# First measurements - very big dip(s)



## jasjenl (Jan 5, 2008)

Hi,

so I've started doing some first REW measurements of the equipment in my livingroom and as I already noticed subjectively I'm having some serious dips in the response. My sub is an SVS PC Ultra 13, my main front speakers are Quadral Aurum 9. Crossover is at 100 hz (yes, I know that's rather high but my receiver has a fixed crossover, so I can't do much about that for the moment). The SVS is tuned at 20 hz (no ports blocked), room comp is set to bypass.

Here's a graph of the response of the sub + mains at the listening position.








You can see the rather big dip a bit past 30 hz and also one a bit past 40 hz.

I also measured the response with the front speakers settings at 'large', so effectively with the subwoofer turned off, for comparison.









Then I did a measurement somewhere else in the room, actually where my kitchen is (which is an open kitchen in the livingroom). Here instead of a big dip you will notice a big peak.









Just for fun, I also did a measurement with the meter very close to the subwoofer, which looked like this








And finally here are all graphs combined









To give you an idea about my livingroom, here's a floorplan of it:








All dimensions are in centimeters, green and red boxes are sofas, the middle of the red one is the main listening spot, so the front speakers are the two black boxes you notice in front of the left wall. The SVS is located in the lower-left corner, so that's left-behind the left front speaker. Half of my livingroom has a diagonal ceiling. This is what my house looks like from the side:










So now I don't have any equalization equipment yet and I haven't touch the built-in eq from the svs yet, but I'm wondering if any of that is going to improve the response shown above. I also tried to relocate the SVS to several places in my room but none really gave any more satisfactory results than the place where it's now. Any thoughts on all this?

Thanks,
Jeroen


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## brucek (Apr 11, 2006)

The first response of the overall sub + mains isn't that bad, but it isn't great either (as you know).

Gee, I don't really know what I would do there. The problem is of course the room. There are just so many different parallel surfaces that will create room modes, it's hard to add them all up.

I do think an equalizer (BFD) could easily bring down all the peaks and clean it up quite a bit though.

Hopefully, someone with more knowledge of what to do in difficult rooms will comment.


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## imbeaujp (Oct 20, 2007)

Hello I think that you should apply some filters on the big peaks and leave the dips because it will be difficult to correct them, anyway they are not really audible (too narrow), I think.

JP


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

> I also tried to relocate the SVS to several places in my room but none really gave any more satisfactory results than the place where it's now.


Have you tried dead in the corner, or this location?







​

I recall Ed Mullen saying once that moving down the wall from the corner something in the 4-8 ft. range can often eliminate some 40-Hz range nulls. So try the corner, and up and down those two walls several feet and let’s see what happens.

Regards,
Wayne


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## jasjenl (Jan 5, 2008)

Wayne A. Pflughaupt said:


> Have you tried dead in the corner, or this location?
> 
> 
> I recall Ed Mullen saying once that moving down the wall from the corner something in the 4-8 ft. range can often eliminate some 40-Hz range nulls. So try the corner, and up and down those two walls several feet and let’s see what happens.
> ...


The sub is currently located pretty tight in the corner. I could try and move it a bit forward in the direction you pointed out, but the problem is that there's also a radiator from the central heating from my house. But I'll try and see what happens if I move it along the two walls. 
Thanks for the tip!

Jeroen


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## clubfoot (Apr 12, 2007)

Also, keep all doors closed when taking measurements to isolate the listening room from the other rooms.


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## jasjenl (Jan 5, 2008)

clubfoot said:


> Also, keep all doors closed when taking measurements to isolate the listening room from the other rooms.


I did have all doors closed, but the stairs you see in the livingroom doesn't have a door, so that's a fairly big open 'space' to the second floor.


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## clubfoot (Apr 12, 2007)

And that could be a deal breaker when your hunting for bass


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## DrWho (Sep 27, 2006)

I would start by moving the sub around. If you can't find anything acceptable, then you might try moving the listening position around a bit too. Even as little as a foot can make some wild differences.


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## jasjenl (Jan 5, 2008)

I'll do some more experimenting this weekend, I'll post here again about my experiences.


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## Hannes_F (Feb 5, 2008)

Question: Do you folks in this forum really think such a frequency response can be repaired with EQing?


