Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 81 - AVS Forum
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post #2401 of 12737 Old 04-05-2013, 12:22 AM
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Originally Posted by jkasanic View Post

Not Sanjay but I'm guessing he'll ask if it's possible to move your speaker further from the wall to eliminate boundary effects?
Have I become that predictable? (You posted exactly what I was going to ask.)

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post #2402 of 12737 Old 04-05-2013, 12:43 AM
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Originally Posted by kbarnes701 View Post

Hi McS - I am no expert on this but if you can run the Mini-DSP after the AVR can you not then set the sub distances individually? IOW, you will have two subs Y-corded from the single sub output in the AVR, into the MiniDSP and from the MiniDSP (I assume) two sub outputs, one to each sub?  This will then let you set the delays for the two subs independently via the MiniDSP. Failing that, I'd see what you can do with adjusting delays of the combined pair of subs. You will need measuring gear to do this - it's impossible really to do it any other way because you need to see the plot of the FR at the XO region and see how it changes as you alter the delays.  Hopefully others will chime in to amplify this advice, or even to outright contradict it smile.gif

Hi Keith, that is correct, there are two ins and four outs on the mini dsp I have (2x4 it is called I believe), allowing you to put one sub in and run up to four separate outs, (or 2 ins to 2 outs). I can set a delay on each channel, so based on sdurani's sugestion:
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Use the delays in your miniDSP to optimize the interaction between your three subs. Once you've done that, think of your system as having a single subwoofer. Use the distance setting in your AVR to optimize the blend between your 'subwoofer' and speaker(s).

I will optimize each channel individually to give me the best OVERALL response with the subs, and THEN use the AVR distance to get the best response after that. Thanks for your help guys!
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post #2403 of 12737 Old 04-05-2013, 05:56 AM
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Is that speaker around 1 foot (maybe a bit less) from a neaby wall?
I measure exactly 1 foot from the top center of the speaker to the wall.
If I divide the speed of sound by the frequency of your dip, I will get the wavelength of that frequency. 1130 ÷ 300 = 4 feet. In order to produce a cancellation dip at that frequency, the direct sound from your speaker would have to combine with a copy of the sound (reflection) that is out of phase.

If the reflected path off a wall is 4 feet longer than the direct path from your speaker, the reflection will be in phase with the original 4-foot wave, albeit delayed once cycle (4 feet later). But if the reflected path is only 2 feet longer than direct path, then they will combine with the reflection mid-wave (180° out of phase with the direct sound). Cancellation.

How would the reflected path be 2 feet longer than the path of the direct sound from your speaker? It makes a 1 foot trip to the wall and a 1 foot trip back to the speaker plane to combine with the direct sound. So there must be a boundry 1 foot away, for that 2-foot round trip. 2 feet of extra travel is a half of the 4-foot wavelength. When combined, they'll cancel at that frequency (300Hz).

If you suspect a dip in the frequency response might be due to speaker/boundry interference (rather than modal), look for a wall a quarter wavelength away: ¼ of 4 feet is 1 foot. Easy to remember?
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Originally Posted by Pres2play View Post

I noticed that changing the height of the speaker from two-feet to 37 inches actually improved the response at the MLP. Also, moving the speaker closer to the wall flattened the crossover region and raised the depression at 300 Hz. This is surprising. I thought generally placing a speaker farther away from the walls improved performance.
If you pull the speaker far enough out, say 4 feet from the wall, then there are some interesting benefits. Since 4 feet would be the quarter wavelength, the full wavelength would be 16 feet. 1130 ÷ 16 = 70Hz. Pulling the speaker out that far into the room will not only reduce the energy of the reflection (smaller dip) but will lower the frequency of the cancellation (below the crossover point). Pretty neat.

By comparison, moving a subwoofer closer to a boundry (aside from the ususal benefits) will slide its boundry cancellation dip higher up the frequency range, hopefully well above its crossover point. Convenient how that works out.

Since I have no intention, let alone space, to place my L/C/R speakers 4 feet into the room, I instead loaded up my front wall with absorbtion. No reflections = no boundry cancellation. There's more than one way to skin a cat.

 

Yet another fabulously useful post from you, Sanjay. I learn something new every day on AVS, and often from your own posts. Forgive me if I sound a little sycophantic but I do believe it is just as important to  offer thanks and positive comments as it is to offer brickbats (and I do sometimes offer the latter I know).

