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LarsS

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#22
@dallas & @RayDunzl , just started my room correction journey.

Having read up and watched a couple of YouTube videos on REW. Suggestion seems to be that room eq should not be done over 500Hz - 1kHz.
Your view on this?

eg. this
 
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dallasjustice

dallasjustice

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Thread Starter #23
@dallas & @RayDunzl , just started my room correction journey.

Having read up and watched a couple of YouTube videos on REW. Suggestion seems to be that room eq should not be done over 500Hz - 1kHz.
Your view on this?
Try it both ways and see which one you like better. There are no hard and fast rules like that. IMO, you’ll end up with a target curve which rolls the highs off a little. That’s also what others have found. Harman did some testing on this came to the same conclusion. Most of the advanced DSP software uses very good windowing so that there’s no overcorrction problems like others may have experienced with more basic DSP software.
 
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dallasjustice

dallasjustice

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Thread Starter #24
I watched that video. “More Flatter” response. I love it!

I would only use REW generated filters for low frequency.
 
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dallasjustice

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Thread Starter #25
B1C68AF2-9DD1-45FC-B4BE-60C3C00789C3.jpeg
EB0C7218-CD09-46E1-AA8F-05A75C89EF6A.jpeg
The mighty little Rythmik F8 subs. They use Hypex UCD amps and have an auto off/on setting. The only annoying thing about them is the mini XLR.
 

Brad

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#26
Sorry, I didn’t understand your question and I probably didn’t explain the flanking sub idea in sufficient detail.

Any “small” room will have Allison effect “nulls” from the boundaries adjacent to the speaker setup at listening position. The primary problem areas are:
If I understand correctly, these are also called SBIR (speaker boundary interference reflections)

The deep nulls I have measured with acourate filters when measuring L+R are not the 'grass' typically measured, as a L+R measurement averages that out. What I am referring to is a 20dB dip that is ~5kHz wide (FWHM)
 
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dallasjustice

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Thread Starter #27
If I understand correctly, these are also called SBIR (speaker boundary interference reflections)

The deep nulls I have measured with acourate filters when measuring L+R are not the 'grass' typically measured, as a L+R measurement averages that out. What I am referring to is a 20dB dip that is ~5kHz wide (FWHM)
Fortunately the human head is slightly larger than the tip of a mic.

Have you tried moving the mic a few inches and retake measurements?
 

Brad

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#28
Yes I have tried moving the mic a head width - it just shifts the position of the null. It looks like interference.

But I don't measure this with all the filters I have made. Hence I am curious to find out if this is typical for coherent sources or aberrant.
 
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dallasjustice

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Thread Starter #29
Yes I have tried moving the mic a head width - it just shifts the position of the null. It looks like interference.

But I don't measure this with all the filters I have made. Hence I am curious to find out if this is typical for coherent sources or aberrant.
I don’t know. I’ve never measured both channels at the same time. Post a screenshot.
 

RayDunzl

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#30
With two spealers playing the same signal, if the microphone is perfectly centered, and a nice "flat/smooth" response is obtained...

Moving the microphone away from center will induce static "waves" in the response (though the waves move as the mic moves), starting with the highest frequencies, and working their way down to as low as the distance moved demands.

The bulk of the effect is combing between the speakers (direct sound), though there would also be some effect (probably similar) if the reflections are strong enough.

I uploaded a movie of it... someplace here...
 
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Brad

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#31
upload_2018-2-20_20-1-39.png


Here's a 1/12 octave smoothed measurement of L+R. Red is mic centred, Green is mic moved about 5cm sideways.
Do you think that's expected?
 

RayDunzl

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#33
Do you think that's expected?
Green has a set of "waves" (as expected) off center...

How did you come up with the center position for the red?

---

With REW, I use pink noise, doesn't have to be real loud. As I move the mic, slowly, past center, I see the save action "reverse".

Then can get closer, with L then R impulse response alignment, using acoustic reference timing of the L speaker...
 

