# Sound Card To Sub Measurements



## Intuition (May 5, 2009)

Hi,

Can anyone clarify what kind of problems i would run into if i have no a/v receiver in the chain to my sub?
I have ordered a galaxy cm-140 check mate and want to avoid any obvious or even less obvious errors in physical connections to and from my mfw-15/sound blaster x-fi extrememusic setup.
Problem is the only way for me to LFE to my sub is to enable 5.1 channel audio and then enabling Bass Redirection to remove non-LFE spectrum otherwise i end up measuring 200hz and up.
I have read that multi channel audio is a no-no but here is my setup anyways :

X-Fi=>Center-Sub-Outputs Split up (Center RCA goes to Center amplfier then to Center speaker, Sub RCA goes to MFW-15 LFE) then when i receive my CM-140 can i loopback from X-Fi-Sub-Out to Line-In to create soundcard cal file for accurate measurements or do i have to front left/right channel to Line-In left/right? or what else can i try if no receiver present?

Thanks

Ian Bester


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## brucek (Apr 11, 2006)

> what else can i try if no receiver present?


You'll need to use your computer for REW just the same as you would if you had a receiver (with soundcard in stereo mode and using SPL meter to line-in and line-out to receiver). Except in your case, you will feed the line-out to your subwoofer. Yes, you will not have the advantage of a crossover from a receiver, but this isn't that big a deal. It will allow you to test your sub though.

brucek


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## Intuition (May 5, 2009)

Thanks for the reply brucek, i have been taking measurements with my multichannel setup and a cheap computer mic and it looks kinda "accurate" when i listen to the signal generator tracking the cursor from say 15 to 200hz but i dont know how many dBs this mic might be "out" but peaks sound like peaks and nulls like they should so maby if i try to see the difference between the 2 setups and if i dont find any i might stay at multi channel? or might i find strange problems later down the line?


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## brucek (Apr 11, 2006)

> i might stay at multi channel?


REW is not to be used with multichannel, as it's a mono signal. The use of soundfields produce unreliable results. The only mode REW should be used in is stereo.



> cheap computer mic and it looks kinda "accurate"


Very unlikely that it's accurate. You require a mic or SPL meter with a calibration file.

You're not getting reliable or accurate results. If you're measuring, then the goal should be to get a generally accurate return that can be used to facilitate EQ or speaker placement or treatments, etc. What would be the use in measuring otherwise?

brucek


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## Intuition (May 5, 2009)

firstly "accurate" doesnt mean accurate thats why i put it in quotes and like i said the cm-140 is on its way though it sounds pretty accurate to the ear (cheap mic) turning off the amplifier and measuring only the sub doesnt have a real difference in the 15-80hz range, also it is mono cause its only the sub playing and not the center, right, left, right-rear or left-rear so i need a justifyable reason not to use the sub out and only the stereo left/right out to my sub for measurements and i do have equivalent OC703 treatment in the room but a really big peak at 44hz aka. my room's mode :bigsmile:

the question is if i get the same measurement using the cm-140 with the 5.1 sub out against the stereo left/right out to my sub why keep it stereo?

thanks again for the response :wave:


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## brucek (Apr 11, 2006)

You're attempting to use your computer as both (the test tool that REW runs in), (and the unit under test). This can't be done, since you require the single channel line-out to line-in connection for soundcard calibration. You need to use the PC as the calibrated test tool, and then feed the sub on its own as an external unit under test.

brucek


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## Intuition (May 5, 2009)

what do you mean feed the sub on its own as an external unit under test?


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## brucek (Apr 11, 2006)

First calibrate the soundcard by creating a soundcard calibration file by using the right front line-out and line-in channel (with PC in stereo only mode).

Use that calibrated right front line-out channel as the signal feed to the input to the subwoofer. Use the calibrated right line-in channel for the SPL meter.

brucek


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## Intuition (May 5, 2009)

Thanks, having received my CM-140 im having more problems :

my sub sound isnt very loud and the cm-140 is already registering around 86db on dbC - 50-100 - slow (fluctuating alot)
and if i decrease my sub gain my Line-In input level becomes alot less than -18db so im struggling to get a good balance all together my line-in record level is at 100% also if i do a sub measurement its asif all frequencies that are measured arent consistent eg 30hz is 5db louder on signal generator than the 200hz sweep while 100hz is only 2db louder on signal generator giving me a red flag on accuracy. im using the cal file on download section with c weighting unticked following all help instructions to the letter. Please Help hehe using windows 7 ultimate so the volume levels are different than windows xp with wave volume at 100% and main volume at 50% isnt possible with vista/7. just cant get it calibrated to 75db with high enough input level (-18db in and out), 86 is best i can do atm.

o btw when i calibrate the sound card to stereo loopback on right channels (-18db in -18db out) and do a sweep on loopback with no cal file i get a 87db response from the loopback, isnt that too loud ?