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post #2404 of 12737 Old 04-05-2013, 09:01 AM
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Originally Posted by amirm View Post

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Originally Posted by jkasanic View Post

Be wary of those "purists" purporting all the negative effects of Audyssey...everything in moderation!
No a purist, nor purporting anything smile.gif. The negative effects show up in blind listening tests and analysis of what it does to the sound going through it. See here for a taste of it: http://www.avsforum.com/t/1439769/any-suggestions-for-cables/150#post_22719249.

Now that you guys know how to measure your own systems, you can confirm the problems that were documented in the above research. Here is a quick example of someone's measurement from a page back:

900x900px-LL-f2fe6df2_lrsubfullrangefrwandwoaudyssey.jpeg

Note how the red graph has a very well defined droop 1K and 5K. The no-Audyssey green curve had a dip between 1K and 2K (common problem in the crossover region of speakers due to directivty problem) but nothing remotely the same as what Audyssey did to it. When frequency response variations become wide, our sensitivity to them increases sharply to as low as 0.5 db. Yes, half a dB. Now look at the red graph. The red graph has 10X more variation in it at 6 to 8 dB! We took a speaker that had a small problem in the crossover and made it a much bigger one on purpose!

If there is one area you want to get right is the mid frequencies where our ears are most sensitive and so many instruments, voices, etc. all hit on this.

Now look at the bass response. It is flatter which is fine. But once you take out that peak at 30 Hz, the bass will sound anemic. It just will. You have taken out some of the energy that was being pumped into the room. Research says we need to compensate with having a sloping down response.

For people who have no other way to smooth the frequency response, it is a useful tool to try and see if it overall improves fidelity. But let's get educated and learn how we can do better.

 

Amir, are you implying that the human ear is more sensitive to 1/2 dB changes over a given frequency range than it is to overall sound level output (where I believe 3dB is the consensus for changes perceptible to human hearing)?  I've taken the liberty of creating a new graph focusing on the higher frequencies 500Hz - 20kHz:

 

 

At 1 kHz, the measured response is nearly the same between both curves (approx. 81 dB with a signal output of approx. 80 dB according to my SPL meter).  At 2 kHz, the measured responses are 75 dB (red with Audyssey) and 73.5 dB (green without).  At approx. 1.7kHz, both plots reach minimum values for their respective curves:

 

70.7 dB up to 15.3kHz for the green curve:

 

and 74.6 dB up to 17.6kHz for the red curve:

 

The total variation of over this same frequency range (1.7kHz - 17.6kHz) is:

 

74.6 dB - 83.3 dB for the red curve:

 

70.7 dB - 78.5 dB for the green curve:

 

So if I understand your premise correctly, the total variation over the 1.7kHz - 17.6kHz frequency range of 8.7 dB for the red curve as compared to 7.8 dB for the green curve is more objectionable than the fact that the green non-corrected response varies in total output of 81dB - 70.7dB = 10.3dB vs. the Audyssey corrected response of 81dB - 74.6dB = 6.4dB?

 

On the matter of the "energy in the room" at 30-40 Hz, are you implying that a flat bass response is less desirable than one with a massive gain that also induces ringing?  See the comparison:

 

No Audyssey decay plot (green curve above):

 

Audyssey corrected response (red curve above):

 

Of course, there are other ways to deal with this issue (e.g. room treatments) but I have a hard time believing (objectively) from these plots that the pros don't far outweight the cons.  Subjectively, it's not even close...

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post #2405 of 12737 Old 04-05-2013, 09:33 AM
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Quote:
Originally Posted by kbarnes701 View Post

Here's my Guide to the sub distance tweak, compiled from all known reports...

Audyssey Sub Distance Tweak Procedure.pdf 1121k .pdf file

here is the result from the procedure from the thread in hometheatershack.
http://www.hometheatershack.com/forums/rew-forum/66619-can-rew-predict-2sub-performance-single-sub-measurements-3.html#post603658
The procedure is not that much different from the one referred to by Keith.

On my system, I cannot set the distance for the subwoofer. So I changed the distance for the center speaker relative to the physical distance. Then I look for the setting where the SPL is maximized. I was surprised about the clearly visible results. SPL is max for a setting of 1.5m, whereas physical distance is 2.7m. I have to check left and right as well, but just based on the center speaker, I would have to reduce all physical distances by an offset of 1.2m.

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post #2406 of 12737 Old 04-05-2013, 09:35 AM
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Amir, so you are saying the red curve "should" sound worse then the green curve.  Interesting, I never would have come to this conclusion by looking at the graphs.  Of course, the real test is in listening, and unfortunately I can't do that.

 

Edit:  That is your graph, jkasanic.  Which sounds better to you, the red or the green?

