# Sweep vs. signal generator



## hunt (May 19, 2012)

I'm new to REW (and room measuring in general) and am trying to understand a couple of things.

When I run the measurement sweep and look at the resulting frequency response, I see several frequencies where the level drops way down (~50dB drop). If I then use the signal generator and set it to those same frequencies, my spl meter does not show the same drop. Even when I try frequencies around there (+/- 10Hz), I don't see it.

Is it because with the sweep, neighboring frequencies are interacting more?

Also, regarding the multiple dips from the sweep, they are evenly spaced by 904Hz starting from around 452Hz all the way up to 20 KHz. I have tried moving my meter up, down, left, and right a couple of feet. I have also tried moving the speaker (a full range front speaker) around and while I see changes in the response, the big dips are always present at the same frequencies.

Does the fact that these big dips are always at the same frequencies mean that my speaker is at fault? My meter is one of the old radio shack ones.

Thanks,
Eric


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## EarlK (Jan 1, 2010)

Eric said:


> I'm new to REW (and room measuring in general) and am trying to understand a couple of things.
> 
> When I run the measurement sweep and look at the resulting frequency response, I see several frequencies where the level drops way down (~50dB drop). If I then use the signal generator and set it to those same frequencies, my spl meter does not show the same drop. Even when I try frequencies around there (+/- 10Hz), I don't see it.
> 
> Is it because with the sweep, neighboring frequencies are interacting more?


If the "hole" in the response is really there and you set REWs signal generator to output a Sine Wave at the exact same frequency ( where a trace indicates there's a dip/hole ) , then the SPL meter should/will display a lower level than frequencies seen on the trace/graph that are higher in level ( if you got a good capture of data ) . 

Note ; I said *"if the hole"* in response is really there . See the next part for the *"if" challenge* .




> Also, regarding the multiple dips from the sweep, they are evenly spaced by 904Hz starting from around 452Hz all the way up to 20 KHz. I have tried moving my meter up, down, left, and right a couple of feet. I have also tried moving the speaker (a full range front speaker) around and while I see changes in the response, the big dips are always present at the same frequencies.


Perform the soundcard calibration as required in the REW Guide ( with a loopback cable in place ) and compare it to the following picture ;










If you see anything like this ( my display is set to "log" rather than linear ) it means you have some softwware based feedback happening ( ie; you are sending input back to output which is a "no-no" ) which contaminates the results rendering them useless .

The regularity ( of the nulls ) which you've mentioned can sometimes be attributed to the delay inherent within a software based mixer ( most likely where the the feedback loop gets created ) . 

IE ; the regularity of the nulls is highly suspicious ( especially if/when they don't change after new data captures have been done at different locations ) .

:sn:


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## hunt (May 19, 2012)

I calibrated my soundcard before running any measurements. I followed the steps in the guide, set the results as my calibration file (Step 6), and performed the check to see if it was successful (Step 8). Everything seemed to go along okay.

The card is a Sound Blaster Live and while the response was not great, it seemed to be decent enough.

Assuming the calibration is okay, are there any other reasons this could happen?

I was wondering that since the sweep runs through all the freqs pretty fast, wouldn't the response from different frequencies start to interact, since their decay times are not 0? Is there a way to slow down the measurement sweep?

My acoustic math is not that strong, but I think the 904 Hz translates to a 1.25' wavelength. Is that correct? My speaker is a floorstanding ported kind that has a low end driver, a mid driver, and tweeter up top. The low and mid drivers are about 2.5' up from the bottom and the port is at the bottom. I've been reading about comb filtering that can occur when you have a speaker with multiple drivers. Could that be an issue here?


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## JohnM (Apr 11, 2006)

The most likely problem is with the soundcard cal, as Earl has said. When you measure the card the response should be almost perfectly flat apart from at the very lowest and very highest frequencies, was it?


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## hunt (May 19, 2012)

The response was very flat except at the edges. For reference, my measurement was similar to that in the help file at Step 3 on page 21 of the pdf, except the -3dB point on the low end was at a lower frequency. It was virtually flat end-to-end after calibration.

I will try to post a picture of it (and the speaker response) once I'm back at home. I will also try a different speaker and room to see if I get the same behavior.


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## hunt (May 19, 2012)

This is a picture showing the response along with the mic-meter and sound card calibration curves.

