappeared and the graph is much more readable.

Should i remove the loopbackcable, especially for these graphs (may-be for waterfalls and RT60 the readout is better, I don't know...)

Thnks!


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## Wayne A. Pflughaupt (Apr 13, 2006)

How did you do a measurement with a loop-back cable in place? That would mean no mic was plugged into the sound card, and no audio from REW was going to the sound system. :huh:

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Wayne A. Pflughaupt said:


> It’s best not to use smoothing on subwoofer graphs. However when you get too the upper frequencies, often the measurement looks really “ragged,” which is due to come filtering. Smoothing takes away some of the “raggedness” and lets you see the underlying trend in response.
> 
> Yes, for any EQ not specificially listed in REW, use the Generic setting. However, the filters of different equalizers have different response characteristics. So, if you take a measurement, tweak a filter on screen, apply it to the equalizer and re-measure, you may well find that the results aren’t what was predicted. So, I’ve found it best to use REW’s RTA feature for other equalizers. That way you can tweak filters and see the results in real time.
> 
> ...




Wayne, thanks for the detailed info. As always, your help is much appreciated...

I have been using the generic setting for my sub. I did find that, at least with EletriQ software parametric EQ plugin for foobar, the REW filters work quite well. I ran REW RTA through foobar after I applied the filters and compared it to the original, un-filtered measurement. For the 15 to 200 Hz, it clearly matched the "predicted" REW graph which is pretty cool... The sound also improved within that range, but I did have to turn up the sub a bit.


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## Joppnl (Nov 2, 2010)

Wayne A. Pflughaupt said:


> How did you do a measurement with a loop-back cable in place? That would mean no mic was plugged into the sound card, and no audio from REW was going to the sound system. :huh:
> 
> Regards,
> Wayne



Well: sound card is stereo and what i read in the manual is that right output is the output to the amplifier, right input is the input from the measurement mic (or SPL meter) and left in-out is connected to each other by a loop-back cable (and as I read REW is using this for some kind of correction)

But with left in/out loopback cable I get a very spiky graph in the higher freq regions (as you can see on the graphs on page 1 of this thread) and without loopback (and telling the sotware that there is no loopback (preferences-analysis-'use loopback as timing reference) the graph is much more clearer : without all the spiky stuff.


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## Kenneth R. (Oct 20, 2010)

Hi Joppnl, the idea of the loopback is that you're measuring the quality of your sound card on the input and output that you want to measure with. So if you did the calibration with the left in and out loopback, you should disconnect that and connect your SPL meter or mic to the left input and have the left output go to your speaker or receiver deck. This way, you know what you're seeing is not influenced by any distortion in the sound card. 

If you get a really spiky graph, make sure that your sound card monitoring is off for that channel.


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## Joppnl (Nov 2, 2010)

first thank you all for your effort to reply.

But: I do not agree.

OK: you can loop the in/output you use for input (the microphone or spl) and the output (to the amp) and make a calibration file REW uses during measurement where this input is connected to the mike and the output to the amp.

However: the mic/amp connected as said you can loop the other in/output during measurement.

Have a look at the REW helpfile, -> getting started with REW -> Connections.(I use the latest REW version, V5)

Look at the section starts with : .The other input and output channels do not need to be used for basic measurement. (but read on....)
However, it is also possible to use a loopback connection from the soundcard's left output to its left input as a timing reference....(continues).

But when I do this the graph is very much spiky, as is on the graphs on the first page of this thread. When I remove this loopback it's perfect (well, besides the normal going up/down of the line because of the room and speaker acoustics)

But if you remove the extra loopback you have to uncheck the 'loop-back' box in the software as well.

PS: if I have a spiky graph I can use the smooth function which will make it disappear.

So the question is: which is the best way: without loopback and without the need of smoothing or with loopback and smoothing...


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hard to say since you haven’t given us any graphs to look at for analysis. There might be some crosstalk between the two channels of the sound card, if they’re poorly isolated. However, once the sound card calibration is completed there’s no need for loopback. I wouldn’t measure with it in place, especially if you’re getting something like this.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Joppnl, can you post both your soundcard calibration graph and the "spiky" measurement graph you are trying to describe? Perhaps we can better understand the issue...

Could it be that you are talking about this method in the "cabling" how-to section??