 

I know Barnes is laughing his you know what off right about now...as I've taken the bait! biggrin.gif  Seriously though, I plan to do some A/B listening this weekend with some known content if only in an attempt to quantify subjectively the improvement Audyssey creates.

 

FWIW, I do subscribe to the theory that providing Audyssey a better starting point is very advantageous and a bad starting point could result in less than desirable results (although I'd be interested to see some measurements that actually show a worse plot over the "entire" 15-20kHz range).  Take a look at a pair of before and after Audyssey plots:

 

 

The top pair was offset +5dB and the bottom pair -5dB to give some separation and make the comparison easier.  The difference in the blue and green No Audyssey responses is my sub location (right front corner with blue and mid front wall with green).  Also note that the orange blue and green responses didn't have the benefit in the XO region of the sub distance tweak I performed with the red response.  As mentioned previously, I plan to experiment more with my XO setting to improve the dip from 70-150Hz.

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post #2407 of 12737 Old 04-05-2013, 09:37 AM
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Originally Posted by jkasanic View Post

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Amir, are you implying that the human ear is more sensitive to 1/2 dB changes over a given frequency range than it is to overall sound level output (where I believe 3dB is the consensus for changes perceptible to human hearing)? 
As I explained, the sensitivity is bandwidth dependent. It ranges from 2-3 dB for sharp changes to 0.5 for wide variations. The reason is that when a variation spans many frequencies, there is much better chance of what we play to hit that region and hence be heard. See the Audibility of Resonances in my article here: http://www.madronadigital.com/Library/AudibilityofSmallDistortions.html. For more detail you can see Dr. Toole's reference.
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I've taken the liberty of creating a new graph focusing on the higher frequencies 500Hz - 20kHz:
For this type of work we want to perform "macro" analysis. That is, we want to step back and look at the overall shape of the graph. In that sense, zooming in is not as helpful as it removes the trends and at any rate, we don't want to analyze any single frequency. Research and listening tests show that we do not want to have a droop in the mid-frequencies. To see if we have that, we need to make sure we are at 1/6 or even 1/3 smoothing. Then we step back and look at the full picture to see if we have variations in response. This is much harder to do when zoomed in and with less filtering.
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At 1 kHz, the measured response is nearly the same between both curves (approx. 81 dB with a signal output of approx. 80 dB according to my SPL meter). 
So take that as a common reference for both since they are roughly equal and now fast forward to 4-5 Khz. We see that the graph without EQ had its level keep going down. The one with EQ in red, has a boost in the latter region. That creates the droop that I was talking about. Here are the results of formal listening tests:

image?pagenumber=24&w=800

The teal color at the bottom is Audyssey. Compare it to the dashed black line which had no EQ. Start at the same 1K point and move forward and you see the same effect as yours. A boost is planted there at 3 to 4 Khz. And that lead listeners to complain about mid-range performance post EQ:

image?pagenumber=18&w=800
Again RC6 (this time in yellow) is Audyssey. RC4 is doing nothing. Note that the correction in low frequencies caused Audyssey to garner scores of too little bass.
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On the matter of the "energy in the room" at 30-40 Hz, are you implying that a flat bass response is less desirable than one with a massive gain that also induces ringing?  See the comparison:
No. You do want to correct for what you call "ringing" (really, minimum phase correction of frequency response which also reduces time domain ringing). But once you are done, you need to overlay a boost for all the frequencies below crossover frequency. Without it, the bass will at the same time be "accurate" and "anemic." You get something good, but you lose something else subjectivity. So the answer is NOT that you want to not EQ. You absolutely want to dial out the variations. Smooth response is a requirement for good performance. "Flat" does not. A simple fix by the way without EQ is to simply increase the sub response by a few dB, tested by ear.
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Of course, there are other ways to deal with this issue (e.g. room treatments) but I have a hard time believing (objectively) from these plots that the pros don't far outweight the cons.  Subjectively, it's not even close...
As I said, it is possible that on balance the subjective results are better with Audyssey. So I am not making an absolute statement. Rather, I hope that now that we know how to measure, we zoom in and look at these issues and get insights into what these systems are doing, and whether our results match that of research. I suggest turning off all of your mains for example and just play the sub. Then turn the EQ on and off. Does it get a lot more quiet all of a sudden when EQ is on? What is the trade off between that and the "ringing" going away? Don't try to convince yourself that it is better just because the graph said so smile.gif. If subjectively you think you have less bass performance, then you do! Bring someone else in the room that doesn't know what you are doing and perform that AB test. If everyone says you have made things worse, then you have smile.gif.

Testing for the mid-range is harder. But you can try. Turn off the sub and just listen to mid frequencies. Is there a sudden change in them? Is that better or worse?

