# Problems with measuring frequency response



## shifty_eyes (Oct 24, 2010)

I’ll start my forum life with a question. I bought Radio Shack SPL meter I tried to measure frequency response using REW. I have mains set as small and crossover set to 80 Hz. First I calibrated my soundcard. Result was:










Calibration curve

That looks like it should? Then I checked the calibration by making a measurement with the loopback connection from line output to line input in place. Result was:










Soundcard frequency response

That should be practically a straight line, right? Something is wrong here. Any ideas what is the problem? In meter settings I had C weighted SPL meter unchecked and there was no meter calibration file loaded. 

I made also one actual measurement anyway. I checked the C weighted SPL meter and loaded meter calibration file which I found from this site. Please ignore high frequency results. I know this meter doesn’t give good results for high frequencies. But the low end doesn’t look good either. I think really low has the same problem as the previous measurement. I was also expecting much higher peaks. I had to put Line In to mute from computer sound level window before I was able to connect both SPL meter and receiver to the soundcard (otherwise sound started going on a loop between meter and receiver, I don’t know English word for this). Is this a problem?










Measurement

I would appreciate any advice how to make this measurement right.

edit: hmm.. maybe my pictures were too big or there is something wrong with my computer. Anyway, I put links also so you can see what's happening.


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

Welcome to the Forum, shifty!

You have to have 5 posts befofe you can use “img” links. You can use our post padding thread to rack up your minimum. Alternately, you can just upload your graphs straight to your post, if they are 800 by 800 or smaller. This post will tell you how to do that.

Regards,
Wayne


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## JohnM (Apr 11, 2006)

shifty_eyes said:


> That should be practically a straight line, right? Something is wrong here. Any ideas what is the problem? In meter settings I had C weighted SPL meter unchecked and there was no meter calibration file loaded.


The soundcard calibration is basically OK, but the REW impulse response window setting needs to be made longer to smooth out the low end and generate a better calibration curve. Go through the soundcard cal process until REW has made its measurement. The click on the IR Windows button and set the Right window duration to 2000ms, and click Apply Windows, that should smooth out the ripples at low frequencies. After that click the button to make the calibration file. Alternatively go through the process in the REW V5 beta, which will automatically use a longer impulse response window.


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## shifty_eyes (Oct 24, 2010)

JohnM said:


> The soundcard calibration is basically OK, but the REW impulse response window setting needs to be made longer to smooth out the low end and generate a better calibration curve. Go through the soundcard cal process until REW has made its measurement. The click on the IR Windows button and set the Right window duration to 2000ms, and click Apply Windows, that should smooth out the ripples at low frequencies. After that click the button to make the calibration file. Alternatively go through the process in the REW V5 beta, which will automatically use a longer impulse response window.


Ok, I tried what you suggested but it had no effect. Then I installed REW V5 and tried with that. I got the same result:









Any more ideas? :help:


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## JohnM (Apr 11, 2006)

Please post a screenshot of the impulse response graph and what the IR window settings are.


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## shifty_eyes (Oct 24, 2010)

Today I made another attempt. My soundcard settings during calibration were:









And the calibration curve looked like this:









IR Windows was:









And impulse graph:









Frequency response measurement with loopback connection in place looked like this:









with IR windows and impulse graph:

















I think now that looks better. Deviation is max 0.4dB. Instruction said that it shoud be within 0.1dB but does that make any difference? I did everything the same way as yesterday so I don't know why different result today.

Then I made an actual measurement. Response looked like this:









Does that look reasonable? Those very low frequencies are probably off. I made another measurement with different sub phase. Comparison is here:









Does that look like it could be correct? Maybe I need to verify the results by making some measurements manually. (if you see a picture after this text, forget it. I don't know how to remove it )


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## JohnM (Apr 11, 2006)

All looks good.


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