_Alternatively, for more advanced users, you may loopback connect the left channel line-out of the soundcard to the left channel line-in of the soundcard, and use the right channel for measuring and this will serve as the soundcard calibration in lieu of a calibration file. To use this special feature, you must check the 'Use Left Channel as Calibration Reference' in the REW Settings page. This method is not recommended. The standard cal file method is recommended. It's simpler, uses less cables and so less chance of problems, offers a bit better S/N ratio, and provides vetting of your setup and connections. *The Left channel calibrate method is best used once you've become familiar with REW and want to test System Delays. Other than that, stick to the standard soundcard calibration file method.*

Read more: REW Cabling and Connection Basics - Home Theater Forum and Systems - HomeTheaterShack.com _

For sound card calibration, you should just have the 3.5mm jack in connected to the 3.5mm jack out of the soundcard. This is the loop as we all understand it. Then, get your stereo 3.5 mm to rca adapters, and after the soundcard calibration, connect the meter to the in jack's right rca and the processor to the out jack's right rca.

You leave both lefts alone...


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## laser188139 (Sep 19, 2009)

ddgtr said:


> Joppnl, can you post both your soundcard calibration graph and the "spiky" measurement graph you are trying to describe? Perhaps we can better understand the issue...
> 
> Could it be that you are talking about this method in the "cabling" how-to section??
> 
> ...


You are right, Danny, that pictures might help. 

Your hypothesis occurred to me, too, that using the loopback as the calibration instead of a soundcard file might explain the symptoms, but I discarded it looking back at the earlier graphs, which appear to be from REW v5. REW v5 uses the loopback cable differently, for timing, and still uses the soundcard calibration file. I missed the fact that the pictures were yours and this is a different user, Joppnl, and he might be using REW v4. 

Bill


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## Joppnl (Nov 2, 2010)

That's it!

As Danny posted: that's the thing!

I use V5. 

But in my help file i didn't read that it was not recommended and only for timing measurements.

What does "in lieu" mean (I'm not native english speaking) Is it "in stead of" or do they mean that you use and the loopback cable and the soundcard cal file?

I used both.

When I found out that removing the cable made the spikes disappear (see on page 1 of this thread: Wayne calls them "raggedness") I did delete all the measurements with the spikes so I can't post it now (but if you want I can make new ones)

Wayne says that the spikes (or "raggedness" comes from reflections in the room (see the story and pictures he posts for a hard, reflecting room (with the 'spikes') and the picture of a more dampened room

So: what to do for a correct measurement in the frequency domain (dB on vert axis, freq on hor. axis):

- with loopback cable and with soundcard cal file
- with loopback cable without soundcard cal file
- without loopback cable with soundcard cal file


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## JohnM (Apr 11, 2006)

Always use the soundcard cal file. If you need an absolute time reference for phase measurements or comparing timing on different measurements, connect the loopback. If you do not need that, there is no need for the loopback. The loopback should not make any difference to the appearance of your measurements. Please post images of a measurement so we don't have to guess what they might look like.


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## Joppnl (Nov 2, 2010)

OK: here are the pictures.

Setup: home th PC, USB soundcard (Behringer UCA202) Yamaha RX-V1700 receiver, Infinity Overture 2 speakers, RadioShack SPL meter

1st graph: with loopback on left in/output of the USB soundcard, software checkbox checked for loopback (in pref's), no smoothing










2nd graph: same setup but without loopback (and in setup the loop-box unchecked)










As you can see: the first graph there are a lot more 'spikes' or otherwise said, the graph looks ragged.

2nd graph is much more 'readable' now without the loopback

What are these spikes? Are these reflections (my living room is 'hard': wooden floor, no carpet, little curtains etc)

Last graph: with some roomcorrection from 30 - 120Hz:
Setup: home th PC -> optical out to Behringer DEQ2496 -> optical to Yamaha receiver. USB souncard is only used as analogue input for the SPL (as I can not use a loopback on the Behringer (since there is no output signal on the analogue out (as it's coming via optical) no spikes as well))










As you can see I managed to get rid of some bass-peaks but tweaking can be done a lot better (I think)

Problem is that when I tell REW to auto-eq 30-120Hz it uses 9 out of the 10 available PEQ's so now I've only 1 left (as the DEQ2496 has 10)

Tweaking manually (and using 1 PEQ to get 1 peak down) wasn't succesful: the peak became smaller but the level left/right from the peak also went down. Making the PEQ smaller didn't work because then the peak in the middle was ok but left/right from the middle appeared 2 (smaller) peaks because the PEQ was to small....