So the larger point here is to use one's ears in collaboration with measurements. The two have to agree at some level. Otherwise we have lost sight of why we are doing this work smile.gif.

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post #2408 of 12737 Old 04-05-2013, 09:41 AM
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I read through this....IMO I'm not sure that I'd want to use the speaker distance adjustment for post-Audyssey adjustment, but focus on the subwoofer distance adjustment primarily. It's been a while, but from what I remember from either the Audyssey or Audyssey Pro thread, the focus of distance tweaks-- which are really delay tweaks IIRC - has been sub-only because there's concern that by manipulating the distance of the mains, you might create distortion in the imaging of the speakers unless it's a marginal adjustment (which IIRC was about 1 ft.). If you're not running Audyssey, I'm not sure it matters because you're presumably doing manual adjustments prior to anything you do with REW or a supported PEQ.

HST, I found that I got a flatter response in the subwoofer/satellite splice region when I adjusted my center channel about 8" from what Audyssey set, with no ill effects AFAIK with my imaging from the center. Having a relatively high crossover (100 Hz) for my powered center may have something to do with this. However, I'm not sure I'd do this for my L/R speakers, but YMMV. Keep in mind that like changing the trim in the AVR after an Audyssey run, this is "preference" vs. "reference".

Just to be clear, is the adjustment in the HTS article in meters?

hello
I think the article in HTS was written for my specific situation. I do not have a distance setting for subwoofer in my AVR. So the article describes how to adjust the mains to align with the subwoofer, without changing the alignment between the mains.
And the distance is in meters.

 

Sorry, I should've read it more closely as I missed the part about not being able to adjust your sub distance.  I presume that something along these lines could be applied to the sub distance tweak procedure to help reduce the number of iterations BICBW.  Maybe it could at least provide a better starting point?

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post #2409 of 12737 Old 04-05-2013, 09:41 AM
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Originally Posted by jkasanic View Post

 

So if I understand your premise correctly, the total variation over the 1.7kHz - 17.6kHz frequency range of 8.7 dB for the red curve as compared to 7.8 dB for the green curve is more objectionable than the fact that the green non-corrected response varies in total output of 81dB - 70.7dB = 10.3dB vs. the Audyssey corrected response of 81dB - 74.6dB = 6.4dB?

 

On the matter of the "energy in the room" at 30-40 Hz, are you implying that a flat bass response is less desirable than one with a massive gain that also induces ringing?  See the comparison:

 

No Audyssey decay plot (green curve above):

 

Audyssey corrected response (red curve above):

 

Of course, there are other ways to deal with this issue (e.g. room treatments) but I have a hard time believing (objectively) from these plots that the pros don't far outweight the cons.  Subjectively, it's not even close...

 

I would think it extraordinarily unlikely that the "total variation over the 1.7kHz - 17.6kHz frequency range of 8.7 dB for the red curve as compared to 7.8 dB for the green curve is more objectionable than the fact that the green non-corrected response varies in total output of 81dB - 70.7dB = 10.3dB vs. the Audyssey corrected response of 81dB - 74.6dB = 6.4dB?"  The green curve would appear to me to be likely to sound worse.  

 

But this is an Audyssey vs No Audyssey comparison isn't it?  You have to remember that for some people the idea of a working room correction system is anathema and they will discredit it whenever possible in order to comply with an agenda that you may or may not know about. Personally, I take the pronouncements of those with hidden agendas with a very large grain of salt.

 

Now HST, I also do not favour a flat bass response because my experience is that it sounds anaemic and if you recall my graphs posted here they all show a definite 'house curve' with a good boost rising from the XO frequency to about 20-30Hz and then, ideally, a small tail-off thereafter. I am not sure if DEQ compensates a little for the flat Audyssey curve but of course it won't if you are listening at Reference - but then at Reference the need for a house curve is less pronounced, so maybe Audyssey expects DEQ to take care of it. IDK.

 

And HST, I agree with you that I would prefer a flatter bass if the necessary trade-off for a rising bass was more ringing. There is no point exaggerating a bass response that already has problems.  Of course, if it is possible to control the ringing by sub placement, multiple subs, room treatments etc, then that is the way to go, only applying the house curve when the problems have been resolved.

 

The decay charts above leave me in no doubt at all that Audyssey ON will be hugely preferable to Audyssey OFF ;)

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post #2410 of 12737 Old 04-05-2013, 09:46 AM
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BTW, one of the problems with Audyssey is that you can't independently test the effect of its EQ in different areas. If you use a separate EQ, you can easily turn filters on and off and hear he effect. Doing so is a great learning tool as we are not normally good at correlating what we hear with specific frequency ranges. Download the software referenced in this thread on how to listen and see how good you are. I suspect you will fail well below that of expert listeners smile.gif. But if you play with the tool a few times as I have, you improve quite rapidly. I still lose to Sean Olive and crew but can outperform average listeners with just that bit of practice.

