Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 397 - AVS Forum
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post #11881 of 11894 Old Today, 08:05 AM
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Originally Posted by Bluescale View Post
That's because there's so little output below 30Hz. If he dropped his volume 10-15 dB on the red graph, he'd see a similar decay from 30Hz down.
I'd measure that to verify. I know in my room, even though none of my subs are rated for much output below 35Hz, I still got a nasty resonance at 31Hz. I mean, like 20dB higher than adjacent frequencies.

Room modes are evil.
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post #11882 of 11894 Old Today, 08:06 AM
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Originally Posted by artur9 View Post
Then below 30Hz the resonance is less for the green than it is for the red. The red doesn't seem to decay much. Probably because the green isn't exciting the room as much at those frequencies.

IME, I hear that lack of decay below 40Hz as if a bird were beating on my eardrums. Can't stand that.
Resonance below 20-30Hz has been discussed quite a bit in this thread. I believe the general consensus was that it's not that easy for most to hear the direct sound of this wavelength let alone the resonances. Therefore and also because it's very difficult to deal with these long wavelengths, we've pretty much limited our discussion of room treatments to above 30Hz. I think depending on your viewing habits, you're more likely to miss the null than notice the additional extension in the red response but that's just my opinion. For those down in the single digits with their extension, I'm sure they'd feel otherwise. Also, to what 3ll3d00d said, the bass decay is more similar below 30Hz than it appears. The major difference is the lower SPL on the green plot. The higher the amplitude (SPL) the more it will resonate for a given frequency.

EDIT: +1 to what Bluescale said above which was also the point I was trying to make. IMHO, the biggest improvement in the waterfall between the two is a result of eliminating the null in the 40-60Hz range.

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post #11883 of 11894 Old Today, 08:20 AM
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Originally Posted by jkasanic View Post
.. it's not that easy for most to hear the direct sound of this wavelength...
Maybe I shouldn't have used the word hear. I know I can't hear anything that low.

But I have felt that beating of the bird wings on my eardrums. That's what prompted me to do room treatments and learn REW.
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post #11884 of 11894 Old Today, 08:36 AM
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Maybe I shouldn't have used the word hear. I know I can't hear anything that low.

But I have felt that beating of the bird wings on my eardrums. That's what prompted me to do room treatments and learn REW.
Sorry, just to be clear, I wasn't directing my response towards your post even though mine was subsequent to yours. I got busy and didn't finish responding before you and Bluescale made your posts. Having recently added about 2' of pink fluffy behind my screen and measuring no real improvement in bass decay below 40Hz, I'd be interested to know how you tamed your resonance at 31Hz?
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post #11885 of 11894 Old Today, 09:10 AM
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Originally Posted by AustinJerry View Post
I agree that the best solution would be to get rid of the 60Hz dip AND preserve the bass extension, but evidently that isn't as easy as it sounds.
Your telling me! Been screwing with these subs for over a year trying to get the most out of them I can.

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what do you mean by that? the only visible difference in those waterfalls to my eye is that one has greater output, the rate of decay looks pretty much the same, i.e. down by ~10dB in the first ~100ms and then more slowly decays from there.
My thoughts exactly.

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I'm also assuming that the bit of resonance that never finishes is something like refrigerator hum.
Either the fridge or my A/C.

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I think depending on your viewing habits, you're more likely to miss the null than notice the additional extension in the red response but that's just my opinion.
Thanks for all the responses guys!

I've got a plan...I'm going to try the 2 front/2 back config again and attempt to get back the ULF with phase and time alignment adjustments with a MiniDSP. I had issues with the MiniDSP and clipping before, so I just took it out of the chain. I'm going to try and overcome that problem with gain/trim adjustments. I'll report back!
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post #11886 of 11894 Old Today, 10:06 AM
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Alan,

I lean towards the green graph. But what is being mostly played through the system should be a prime considering factor.


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post #11887 of 11894 Old Today, 10:07 AM
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I think you are splitting hairs (not you, Jim). Above 30Hz, both waterfalls look fine. Below 30Hz, one would expect the level of ringing to be slightly higher on the red graph because the bass extension is greater. Regardless, IMO the dip at 60Hz poses the biggest problem, which is why I prefer the green response curve (if nothing else can be done to improve the dip).

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post #11888 of 11894 Old Today, 10:11 AM
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Originally Posted by jkasanic View Post
Having recently added about 2' of pink fluffy behind my screen and measuring no real improvement in bass decay below 40Hz, I'd be interested to know how you tamed your resonance at 31Hz?
My room treatments are
  • almost 12 linear feet of 4" Roxul SafeNSound behind the MLP, for something like 48 ft^2 of panels.
  • I also filled the wall cavities with more Roxul. I had placed the mic into the wall cavities through some access panels and it picked up a fair amount of sound. One wall cavity has 12" of Roxul and the other has 4".
  • The ceiling has 1" of high density stuff but I don't know exactly which one (OC7??).
  • 2 other 2x4x4 panels in a kind of nook above the MLP.
  • A really large cushy sofa.