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

Well, if you’re talking about the first graph from the listening position...












...I’d say the general consensus here is, “it doesn’t look promising...”

Regards,
Wayne


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## Sonnie (Apr 11, 2006)

Awh... that ain't all that bad. :sarcastic: 

I definitely think you can improve on it. Perfect? No! Much better? Yes!

It looks wicked at full resolution, but knock it down to 1/6 octave and it ain't so wicked anymore.


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## DrWho (Sep 27, 2006)

Hannes_F said:


> Question: Do you folks in this forum really think such a frequency response can be repaired with EQing?


To be honest, I don't think EQ "fixes" anything 

I think it can be used to attenuate some problems, but it's not changing the source of the problem...and I really don't mean it as a semantic argument either.

One thing I'd like to do though is further explore the effects of filter Q and all that on the time domain. John claims they totally nullify out, but I'm not sure if I buy it completely since the room effects causing the standing waves and resonances and all that take some time to build up to their full amplitude.


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## brucek (Apr 11, 2006)

> I'm not sure if I buy it completely since the room effects causing the standing waves and resonances and all that take some time to build up to their full amplitude.


Yeah, I had trouble buying into that too, but John has made the point on many occasions that room mode resonances basically act the same as 2nd order biquad filters, and so the rise and decay time reaction to stimulus in a room will behave with those filter characteristics at the listening position (point of correction). If you counteract with a similar 2nd order biquad correction filters (used in the BFD equalizer), the mode is nullified.

So, I suppose what you're questioning is Johns assertion that room modes (minimum phase phenomena) behave as 2nd order biquads? I've attempted some reading on the subject, but it gets pretty bogged down in the math I'm afraid.

brucek


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

> John claims they totally nullify out, but I'm not sure if I buy it completely since the room effects causing the standing waves and resonances and all that take some time to build up to their full amplitude.


It should be easy enough to run some in-room tests with REW and see, but no one seems inclined to. I keep meaning to, but I seem to keep finding other things to do instead… :whistling:

Regards,
Wayne


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## DrWho (Sep 27, 2006)

brucek said:


> Yeah, I had trouble buying into that too, but John has made the point on many occasions that room mode resonances basically act the same as 2nd order biquad filters, and so the rise and decay time reaction to stimulus in a room will behave with those filter characteristics at the listening position (point of correction). If you counteract with a similar 2nd order biquad correction filters (used in the BFD equalizer), the mode is nullified.
> 
> So, I suppose what you're questioning is Johns assertion that room modes (minimum phase phenomena) behave as 2nd order biquads? I've attempted some reading on the subject, but it gets pretty bogged down in the math I'm afraid.
> 
> brucek


I would really love to see some measurements of a real system showing that it works since that is very possible and easy to do...and then we wouldn't have to deal with the math 

I'm all up for tackling the math though, but the problem with math in acoustics is that it is full of assumptions and simplifications to make the math possible to do in the first place. I had to quit taking grad level acoustics in school because the prof would make claims that I could easily measure and verify to be false :yikes: Theory world is fun and all, but I prefer to ground myself in reality first. I have since learned that the simplifications the prof was making actually happen to be valid in the biomedical imaging research he does, but he was using audio and room acoustics analogies instead of biomedical analogies....sheesh, who opened this can of worms? lol.

Anyways, my point is that an argument based on math may not be the best approach since it is going to revolve around making sure all of the assumptions don't affect the results in a manner that doesn't correlate to reality. And the only way to verify a correlation to reality is to measure - so why not directly test claims through measurements instead of getting lost in a sea of math?

I was just thinking the other day...let's say I have two test tones in my arsenal, one is at 50Hz and lasts 10ms. The other is at 50Hz, but lasts 100ms. If my room has a standing wave at 50Hz and I put the measurement microphone at the peak of this standing wave, then I would assert that I would measure two different amplitudes at 50Hz. Namely, the tone that lasts 10ms will have a smaller amplitude because it will provide less total energy to the standing wave. The Q of this standing wave should measure the same though, but how do you know how much attenuation to provide? ****, if the tone is shorter than the pathlength of the first reflection then there will be no measured gain at all...just a signal that seems to last longer than 10ms.