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post #2411 of 12737 Old 04-05-2013, 10:49 AM
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Originally Posted by jkasanic View Post

Not Sanjay but I'm guessing he'll ask if it's possible to move your speaker further from the wall to eliminate boundary effects?
Have I become that predictable? (You posted exactly what I was going to ask.)

 

Haha...no, I think I'm just an above average student! tongue.gif

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post #2412 of 12737 Old 04-05-2013, 10:51 AM
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Originally Posted by kbarnes701 View Post

Yet another fabulously useful post from you, Sanjay. I learn something new every day on AVS, and often from your own posts. Forgive me if I sound a little sycophantic but I do believe it is just as important to  offer thanks and positive comments as it is to offer brickbats (and I do sometimes offer the latter I know).

 

+1  Thanks again Sanjay for sharing with us.  I've learned a tremendous amount on how to improve the SQ in my home theater and I owe a lot of that to your participation here at AVS!

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post #2413 of 12737 Old 04-05-2013, 11:02 AM
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I learn something new every day on AVS, and often from your own posts.
Thanx, had to massage that post to make it as non-technical as possible and still have it make sense.

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post #2414 of 12737 Old 04-05-2013, 11:18 AM
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Originally Posted by amirm View Post
 

You absolutely want to dial out the variations. Smooth response is a requirement for good performance. "Flat" does not. A simple fix by the way without EQ is to simply increase the sub response by a few dB, tested by ear.

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Originally Posted by kbarnes701 View Post

 

Now HST, I also do not favour a flat bass response because my experience is that it sounds anaemic and if you recall my graphs posted here they all show a definite 'house curve' with a good boost rising from the XO frequency to about 20-30Hz and then, ideally, a small tail-off thereafter. I am not sure if DEQ compensates a little for the flat Audyssey curve but of course it won't if you are listening at Reference - but then at Reference the need for a house curve is less pronounced, so maybe Audyssey expects DEQ to take care of it. IDK.

 

And HST, I agree with you that I would prefer a flatter bass if the necessary trade-off for a rising bass was more ringing. There is no point exaggerating a bass response that already has problems.  Of course, if it is possible to control the ringing by sub placement, multiple subs, room treatments etc, then that is the way to go, only applying the house curve when the problems have been resolved.

 

Just to clarify my earlier statements, I didn't mean to imply that a ruler flat bass response was the objective.  My point was simply to call out that a nearly 20dB boost over the 20-40Hz region that remains untamed does nothing imho to improve the overall SQ in the bass region.  I can say definitively that my overall bass response is significantly improved with the relocation of the sub and further improved with the Audyssey filters as well as sub distance tweak.  Looking at the pre-Audyssey, right front wall corner loaded sub location vs. the post Audyssey, mid wall sub location with no smoothing, I can't see how anyone would conclude otherwise?

 

 

In fact, if I apply 1/6 smoothing to these responses, then one can see a 3-4dB boost in the 35-70Hz region (I've already mentioned that I would like to improve the 80-110Hz region):

 

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post #2415 of 12737 Old 04-05-2013, 12:36 PM
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Just to clarify my earlier statements, I didn't mean to imply that a ruler flat bass response was the objective.  My point was simply to call out that a nearly 20dB boost over the 20-40Hz region that remains untamed does nothing imho to improve the overall SQ in the bass region.  I can say definitively that my overall bass response is significantly improved with the relocation of the sub and further improved with the Audyssey filters as well as sub distance tweak.  Looking at the pre-Audyssey, right front wall corner loaded sub location vs. the post Audyssey, mid wall sub location with no smoothing, I can't see how anyone would conclude otherwise?




In fact, if I apply 1/6 smoothing to these responses, then one can see a 3-4dB boost in the 35-70Hz region (I've already mentioned that I would like to improve the 80-110Hz region):


As I mentioned, the conversation we are having is a "macro" level look at the overall response. To know if the bass level is right, you need to look at it in the context of the full response and not just up to 300 Hz. Let's look at the previous research graph I posted:

image?pagenumber=24&w=800
These graphs have been shifted in level so that they don't overlap. So read them as if they were on top of each other. Now look at the top two performing EQ systems on top. You see a downward slope from low frequencies to high. If you just looked at the range up to 300 Hz you would not see that effect. If we now look at your full spectrum response:

900x900px-LL-f2fe6df2_lrsubfullrangefrwandwoaudyssey.jpeg

We see that at 400 Hz and 7 KHz (my eyeballing it) post EQ in red, you have higher levels than your bass region. That doesn't follow the curve of the winning "target curves" where we have a generally slopping down response from sub to high frequencies.