But using 3 subs and adjusting their gains and crossovers according to Geddes got rid of it ultimately. I think I could have gotten away with 2, though.

Current placement with my 4 subs are:
  • 2 up front flanking my bookshelf speakers. I like the effect of stereo subs crossed over fairly high @ 100Hz. The LHS sub has a pretty nice FR but the RHS sub could use some EQ.
  • One at the end of a half wall. The room is oddly shaped so that ends up being something like 1/3 of the way into the room and maybe 5/8 of the way along the back wall. When I had only one sub this was the best place for it.
  • One more or less opposite the previous one along the RHS wall. This is the second best place for a single sub.

WAF works out because the 2 flanking subs "appear" as part of the bookshelves. The half-wall sub is a slim model (external amp) and seems invisible and the last sub is hidden by the sofa.
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post #11889 of 11894 Old Today, 10:30 AM
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I find the sub 30Hz response of the green trace slightly odd. It has the same notch in the mid teens which is presumably room induced yet falls off at about 18dB/octave. It is not obvious to be why putting 2 subs at the back would do that given the effect seems to extend beyond the modal region. Where is your 1st mode?

Similarly I wonder whether the 60Hz dip is room induced or results from the way the subs interact in those positions. Do you know? Do your mains help out at all in that area?

I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.

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post #11890 of 11894 Old Today, 10:35 AM
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But using 3 subs and adjusting their gains and crossovers according to Geddes got rid of it ultimately. I think I could have gotten away with 2, though.
I suspected this to be the case and likely what I'll need to do in my space (i.e. add more subs to even the response which has the byproduct of reducing the ringing). Thanks.
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post #11891 of 11894 Old Today, 10:48 AM
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Originally Posted by 3ll3d00d View Post
I find the sub 30Hz response of the green trace slightly odd. It has the same notch in the mid teens which is presumably room induced yet falls off at about 18dB/octave. It is not obvious to be why putting 2 subs at the back would do that given the effect seems to extend beyond the modal region. Where is your 1st mode?
Yes, I agree that the FRs are very similar but the slope on the green just drops off a cliff...I have no idea why (except for phase/delay issues).

Where is my 1st mode? I also have no idea.

Here's a diagram of my room (with all possible sub placements, I have measurements for all placements in the thread I linked to before). Subs are currently stacked at positions 1/2 (red trace). The front/rear config (green trace) is positions 1/2/5/6.



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Similarly I wonder whether the 60Hz dip is room induced or results from the way the subs interact in those positions. Do you know? Do your mains help out at all in that area?
I know it's room/MLP location induced (which I really don't want to move). And, no, the mains most certainly do not help out...in fact they are causing a lot of harm. Unfortunately they are corner horns and cannot be moved.

Here's a measurement of my mains:



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I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.
Could you explain what is bolded? I don't understand what you mean.

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post #11892 of 11894 Old Today, 11:20 AM
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I do think you are a good candidate for doing the delay experiments offline with time aware measurements. You may want to look back at bluescale's recent attempt to use Holm for this.
I was thinking the same thing.

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Could you explain what is bolded? I don't understand what you mean.
Go back and take a look at pages 393 and 394. There was quite a bit of discussion around using HolmImpulse to try to time align speakers. It was a bit over complicated in my scenario, although I kept playing with it after the conversation in this thread ended, and was able to get it very close to correct. The biggest problem I faced was the amount of time it took me between measurements. If you're not using a USB mic, that won't be an issue.

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post #11893 of 11894 Old Today, 11:23 AM
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Could you explain what is bolded? I don't understand what you mean.
if you have measurements that reference a known time source (loopback) then it is possible to take a measurement at each locations and then use trace arithmetic to combine them into an accurate projection of the combined response. You can then apply delays to each one individually offline, sum them again and you still end up with a valid projection of the expected response if you applied those delays. The benefit to you is twofold; firstly you get to try out all possible combinations, even those your minidsp can't implement, without having to go through the tedious process of moving subs, changing delays and measuring a million times & secondly you could share those measurements and perhaps you'd get some different ideas from people playing with them themselves (or maybe you won't, who knows!).

The problem is you have a USB mic and it seems this means the timelock feature (http://www.diysubwoofers.org/misc/holmimpulse/ta.html) is fallible over time (http://techtalk.parts-express.com/sh...reeping-offset), this would render the calculations void. However it seems that the offset is small initially so perhaps it will still work ok as long as you get the measurements done within x seconds. Alternatively, if you have the means to use an SPL meter for measurements, then that would suffice for this purpose.
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post #11894 of 11894 Old Today, 11:43 AM
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I suspected this to be the case and likely what I'll need to do in my space (i.e. add more subs to even the response which has the byproduct of reducing the ringing). Thanks.
This page has a wealth of information for when you get to that point.
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