Well there's me hypothesis...once I get this bloody DSP code debugged I'll go test it for myself and post the results...though I would be interested in the results posted by others too of course.

Man, talk about a thread tangent


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## Hannes_F (Feb 5, 2008)

DrWho said:


> To be honest, I don't think EQ "fixes" anything
> 
> I think it can be used to attenuate some problems, but it's not changing the source of the problem...and I really don't mean it as a semantic argument either.
> 
> One thing I'd like to do though is further explore the effects of filter Q and all that on the time domain. John claims they totally nullify out, but I'm not sure if I buy it completely since the room effects causing the standing waves and resonances and all that take some time to build up to their full amplitude.


I would be thankful if you could point me to that explanation directly.

For my understanding standing waves (room resonances, modes, however you call them) mainly influence the reverberation time and can be seen in the waterfall diagram as long foothills.

However dips and peaks that come from direct reflection with a wall influence the frequency response curve (the starting curve of the waterfall diagram, the ridge so to say, and they happen almost instantly.

Both phenomenons are similar but not the same. They have nearly the same frequency sometimes, but often only close. This can be seen in the waterfall diagrams. If you wondered why you saw a peak in the ridge and then a long decay nearby but not really at the same frequency this is why.

I personally think that the right order of action should be to treat the room with absorbers first and then use EQing for some fine adjustion. For an example look at the waterfall of my room that has been heavily treated (full range speaker, no sub). There is no smoothing applied to the data. The deviation is in the range of +/- 6 dB, the decay is very even and short (in the range of 0.2 sec).










Starting from that I would expect an EQ to have a chance for smoothing out a bit of the rest.

Hannes


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## JohnM (Apr 11, 2006)

DrWho said:


> ...let's say I have two test tones in my arsenal, one is at 50Hz and lasts 10ms. The other is at 50Hz, but lasts 100ms. If my room has a standing wave at 50Hz and I put the measurement microphone at the peak of this standing wave, then I would assert that I would measure two different amplitudes at 50Hz. Namely, the tone that lasts 10ms will have a smaller amplitude because it will provide less total energy to the standing wave. The Q of this standing wave should measure the same though, but how do you know how much attenuation to provide? ****, if the tone is shorter than the pathlength of the first reflection then there will be no measured gain at all...just a signal that seems to last longer than 10ms.


The impulse response will show exactly how the room will respond in both cases, indeed how it will react to any test signal you care to convolve with it. The sweep measurement lasts long enough to capture all the room's response (including a capture period that continues after the sweep ends) and the resulting impulse response fully represents how the room reacts.


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## DrWho (Sep 27, 2006)

JohnM said:


> The impulse response will show exactly how the room will respond in both cases, indeed how it will react to any test signal you care to convolve with it.


I agree.


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## glaufman (Nov 25, 2007)

Question, as long as Jon's here and talking about how REW operates...
During the sweep, and the capture period thereafter, the listening part of REW captures all audio coming into the it, as opposed to just monitoring frequencies near where REW is in the sweep, correct? So theoretically it would be possible if some moron (read: me) wanted to play around with using an externally generated sweep and using REW to capture the response?


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## brucek (Apr 11, 2006)

> wanted to play around with using an externally generated sweep and using REW to capture the response?


Yes, the new version allows external signals to be used in the new Spectrum/RTA panel. See here. Any question on it should probably go to a new thread. 

brucek


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## JohnM (Apr 11, 2006)

glaufman said:


> Question, as long as Jon's here and talking about how REW operates...
> During the sweep, and the capture period thereafter, the listening part of REW captures all audio coming into the it, as opposed to just monitoring frequencies near where REW is in the sweep, correct? So theoretically it would be possible if some moron (read: me) wanted to play around with using an externally generated sweep and using REW to capture the response?


No, to use a sweep as a measurement signal the software needs to know the test signal so that it can determine the room's response to it - it generates the frequency response by dividing the FFT of the room measurement by the FFT of the test signal, without the test signal it can't do that and if you use a signal of your own REW will give an incorrect result. The external signals you can use are pink noise or (preferably) pink periodic noise and look at the results on the Spectrum/RTA plot as Bruce says.


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