Now if you look at the green no EQ curve of your room, it doesn't have that peaking at 7 Khz so follows the preferred graph better in how its high frequencies keep declining. And it has 7 to 8 dB boost in ~30 Hz region which helps it establish the subjective feel of of higher bass. Of course it has fluctuations which are bad but we tend to be bothered by poor spectrum balance per study report.

As I mentioned this is not at all an argument against EQ. In my book, EQ is mandatory in proper room optimization. To wit, I own the system that is behind the top two performing auto EQ systems in the research study smile.gif. The ideal goals for an EQ is to smooth out the response for us while following the right guidelines as far as the spectrum generally.

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post #2416 of 12737 Old 04-05-2013, 02:24 PM
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Amir,

Why do you claim that the graph should slope down from low to high frequencies and that "flat" is not good?
Is this slope supposed to be the equivalent of the THX equalization recommended for movie audio tracks when played back in home environments?

What document are you getting the plots from?

I would have expected that appropriate adjustments of the relative sound levels would already have been made by the person doing the audio mixing for a movie, A flat trace is supposed to indicate that the audio system will accurately reproduce what the person doing the audio mix intended. (Reference instead of personal preference.) A downward slope indicates that higher frequencies are being progressively more attenuated relative to the lower frequencies.

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post #2417 of 12737 Old 04-05-2013, 02:33 PM
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Quote:
Originally Posted by amirm View Post
 
As I mentioned this is not at all an argument against EQ. In my book, EQ is mandatory in proper room optimization. To wit, I own the system that is behind the top two performing auto EQ systems in the research study smile.gif. The ideal goals for an EQ is to smooth out the response for us while following the right guidelines as far as the spectrum generally.

 

@Selden Ball:  Call me a skeptic, but I think this answers your question...

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post #2418 of 12737 Old 04-05-2013, 02:40 PM
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For some reason I can't find the "research study" being mentioned. A pointer to it probably is right in front of me and I'm not seeing it.

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post #2419 of 12737 Old 04-05-2013, 02:52 PM
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Originally Posted by sdurani View Post

If I divide the speed of sound by the frequency of your dip, I will get the wavelength of that frequency. 1130 ÷ 300 = 4 feet. In order to produce a cancellation dip at that frequency, the direct sound from your speaker would have to combine with a copy of the sound (reflection) that is out of phase.

If the reflected path off a wall is 4 feet longer than the direct path from your speaker, the reflection will be in phase with the original 4-foot wave, albeit delayed once cycle (4 feet later). But if the reflected path is only 2 feet longer than direct path, then they will combine with the reflection mid-wave (180° out of phase with the direct sound). Cancellation.

How would the reflected path be 2 feet longer than the path of the direct sound from your speaker? It makes a 1 foot trip to the wall and a 1 foot trip back to the speaker plane to combine with the direct sound. So there must be a boundry 1 foot away, for that 2-foot round trip. 2 feet of extra travel is a half of the 4-foot wavelength. When combined, they'll cancel at that frequency (300Hz).

If you suspect a dip in the frequency response might be due to speaker/boundry interference (rather than modal), look for a wall a quarter wavelength away: ¼ of 4 feet is 1 foot. Easy to remember?
If you pull the speaker far enough out, say 4 feet from the wall, then there are some interesting benefits. Since 4 feet would be the quarter wavelength, the full wavelength would be 16 feet. 1130 ÷ 16 = 70Hz. Pulling the speaker out that far into the room will not only reduce the energy of the reflection (smaller dip) but will lower the frequency of the cancellation (below the crossover point). Pretty neat.

By comparison, moving a subwoofer closer to a boundry (aside from the ususal benefits) will slide its boundry cancellation dip higher up the frequency range, hopefully well above its crossover point. Convenient how that works out.

Since I have no intention, let alone space, to place my L/C/R speakers 4 feet into the room, I instead loaded up my front wall with absorbtion. No reflections = no boundry cancellation. There's more than one way to skin a cat.

Never considerd reflections from the back of the speaker cancelling sound coming from the front. That's impressive, sdurani.

