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post #51 of 70 Old 02-20-15, 03:10 PM Thread Starter
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Re: Room simulation

REW's target match will not try to correct notches in the response, which is generally where the non minimum phase behaviour is to be found. Whether a section of response is minimum phase or not doesn't prevent EQ being applied to it, and it might even be possible to correct the magnitude (SPL) response in such a region back to flat, but the phase response will not be corrected so the overall effect of the EQ would not restore the timing of the signal even if the magnitude is flattened.
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post #52 of 70 Old 02-20-15, 07:36 PM
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REW's target match will not try to correct notches in the response, which is generally where the non minimum phase behaviour is to be found. Whether a section of response is minimum phase or not doesn't prevent EQ being applied to it, and it might even be possible to correct the magnitude (SPL) response in such a region back to flat, but the phase response will not be corrected so the overall effect of the EQ would not restore the timing of the signal even if the magnitude is flattened.
That is true. But I thought that correcting the amplitude out side the min phase region will result in worse audio quality than before and should be avoided.

Do you agree?

Also how does it sound when frequency curve is equalized in non-minimum phase regions (and phase could not be corrected?)

Last edited by shkumar4963; 02-21-15 at 01:21 AM.
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post #53 of 70 Old 02-21-15, 06:33 AM Thread Starter
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Re: Room simulation

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I thought that correcting the amplitude out side the min phase region will result in worse audio quality than before and should be avoided.

Do you agree?
Not really, no, that's too much of a generalisation. There are two kinds of feature in the response that can correspond to non-minimum phase behaviour. The first is partial cancellation of sounds travelling different paths to the measurement point, which causes sharp dips in the response and also shows up as sharp spikes in the Excess Group Delay plot. There is nothing to be gained by trying to raise those dips with EQ and trying to do so would be detrimental. The second is peaks caused by sounds on different paths combining to cause reinforcement at some frequency, which shows up on the group delay plot as broader bumps. Those can be addressed by EQ to reduce or eliminate the emphasis that would otherwise occur at that frequency which would typically be better than doing nothing, though the benefit may be restricted to a fairly small region near the measurement point.

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Also how does it sound when frequency curve is equalized in non-minimum phase regions (and phase could not be corrected?)
I don't know that there is any meaningful answer to that. How something sounds depends on the whole response and the content being reproduced and is very subjective. Trying to assign a label to the 'sound' of an aspect of one small part of the response is likely to be futile.
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post #54 of 70 Old 02-22-15, 11:53 AM
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Thanks John. I recently have been reading about Dirac Live. They claim that it can look ahead in the music and cancel reflections by sending inverse in time domain. The idea sounds feasible but I have not been able to read any papers or books that can explain how this is actually done and how effective it could be. For now I am assuming that this is different than what REW does which is in frequency domain and at min phase. But I may be wrong and Dirac assertions may only be a marketing line. Could you comment and refer to somewhere where I can read and learn more about the technology.
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post #55 of 70 Old 02-22-15, 01:06 PM Thread Starter
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Re: Room simulation

That's how FIR filters operate, there's more info in this wikipedia article and particularly in its references and external links: http://en.wikipedia.org/wiki/Digital_room_correction. There is also relevant discussion in this thread.
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post #56 of 70 Old 02-22-15, 05:53 PM
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Re: Room simulation

Thanks John. There was a lot of great information in that thread. arc2 is very similar to Dirac live. REW is very easy to use and gives great EQ curves. Do you know of a free pc based system that does the same for FIR filters. I would try it and then post results.
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post #57 of 70 Old 02-22-15, 09:05 PM
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Re: Room simulation

John and all - Wow the REW Room Simulator is a fantastic tool. With my room set up I need to sit in middle of a 28 foot room, slightly off center. This of course is not good because of the 20Hz 10-20 dB drop it creates due to being in the middle. However I think I have a workable solution with two HSU subs rated for 18 Hz (VTF-3 MK5).

Please view the screenshots I took of some configurations with Room Simulator. Can you please tell me if this looks like it would provide a viable solution?

There are a number of screenshots available here http://s177.photobucket.com/user/lov...rary/Sub%20Sim . Look at the title shown in for pic for a terse summary of what that configuration consists of.