Question: as you can see in the graphs, from 3kHz and above the level is increasing. Do you think this is due to ehe inaccurateness off the RadioShack digital SPL meter?

Thanks for your help, much appreciated,


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## laser188139 (Sep 19, 2009)

Joppnl said:


> ... As you can see: the first graph there are a lot more 'spikes' or otherwise said, the graph looks ragged.
> 
> 2nd graph is much more 'readable' now without the loopback ...


I know what you are seeing. 

When there is no loopback cable, and Allow 96 PPO Log Spacing is enabled in Preferences -> Analysis, REW massages the impulse response to eliminate information at the high end that is not useful. Part of this massaging includes implicit 1/24 octave smoothing (see Preferences -> Analysis in the helpfile). 

When the loopback cable is connected and the preference are set to use the left channel for timing, REW retains absolute timing information in the impulse data. When retaining this impulse timing information, it ignores the 96 PPO Log Spacing option. 

If you want to verify this explanation, you can go into Preferences -> Analysis and turn off the Allow 96 PPO Log Spacing option, then measure again with no loopback cable. The SPL response curve should appear identical to the one with the loopback cable present. 



Joppnl said:


> ... Question: as you can see in the graphs, from 3kHz and above the level is increasing. Do you think this is due to the inaccurateness off the RadioShack digital SPL meter?


Absolutely, with the RS meter you really cannot be confident in whatever you see above 3kHz. 

Bill


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## Wayne A. Pflughaupt (Apr 13, 2006)

Joppnl said:


> As you can see I managed to get rid of some bass-peaks but tweaking can be done a lot better (I think)
> 
> Problem is that when I tell REW to auto-eq 30-120Hz it uses 9 out of the 10 available PEQ's so now I've only 1 left (as the DEQ2496 has 10)
> 
> Tweaking manually (and using 1 PEQ to get 1 peak down) wasn't succesful: the peak became smaller but the level left/right from the peak also went down. Making the PEQ smaller didn't work because then the peak in the middle was ok but left/right from the middle appeared 2 (smaller) peaks because the PEQ was to small....


It’s highly doubtful that you’re really requiring that many filters. The goal of sub (or any) equalizing is not to get ruler-flat response. I suggest checking out my minimal EQ article (you can find a link to it in my signature).

Regards,
Wayne


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## Joppnl (Nov 2, 2010)

@Bill: YES!! that's it! I verified by unckecking the 96..spacing and, without loopback cable, the 'spikes' where there again.

Thank you for your help.


@Wayne: thank you as well: I read the article and that's exactly what I'm doing: equalizing the equalizer (very well said) Next I'll try to use less eq, play with the goal dB to get as less filters as possible. 

Well...not only REW is a fantastic program also the support is great, thank you for both!


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## JohnM (Apr 11, 2006)

If you would like REW to use fewer filters for its auto EQ just set the ones you want to reserve to Manual instead of Auto, or disable the ones you do not want REW to use.


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## ddgtr (Nov 1, 2009)

Between wife, kids, friends and work I finally got a chance to play with the RTA in REW5 and see how the Auto EQ feature works in Generic mode. For those who do not want to read the entire thread, I am using a software based parametric EQ (Electri-Q) with foobar2000.

So I ran the Auto EQ, got my values (thanks Wayne for the "raising the Target Level" technique), then input them into Electri-Q which has a comparison mode where with one click you can compare against a non eq'd mode. 

Ran the RTA feature and switched back and forth between the eq'd and non eq'd modes and it showed the same result as initially predicted by REW!! I thought that was pretty cool!! It's good to know that both pieces of software were dead on!

The next step is to actually listen to some music and see which one I like better. Then I'll get working on the hard-knee house curve and compare everything... 

I'll post some graphs later tonight.


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## ddgtr (Nov 1, 2009)

Here are a couple of graphs, the first one RTA with no EQ and the second one is with EQ applied. Really nice because it matches the REW predicted graph... 

Still have to do comparative listening...


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## ddgtr (Nov 1, 2009)

@Wayne:

Here is my feeble attempt at a hard-knee curve. I know it doesn't look right...