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post #2420 of 12737 Old 04-05-2013, 03:47 PM
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Quote:
Originally Posted by Selden Ball View Post

For some reason I can't find the "research study" being mentioned. A pointer to it probably is right in front of me and I'm not seeing it.
https://docs.google.com/file/d/0B97zTRsdcJTfY2U4ODhiZmUtNDEyNC00ZDcyLWEzZTAtMGJiODQ1ZTUxMGQ4/edit?hl=en&pli=1
Old and oft-discussed.

For a more recent perspective, see here: http://www.aes.org/e-lib/browse.cfm?elib=16480
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post #2421 of 12737 Old 04-05-2013, 03:56 PM
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Originally Posted by Selden Ball View Post

For some reason I can't find the "research study" being mentioned. A pointer to it probably is right in front of me and I'm not seeing it.
Those graphs represent the Harman studies on preferred frequency responses for music listening.

Interestingly enough, in average sized rooms, a speaker that has a flat anechoic response tends to produce this type of descending response in room. Likewise with live musical instruments played in smaller spaces. Our hearing is accustomed to this and this frequency response sounds more natural, where a flat response sounds more like live instruments played outdoors with no reinforcement. In room, it sounds weak and flat.

You have a point though, about multichannel movie mixes. Those mixes are made for a particular spectral balance that is most accurately reproduced by systems playing at Reference with flat in room responses.


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post #2422 of 12737 Old 04-05-2013, 06:11 PM
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Quote:
Originally Posted by sdurani View Post

If I divide the speed of sound by the frequency of your dip, I will get the wavelength of that frequency. 1130 ÷ 300 = 4 feet. In order to produce a cancellation dip at that frequency, the direct sound from your speaker would have to combine with a copy of the sound (reflection) that is out of phase.

If the reflected path off a wall is 4 feet longer than the direct path from your speaker, the reflection will be in phase with the original 4-foot wave, albeit delayed once cycle (4 feet later). But if the reflected path is only 2 feet longer than direct path, then they will combine with the reflection mid-wave (180° out of phase with the direct sound). Cancellation.

How would the reflected path be 2 feet longer than the path of the direct sound from your speaker? It makes a 1 foot trip to the wall and a 1 foot trip back to the speaker plane to combine with the direct sound. So there must be a boundry 1 foot away, for that 2-foot round trip. 2 feet of extra travel is a half of the 4-foot wavelength. When combined, they'll cancel at that frequency (300Hz).

If you suspect a dip in the frequency response might be due to speaker/boundry interference (rather than modal), look for a wall a quarter wavelength away: ¼ of 4 feet is 1 foot. Easy to remember?
If you pull the speaker far enough out, say 4 feet from the wall, then there are some interesting benefits. Since 4 feet would be the quarter wavelength, the full wavelength would be 16 feet. 1130 ÷ 16 = 70Hz. Pulling the speaker out that far into the room will not only reduce the energy of the reflection (smaller dip) but will lower the frequency of the cancellation (below the crossover point). Pretty neat.

By comparison, moving a subwoofer closer to a boundry (aside from the ususal benefits) will slide its boundry cancellation dip higher up the frequency range, hopefully well above its crossover point. Convenient how that works out.

Since I have no intention, let alone space, to place my L/C/R speakers 4 feet into the room, I instead loaded up my front wall with absorbtion. No reflections = no boundry cancellation. There's more than one way to skin a cat.

Quote:
Originally Posted by Pres2play View Post

Never considerd reflections from the back of the speaker cancelling sound coming from the front. That's impressive, sdurani.

I checked the absorption coefficients for 703/705, which is what I have. For 250 and 500 Hz, OC panel must be 4" thick. That's with panels on the wall and no air space. I'll post my results.

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post #2423 of 12737 Old 04-05-2013, 07:50 PM
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Never considerd reflections from the back of the speaker cancelling sound coming from the front.
Keep in mind that this is only for frequencies that are still radiating somewhat omnidirectionally from your speakers. As frequencies go higher, dispersion usually becomes narrower, and it becomes less of a problem. As you might guess, this will vary from speaker to speaker.

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Originally Posted by djbluemax1 View Post

Those graphs represent the Harman studies on preferred frequency responses for music listening.

Interestingly enough, in average sized rooms, a speaker that has a flat anechoic response tends to produce this type of descending response in room. Likewise with live musical instruments played in smaller spaces. Our hearing is accustomed to this and this frequency response sounds more natural, where a flat response sounds more like live instruments played outdoors with no reinforcement. In room, it sounds weak and flat.

You have a point though, about multichannel movie mixes. Those mixes are made for a particular spectral balance that is most accurately reproduced by systems playing at Reference with flat in room responses.


