# Help measuring/interpreting graphs



## dachness (Feb 17, 2009)

Hello All,

I am have Sony STR-510 AVR, with SVS SCS-01 LCR and HSU VTF-15H sub. Measurement equipment, Macbook Pro, RS Digital SPL meter. I am in the process of getting them level matched and trying to find best location/eq for sub.

I level matched them using the DIY Audio calibration dvd V2.0 found here: http://www.avsforum.com/avs-vb/showthread.php?t=742969

When comparing this to the AVR test tones, it looks like the sub is 10db hot. This is also reflected on REW if I choose calibrate SPL with sub vs Calibrate SPL with speaker. So first question, is that test disc incorrect? 

My other concern is whether my crossover in my AVR is working correctly. I suspect that it isn't (I may have an incorrect sound effect enabled not sure). For REW setup, I have audio going into the AVR into both L & R, speakers set to small crossover at 80hz. 

On the following graph, green is with both speakers and sub playing. Yellow is same measurement with the sub turned off via its amp. The output of the spakers in this lead me to believe the crossover isn't working correctly. It also looks like that is why the lower frequencies are elevated, due to output coming from both the speakers and the sub.










Another concern is the dip at 83hz on the green plot. The sub is oriented in the same direction as the mains.

Here is a graph of an previous measurement I took with the sub facing horizontal to the mains. Note that this was before I did the above level matching with the DIY disc. Notice how I do not have a dip in the 80-90hz area. 










My goals are the setup my sub/mains for proper interaction with each other with best frequency response. This is my first attempt on taking measurements with REW. I would appreciate any advice on what type of measurements I should be taking. I have only measured through the AVR, should I take measurements with output going directly to the sub?

Thank You,

Daniel


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

Hi Daniel,



> When comparing this to the AVR test tones, it looks like the sub is 10db hot. This is also reflected on REW if I choose calibrate SPL with sub vs Calibrate SPL with speaker. So first question, is that test disc incorrect?


Can’t say, knowing nothing about your test disc. However, if it’s only sampling one speaker at a time, that could account for at least some of the discrepancy: When you add the main speakers you’re getting an additional 3 dB or so per speaker in the frequency range where they overlap the sub - ~35-80 Hz in your case.




> My other concern is whether my crossover in my AVR is working correctly. I suspect that it isn't (I may have an incorrect sound effect enabled not sure). For REW setup, I have audio going into the AVR into both L & R, speakers set to small crossover at 80hz.


So many things come into play here. You have to keep in mind that the crossover is not a “brick wall.” Acoustical and other factors can extend response substantially below the crossover frequency, such as the speakers being “oversized” for the room (i.e., with bass output and extension that would be better suited for a large room), being located near corners (that boosts bass output), or even EQ from the receiver. 





> On the following graph, green is with both speakers and sub playing. Yellow is same measurement with the sub turned off via its amp. The output of the spakers in this lead me to believe the crossover isn't working correctly. It also looks like that is why the lower frequencies are elevated, due to output coming from both the speakers and the sub.


I’d agree about the overlapping. Your best bet to tell what’s happening with the crossover is to take separate measurements of the mains with the receiver set for small, and full range. With some receivers, the crossover only works when Dolby Digital is engaged.




> Here is a graph of an previous measurement I took with the sub facing horizontal to the mains. Note that this was before I did the above level matching with the DIY disc. Notice how I do not have a dip in the 80-90hz area.


 It’s best to keep comparisons as consistent as possible, so both measurements should be taken either before or after using the test disc. Naturally, in the end you want to use the sub positioning that gets the smoothest response...




> My goals are the setup my sub/mains for proper interaction with each other with best frequency response. This is my first attempt on taking measurements with REW. I would appreciate any advice on what type of measurements I should be taking.


For what you’re doing, basic frequency response measurements are best. Response measurements will help you with sub positioning, phase-control adjustments, etc. After that, use your preferred method for adjusting the speaker levels.




> I have only measured through the AVR, should I take measurements with output going directly to the sub?


Generally, through the AVR is what you want. You can’t tell sub/mains interaction, the effects of phase adjustments, etc. with the sub only.

Regards,
Wayne


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## laser188139 (Sep 19, 2009)

dachness said:


> ... I level matched them using the DIY Audio calibration dvd V2.0 found here: http://www.avsforum.com/avs-vb/showthread.php?t=742969
> ... So first question, is that test disc incorrect? ...


There are variations among all the test disks on where they define the sub range, i.e., how they filter the pink noise to create a "sub" test signal. When I measured the DIY test disk at my AVR preout, it started a little higher in frequency at the bottom end, and went much higher at the high end, than other test disks -- it appeared flat from over 50Hz - 200Hz, where the AVIA disk was flat 40Hz-70Hz, both with very sharp filters at the bottom end, while my receiver's test signal and the REW signal roll off much more gradually, and are flat 30Hz-90Hz. 

Because DIY disk's spectrum runs so high, and does not go as low as the others, this can bias your results if you have the crossover set to a typical, low frequency in the receiver. If you temporarily raised the crossover in the receiver to its maximum value, 200Hz in your case, adjusted the levels using the DIY disk, then lowered the crossover back to your desired setting, e.g., 80Hz, you will probably end up with the same level that you would have if you used the REW or the Sony receiver's test signals.

The nice thing about the DIY spectrum is it avoids the area where the A and C weighting of the SPL meter has an impact. Your sub goes much lower in frequency than 40Hz. Your RS SPL meter uses C weighting so it will read a little lower than will REW's flat weighting, assuming they read the same on the main speakers. I would trust the REW SPL meter's flat weighting for leveling more than your RS SPL meter's C weighting, but you might find you need the sub's level raised slightly if you don't have an equalizer to introduce a deliberate house curve. 