I've been reading your article and I notice that my problem is that my house curve slope is only 0.4 (30 to 70 Hz) while most midway points have higher values. How do I address this issue in order to create a successful hard-knee curve? Do I shelve at a lower point? 

Here is a copy of my file and a graph: 

30 0.4
35 4.7
40 4.6
50 7.3
55 5.5
60 6.7
70 0.0


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## Wayne A. Pflughaupt (Apr 13, 2006)

ddgtr said:


> @Wayne:
> 
> Here is my feeble attempt at a hard-knee curve. I know it doesn't look right...
> 
> ...


Hey ddgrtr,

Your beginning and ending frequency have the same values – 0 – which is not what we want.
Your beginning frequency (70) should be 0, and the ending frequency (30) should be the amount (i.e. how steep) of a slope you want, such as 6, 7 etc. 

So in your case:
30 5
70 0

All figures in between should be lower than 5.

It’s a tricky “trial and error” process to create a hard-knee file. Using your figures as an example - I started with the two-figure file and used a piece of paper (or some other straight edge)to line-up the 30 and 70 Hz points on the graph (as an temporary straight line). I picked a frequency half-way in between 30 and 70 and created a new file that included it...

30 5
50 3
70 0

... and checked to see what that got me. If 50 Hz was hitting the temporary paper line, then it was a good figure. If it was above or below, I adjusted the “3” figure accordingly and tried again. Once I got 45 Hz nailed down, I chose two additional intermediate frequencies:

30 5
40 xx
50 3
60 xx
70

Again, I experimented with different values to get 40 and 60 Hz aligned with the temporary line from the paper straight edge .

Since writing the article, REW now has a “logarithmic interpolation” feature that can be used instead of a difficult house curve file like this. It basically flattens the line between a simple two-figure house curve (such as the 70 0, 30 5 curve you’re using. Look for the “logarithmic interpolation” box in the Settings window, “House Curve” tab.

Regards,
Wayne


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## ddgtr (Nov 1, 2009)

Great! It's become more clear, I will go ahead and implement that tonight! Thank you again for taking the time to work that scenario out! 

Playing with the test tones I could really see where the nulls are in the room, it was pretty interesting. At the listening position I could hear say a 50Hz just fine, but if I moved around the room I could find one spot where it was almost not audible at all!

When positioning the subwoofer within the room, I went after a flat response at the listening position. However, there are still some standing waves in all four corners where it's louder by some 8db. If I move the sub around a bit, I can get rid of most of the standing waves but the graph is not as flat. What would your recommendation be in regards to this?

Thanks again!


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## Wayne A. Pflughaupt (Apr 13, 2006)

There are a couple of approaches you could try. One would be to simply equalize for the listening position. The other would be to take readings at various locations, and REW could average them, and you could equalize based on the result.

Regards,
Wayne


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## laser188139 (Sep 19, 2009)

I would go for whichever sub position gives the flattest response at the listening position or positions. For most of us in a home environment, the listeners are grouped in one part of the room, so problems in other areas don't matter. You can use REW to measure over all your listening positions, and look at the overall trend or look at the average. Obviously, if your primary use is for parties where people will be all over the room, that is a different situation. 

Sometimes the position that gives the flattest response does not give an adequate volume level without straining the sub. Harman Kardon, among others, have published several articles on the best positioning of multiple subs to give an even response over a larger area. 

Bill


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## ddgtr (Nov 1, 2009)

Thank you Bill and Wayne!

I do use my system equally for movies and 2 channel listening, and it's usually just my wife and I with occasional guests, so it looks like I will go for the listening position scenario. One thing, I do have plenty of headroom with my sub, it's a 15" Velodyne CT-150 model and the volume knob is set at 3 now (from 1 to 10). It looks like even with the EQ in place, going to position 4 should suffice.

Your input is as usual much appreciated!!


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## ddgtr (Nov 1, 2009)

Ran the RTA feature today. Set up the eq as follows:

30.9 hz -5.5 db Q 7.31
69.7 -5.3 7.64
74.5 3.5 3.0
117.0 4.0 4.0
380.0 3.0 3.0
8,208 2.2 0.67

The last filter is as you can see, very broad.

I have not tried to listen to any songs yet, I will do it tonight after the kids go to bed.


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