Is this enough info to determine if the sound card calibration is okay? When I do the loopback test to check the calibration, I get a very flat line.

The picture shows the periodic dips in the response but I can't reproduce them with the signal generator.

Any ideas?

Thanks,
Eric


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## EarlK (Jan 1, 2010)

Well , that's certainly not looking good .

Tell us about your computer ( Make & Model ), plus the OS ( along with your soundcard type ).

Are you measuring one or two speakers at a time ( if 2 , then measure a single up close at 24 inches ) ?

Why are you using a linear scale ( in that window ) ? ( The default is logarithmic ) ?


:sn:


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## hunt (May 19, 2012)

Thanks Earl.

Yeah, it looks bad but it doesn't sound as bad as it looks (at least to my untrained ears).

The pc is an old dell dimension 4300, 1.5GHz P4 running xp sp3. The soundcard is a soundblaster live wave.

I'm measuring only one speaker, my right front. I used a linear scale so I could see the periodicity easier.

I have since tested my left front and it does pretty much the same thing.

I then tested my center channel (which is a different type of speaker) and it did something different. It still had the periodic nulls but the period was 32Hz instead of 904Hz.

So I gotta think that these periodic nulls are related to the speakers, but I still don't understand why the signal generator doesn't act the same as the sweep. When I set the generator to any of the nulls from the sweep, I'm definitely not getting the dB drop.

Eric


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## EarlK (Jan 1, 2010)

Hmmm, I'm stumped,

Acoustic problems ( ie ; diffraction effects from the box & modal nulls from the room ) don't typically repeat over the full-range like you see and just aren't that consistent in their level .

Make sure the RS SPL meter has a strong battery and that you aren't driving it into clipping when measuring .

(That's all I got for the moment / assuming there's no undiscovered feedback loop within the setup ) .

:sn:


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## JohnM (Apr 11, 2006)

hunt said:


> When I set the generator to any of the nulls from the sweep, I'm definitely not getting the dB drop.


Meaning it doesn't drop at all, or it doesn't drop as much as the graph shows? The measurement can show levels far below the ambient noise floor of the room, but you won't be able to see that looking at the SPL meter reading, which shows the sum of the test tone signal plus everything else it is picking up. Try using the RTA while running the signal generator, that will let you see what is happening at the particular frequency of the signal.


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## hunt (May 19, 2012)

Earl, I will double check the battery and spl levels at the meter. I did use the spl tab to set the level the meter was showing when pink noise was at played at -12dB (which was about 79dB).

John, I have my meter range set at 80dB and it can show +6/-10dB around 80. When I play the null frequencies, it may waver a couple of dB, but the needle definitely stays within that range (doesn't peg at the max or min).

I will read up on the RTA and give it a shot.

Thanks to both of you for your help.

Eric


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## hunt (May 19, 2012)

Well, I finally got a chance to experiment this weekend. The battery and spl levels checked out okay. I played around with RTA and while I wasn't sure exactly what to expect if there was a problem, I didn't see that my questionable frequencies (e.g. 452Hz, 904Hz, etc.) looked any different than any other frequencies. That is, they all had a large spike at the frequency under test with much lower level "grass" sloping off to the sides.

One thing I did find, however, is that the number of sweeps seems to affect the results. I had been running with the number of sweeps set to 2, thinking that would give me a sort of averaging of results. I tried setting the number back to 1, and it appears that the problem goes away. I still have dips in the response, but they're much more reasonable (in the 20dB range), they're not evenly spaced as before, and best of all, they match what I see with the signal generator.

Just for fun, I tried setting the number of sweeps to 4 and 8 and the problem got worse with each setting, meaning the bad frequencies were the same as before, but the dB drop was even worse than with 2 sweeps.

So I think I'm going to stick with 1 sweep per measurement from here on as it seems to work well with my setup. Thanks for all the help. This is a great tool and is fun to play with; it just makes me wish I was smarter.

Thanks,
Eric


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## JohnM (Apr 11, 2006)

Thanks for the update, glad to hear it is working properly with single sweeps. Can you please confirm which version of REW you are running and whether you are using Java or ASIO driver setting on the soundcard tab.


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## hunt (May 19, 2012)

Sure thing. REW 5.00, build 2142. I looked all around and didn't see anything about ASIO for the soundcard (I don't even know what it means), so I assume I'm using Java.

Eric


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