Wayne has already mentioned that you need to look at these with the sub and one front speaker at a time for them to appear level, that the sub will appear elevated when driven with two identical signals. 

Below 20Hz, I would not trust that your RS SPL meter matches exactly the generic calibration file. So don't let the apparent peak bother you, unless you hear it with your own ears. 

Like Wayne, I am puzzled that you are not seeing more effect from the front crossover in the yellow curve. That would make sense if you were in Sony's 2ch stereo or analog direct modes, but then you would not hear any output from the sub. 

Regards,
Bill


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## dachness (Feb 17, 2009)

Wayne and Bill, thank you for the feedback. I will test one speaker at a time with the sub and also see what affect changing speakers from small to large has. Will post results when I have them.

Daniel


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## dachness (Feb 17, 2009)

Took measurements with sub and right speaker. Red line is sub and right speaker. Blue is with sub turned off from amp. Purple is with sub off and speaker set to large.










It looks like my right speaker has a dip at 75hz. Wayne, regarding my setup, you are probably correct about room/size of speakers. Room is 12x12x8. 

How should I proceed from here?

Edit: Decided to also measure sub only, by removing right speaker wires from AVR. All measurements in this set are with speaker small crossover at 80hz. Image below shows red: both sub and speakers playing, green: Sub only, purple: speakers only. I also measured the sub only with speakers set to large, and measurement was pretty much the same as speakers set to small.


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## laser188139 (Sep 19, 2009)

It looks as if your sub is still hot relative to the main. 

The main Large vs Small is still peculiar. It is almost as if your receiver implements only a first order high pass filter on the fronts, providing 6db/octave. This may explain part of why the low end sub+main seems so high.

As your receiver provides a preout to the sub, you could measure this directly, with no mic nor mic calibration file, and determine the slope of the sub side of the crossover. If the main slope is only 6db/octave, you would expect to find the sub's low pass filter in the receiver to be either 6db/octave or 12db/octave. 



dachness said:


> ... It looks like my right speaker has a dip at 75hz. ...


This is one of the reasons to use a sub. 75Hz would be consistent with the right speaker being 3'9" from the corner (1/4 wavelength at 75Hz, generating a 1/2 wave reflection). By setting the sub's crossover higher than this, you let the sub fill in for the main below this point, so this dip won't matter so much to you. 

Bill


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

I agree with Bill, it looks like you’re only getting a 6 dB/octave action from your receiver’s crossover. In your first graph from Post #5, notice that the mains signal w/crossover is only about 6 dB different from the full-range signal (40 Hz is one octave below the crossover frequency of 80 Hz). By contrast, most receivers these days have a 24 dB difference an octave below the crossover point.

What you might do: Does the receiver have a bass control that you could set for 80-100 Hz? If it does, and if it has no affect on the subwoofer, you could reduce the bass control. That would help attenuate the main’s bass output and concurrent overlap with the sub.

Regards,
Wayne


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## dachness (Feb 17, 2009)

I will measure the sub output from the AVR with out any calibration files loaded. The AVR only has a Bass and Treble adjustment in +/- dB. I will fool around with that to see what affect it has.

Thank you, will post new graphs when available.


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## dachness (Feb 17, 2009)

Finally had time to measure sub output directly from AVR. It looks like it matches up to a 12db slope.


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## dachness (Feb 17, 2009)

I corrected the graph above with sound card calibration.


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## dachness (Feb 17, 2009)

First image: Sub and speaker, Red is base. From then on I decreased the bass setting in avr by -1 db to -3 db.

Second image: Sub and speaker, Yellow is base. From then on I decreased sub output in AVR by -1, -2 and -4 db.

I am still unsure how I should be level matching the mains to the sub. If I make it level based on REW's measurements(graphs) would it play LFE at correct volume? 

On a side note, I should have a BFD 1124P on its way(I hope). I was the high bidder on one.


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## JohnM (Apr 11, 2006)

dachness said:


> I corrected the graph above with sound card calibration.


The calibration trace rolls off suspiciously early, from the effect it has on the measured output I would say it is not valid - some other setting may have been causing early roll-off.


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## dachness (Feb 17, 2009)

Thanks, will recalibrate the sound card.


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## dachness (Feb 17, 2009)

JohnM said:


> The calibration trace rolls off suspiciously early, from the effect it has on the measured output I would say it is not valid - some other setting may have been causing early roll-off.


Thanks John. There was definitely something funky going on. Recalibrating the sound card by itself didn't seem to fix it. I installed REWb5 as well and sound card calibration was showing -3db at 1khz. 

I ended up doing the master reset and close of REW and then redid calibration which looked good. Attached is the sound card calibration file. Things should look smoother now 30hz and down with the corrected sound card cal.


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## dachness (Feb 17, 2009)

*Got a BFD1124P setup and first EQ attempt*

Wanted to post my results for my first EQ attempt. I measured the sub only through the AVR. The sub output has a 12dB/Octave crossover at 80hz.

I EQ'd the sub for the target curve. I raised the target curve to match up with the slope above 80hz and then dropped the rest of the curve down. I added a couple more filters than what REW recommended one around 31hz and another in the crossover area.

After EQing the sub I then level matched the sub to the left speaker using AVIA.

At a point I thought there was a checklist for minimal EQ on how wide the bandwidth should be and how much gain a filter should have before you should drop it, however I didn't find it. If I should remove any of my filters let me know.

Look forward to hearing feedback.

Daniel


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