DonH56

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#34
Here's a 1/12 octave smoothed measurement of L+R. Red is mic centred, Green is mic moved about 5cm sideways.
Do you think that's expected?
Yes. This is a great example of why room treatment can help, why multipoint room correction systems have evolved, and how small movements can lead to big changes in the frequency response. Fortunately as Amir has repeatedly noted our brains process the sound so we ignore small (narrow) dips though you can probably hear the broader upper midrange (~2 to 8 kHz) dip. Hard to say if that's the room, speaker position, speaker phasing, and/or something else.

I don't have my response and am not sure I have measured off-axis for a while but it does not vary as much due mainly to lots of treatment (absorbers). There are drawbacks, however, as the room is very dead and many (perhaps most) folk prefer a room that is more "live".
 
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dallasjustice

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Thread Starter #35
View attachment 10781

Here's a 1/12 octave smoothed measurement of L+R. Red is mic centred, Green is mic moved about 5cm sideways.
Do you think that's expected?
I still think it’s a nonstandard measurement and you shouldn’t worry about it. But just to test what I’m saying. You can test using REW signal generator. Pick the center frequency of the dip about which you are comcerned. Play through both speakers. Sit at listening position. Move your head around front to back, side to side and vertically one or two feet each time. See if you can hear huge changes in SPL. If you don’t hear really big/obvious changes, then there’s nothing to worry about.
 

RayDunzl

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#36
Not in any ways an expert on this, if you watch the Youtube video I linked to above from ~1:05 - 1:30 he shows the impact of moving the microphone.
I skipped the video (sorry, for some reason, I'm prone to do that)...

But, the 1:05-1:30 range does show (grossly) the "wave" action I intimated. I couldn't find my little video of it.

I moved the mic much more slowly, over a smaller range, and see the wave disappear, then "grow" from the highest freq as the mic crosses center, and I try to locate the center.

Fine positioning after that gets the mic location within a single "sample period" - 1/48000 second time --- (1134*12)/48000 inches --- 0.14 inches of equidistant to the speakers.

Reason?

Since I'm allowing (stuck with?) full range correction, and single point measurement with AcourateDRC, I want the best (repeatable) starting point for measuring the uncorrected signal.

Maybe trying to be that accurate is unnecessary, but it is easy enough to do.

Suggestion seems to be that room eq should not be done over 500Hz - 1kHz.
Your view on this?
The most obvious changes are likely to be in the bass region - and maybe least likely to be mis-corrected because of poor mic placement (big waves).

I can't hear high stuff anyway, so I look at measurements, and ask the opinion of my Audio Buddy, who can hear higher. He doesn't complain, so, either it did a good job, or just changes the little faults to little faults at different frequencies.

In AcourateDRC, you choose (from five choices) how "fine" a correction you want. It (when permitted) uses IIR to correct broad ranges, particularly in the bass, then creates FIR to correct small differences. It isn't documented well, has another control I've never figured out.
 

RayDunzl

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#37
You can test using REW signal generator. Pick the center frequency of the dip about which you are comcerned. Play through both speakers. Sit at listening position. Move your head around front to back, side to side and vertically one or two feet each time. See if you can hear huge changes in SPL. If you don’t hear really big/obvious changes, then there’s nothing to worry about.
Playing correlated pink noise, even I am able to hear changes in the FR as I move my head left and right of center.

Listening to correlated pink, and moving your head, is a good way to find center if you don't know where it is.

With music or a single tone, not so much, though the spatial presentation of stereo music falls away from perfect sweet-spot-ness when off-center.
 

FrantzM

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#38
Them of my posts has been boosterish .. I am not abou tot change it :D

Great Discussion/posts people!!! Keep on posting!! I ( We?) are learning or at least realizing there is so much to learn.
That kind of discussion can only further my knowledge ( and make me realize how ignorant I am .. :( )
 

RayDunzl

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#39
I found my little video...

Correlated pink noise playing through both speakers.

I simulated moving the microphone from exact center more and more off-center - see the mouse and modification of the delay time of the right channel at the lower left of the video.

The change in the displayed RTA frequency response is like moving the mic (changing the relative delay between speakers)


upload_2018-2-20_9-2-30.png


https://www.screencast.com/t/KenwrJj4WGA
 
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