Note that you can see the speaker/sub placements and MLP in the diagram on the top left of each picture.

- It starts with rew-sim-left-sub with is just one sub in the best possible spot. Not a terrible starting spot I assume.

- Next is rew-sim-two-sub. This is with a second sub placed. Wow this really looks great (I think??) as the peaks around 80hz should be easily equalized out.

- Normally we would be done here I suppose, but look at the next pic titled rew-sim-two-sub-with-fronts. This adds in the L/R fronts. Wow that 40 Hz dip is horrendous! Am I correct to assume that I cannot discount this and that indeed you have to look at the total result with the subs AND the fronts included like this?

- Assuming the answer to the above question is "yes, it matters!" then how about this next one rew-sim-two-sub-with-fronts-crossed-120hz... Its the same as the last one except I cross the fronts at 120hz instead of 80hz. The dip at 40hz is much improved and now "only" -10 dB.

- Next up is rew-sim-two-sub-with-fronts-crossed-120hz-5ms-delay. Here I add a 5ms delay to each sub. Now we just have the peak at 80 to (easily) handle. However look at the massive dip at 110hz. Is that a concern?

- Finally is rew-sim-two-sub-with-fronts-crossed-120hz-5ms-delay-subs-absorb. This is the same as the last one, except assuming the room is treated with material with an absorption factor of 0.30. I do plan to use acoustical panels for all walls and ceiling, and to have a carpeted floor. What type of absorbtion factor do acoustical panels provide? I'm not sure is 0.30 is low or high. But assuming its not hard to get 0.60, have a look at the last one rew-sim-two-sub-with-fronts-crossed-120hz-5ms-delay-subs-absorb-60. With this curve there is a little peak at 80 and that's about it.

Hopefully I am on the right track here and am not just adjusting things that can't really be done just to get a pretty curve. Please let me know if this may be realistic to achieve. For example I will have a highish-end AVR like Denon 5200W with its advance multi sub eq. I assume it will find the delays or otherwise allow me to manually set such adjustments?

Thanks!

Last edited by lovingdvd; 02-22-15 at 10:30 PM.
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post #58 of 70 Old 02-23-15, 01:40 PM Thread Starter
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Re: Room simulation

For much content the contribution of the main speakers does matter. Did you try a more symmetric positioning of the speakers across the width? There is a table of absorption coefficients here.
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post #59 of 70 Old 02-23-15, 03:04 PM
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Re: Room simulation

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For much content the contribution of the main speakers does matter. Did you try a more symmetric positioning of the speakers across the width? There is a table of absorption coefficients here.
Thanks John. Yes I experimented with several placements. You can see the various results by viewing this "photo album" of screenshots here http://s177.photobucket.com/user/lov...?sort=2&page=1

In general how does an uneven rectangle affect the results? For instance, the front half of my room is 18.5 feet wide, but then there is a 42.5" bump out (about 17 feet from the front right wall) that runs down the rest of the room on that side, which makes the back half of the room 15 feet wide.

To put it another way, imagine a 18.5' x 17' rectangle, followed by a 15' x 11' rectangle that's flush with the first rectangle on the left-hand side.

I have a lot of confidence in the REW simulator and am happy with the various options it shows that I have as good starting points, but I am unsure of how much of a difference there will be in the real-world due to my room not being a perfect rectangle.

And, I wasn't sure what would be a better model given that I can only draw a rectangle - to do one at 15' wide or to do one at 18.5' wide. I did one at 16' wide too. Anyway, here is the latest one I did for one sub:



and here's one with two subs:


There are based on 18.5' wide model.

From everything you've seen so far in my simulations, do you think its a safe assumption that I can count on getting a great EQ/calibration for my sub, despite sitting in the center of the room (although technically because the room is wider at the top, I am sitting a few feet off center there, which helps avoid a dip I think).

Thanks!
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post #60 of 70 Old 02-24-15, 04:17 PM Thread Starter
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Re: Room simulation

The substantial change in width will affect all the modal paths that encounter those surfaces, so don't put too much store in the simulation output. Even if the room is rectangular there is still no substitute for measurement.
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