Thanks again


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## brucek (Apr 11, 2006)

> when i calibrate the sound card to stereo loopback on right channels (-18db in -18db out) and do a sweep on loopback with no cal file i get a 87db response from the loopback, isnt that too loud ?


When you are measuring a cable from line-out to line-in, there is no actual SPL level reference, so you have to create one. This is why you must run the Calibrate routine after the Check Levels routine and set the level to 75dB. The Calibrate button is located on the REW SPL meter.

In fact, when you take normal measurements with the SPL meter, you must also run the Calibrate routine after the Check Levels routine also. This tells the REW SPL meter what the actual SPL level is or it has no reference.



> if i do a sub measurement its as if all frequencies that are measured aren't consistent eg 30hz is 5db louder


There are a couple factors to consider. You're forgetting that the SPL meter has a C-Weight filter turned on. This is to compensate for the poor human hearing response. The C-Weight filter emulates that. We undo it in REW to make it flat after you take a measurement though.
You also have to consider room effect. It tends to increase the level of low frequencies, especially in corners.

Run and complete the Check Levels routine, setting the level at the listening position to 75dBSPL. Then run the Calibrate routine. Then set the Input Level of REW to -12 to -18dBSPL. If you can't attain the -18dB at that point, it may be a problem with the Mic or the cable or the connectors you're using.

brucek


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## Intuition (May 5, 2009)

Thanks for the reply but how will i ever know that everything is ok and not completely unaccurate if there can be so many factors causing questionable results? If i give you screenshot will you be able to approve if everything is fine or somethings off? are you telling me the measurement graph isnt c weighted but manual signal gen is or is it the other way around? Completely confused at the moment because i dont know if i should be looking at c or non c wighted curve to equalize that result. also i have done everything in order sound card cal -> check levels -> mic cal -> calibrate SPL.

I can get a -18 out/-18 input level by increasing my soundcard main volume control but that would make everything very loud on all levels including games, music, movies...


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## brucek (Apr 11, 2006)

> that everything is ok and not completely unaccurate if there can be so many factors causing questionable results?


Just like any test equipment, REW has to be set up properly to return a reliable result.
There is a fairly foolproof method of checking the accuracy, by running a measurement of the loopback cable that was used to setup the soundcard calibration routine. If the result is a straight line, then you have ensured yourself that the test equipment is accurate.

There can still be inaccuracies though as a result of using generic meter calibration files, but they're fine for home use.



> are you telling me the measurement graph isnt c weighted but manual signal gen is or is it the other way around?


The SPL meter is C-Weighted. REW removes that C-Weight by using a calibration file that applies the opposite filter characteristics. The measurement graph result is a flat response over the bandwidth of the file.



> I can get a -18 out/-18 input level by increasing my soundcard main volume control but that would make everything very loud on all levels including games, music, movies...


The levels we set REW to for taking a response measurement and creating a graph have no influence on the levels you set your AV systems volume control to when you watch movies, etc. They aren't related.

brucek


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## Intuition (May 5, 2009)

Ok im sitting less than a meter away from the rear of the room so that might be the reason the cm-140 is giving such a high fluctuating reading making me struggle to get less than 80db reading because if i move the cm-140 12" to the front of that position i get 3db less on the reading of my huge 43hz peak but why dont i have a huge 86hz peak if 43 is a room mode?


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## brucek (Apr 11, 2006)

> why dont i have a huge 86hz peak if 43 is a room mode?


There could easily be a dip at 86Hz caused by other room dimensions or reflections.........
Under 100Hz you can pretty well throw out any math you might want to apply to the end result of a room response. You have to measure and see what you get...


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## Intuition (May 5, 2009)

I've tried everything but cant attain atleast -18db at 75db SPL it seems impossible because the cm-140 is just too sensitive for the line-input level to reach even close to -18db i need atleast 6db added to get it to -18 @ 75 SPL best i can get is -18 @ 86db SPL


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## brucek (Apr 11, 2006)

I'm afraid I have no ideas to help you out here.

Best thing to do would be to simply measure at 86dB, rather than 75dB...

brucek


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## Intuition (May 5, 2009)

Well thanks anyways bruce but what effect do you think this will have on my measurements?


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## brucek (Apr 11, 2006)

It should be fine. The choice of 75dB is a level that allows large peaks to still be at a reasonable point where a sub won't compress. It's a good mid SPL level for measuring, but it isn't cast in stone. I'm still stumped why your SPL meter won't supply enough signal to a soundcard at 75dB to get a decent input volume - but, that's the way it is......

You can measure at 85dB instead without much problem. If you had a peak of 20db, you can see where that level of 105dB might get to be a bit much for some subs.....

You also have to pick a measurement level that doesn't clip your meter. The Galaxy has a large scale (I believe from 50-100), so picking 85dB still gives decent headroom.

brucek


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