Max

In the last week, I have been concentrating on sub measurements, with the intention of optimizing house curves later. But the comments in the last few posts about descending response had me interested.
Here is the response I measured with center speaker and sub, without EQ corrections (this is from an earlier post about optimizing sub delay):

Disregarding the sub part (level of which has not been optimized yet), the center has a descending slope in my measurement. However, in this review, the slope of the speaker is basically flat:
http://www.soundandvisionmag.com/article/test-report-goldenear-technology-3d-array-ultra-high-performance-soundbar-system?page=0,3

My question is just basic: why do I measure a descending slope for a speaker which is flat? Is this an AVR effect? But my AVR does not have special effect settings. Is it a room effect? If yes, what room effect causes the descending slope?
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post #2425 of 12737 Old 04-05-2013, 09:52 PM
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According to the reviewer, "I would like to have measured the frequency response of the 3D Array with the unit mounted on a wall, but the ersatz wall I attach to my measurement stand for this purpose wasn’t large enough to accommodate the long bar. So I measured it placed atop a 2-meter stand, with the microphone placed at a distance of 2 meters, using quasi-anechoic technique to remove the effects of reflections from nearby objects."

 

This is likely quite different from how you measured the response of your soundbar, and could account for the different results.  I don't see anything particularly concerning about the response you measured.

 

By the way, you continue to emphasize that your measurements are without any EQ.  What brand AVR do you have, and does it have EQ capabilities like Audyssey?  And you mention that you have been working on the sub response.  Can you share what you have been doing to improve the bass response, and do you have any new measurements?

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post #2426 of 12737 Old 04-05-2013, 10:41 PM
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Originally Posted by AustinJerry View Post

According to the reviewer, "I would like to have measured the frequency response of the 3D Array with the unit mounted on a wall, but the ersatz wall I attach to my measurement stand for this purpose wasn’t large enough to accommodate the long bar. So I measured it placed atop a 2-meter stand, with the microphone placed at a distance of 2 meters, using quasi-anechoic technique to remove the effects of reflections from nearby objects."

This is likely quite different from how you measured the response of your soundbar, and could account for the different results.  I don't see anything particularly concerning about the response you measured.

By the way, you continue to emphasize that your measurements are without any EQ.  What brand AVR do you have, and does it have EQ capabilities like Audyssey?  And you mention that you have been working on the sub response.  Can you share what you have been doing to improve the bass response, and do you have any new measurements?
hello Jerry
Thanks for your clarification. I mention that my measurements are without Audyssey, because in this thread the use of Audyssey seems to be so ubiquitous. I have an entry level Cambridge Audio Azur 351R receiver, which does not have Audyssey. I bought this before I started learning about REW, room influence and Equalizing, and when I bought it the slimline profile of the Azur 351R fitted the small space I had. Now I regret maybe not having Audyssey. So I am thinking now what I could do, like putting a miniDSP on the subwoofer line.

I will come back with another post with some more bass response measurements. Have to go now.
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post #2427 of 12737 Old 04-06-2013, 05:25 AM
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Quote:
Originally Posted by jkasanic View Post

Quote:
Originally Posted by amirm View Post
 
As I mentioned this is not at all an argument against EQ. In my book, EQ is mandatory in proper room optimization. To wit, I own the system that is behind the top two performing auto EQ systems in the research study smile.gif. The ideal goals for an EQ is to smooth out the response for us while following the right guidelines as far as the spectrum generally.

 

@Selden Ball:  Call me a skeptic, but I think this answers your question...

 

I'm intrigued how he knows which system under test he owns because the results were anonymised and AFAIK the brands under test were not named in the results. Amir seizes on this report all the time because, according to him, Audyssey did badly in it (qv how he knows this).

 

I am always suspicious when Audyssey XT32 (used by most of us here) is claimed to have this problem and that problem if the person making the claim also sells 'exotic' cables and 'high end' mumbo-jumbo equipment like Lexicon and so on - aptly named because they were the people who took an Oppo BD player, wrapped it in its entirity in a new case, added 3,000 dollars to the price and then claimed it had huge performance benefits over other BD players...

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post #2428 of 12737 Old 04-06-2013, 05:48 AM
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Quote:
Originally Posted by Selden Ball View Post

I would have expected that appropriate adjustments of the relative sound levels would already have been made by the person doing the audio mixing for a movie, A flat trace is supposed to indicate that the audio system will accurately reproduce what the person doing the audio mix intended. (Reference instead of personal preference.) A downward slope indicates that higher frequencies are being progressively more attenuated relative to the lower frequencies.

 

For movies, the mixer will have made the appropriate adjustments but remember that he is mixing for Reference Level, which is rarely used in a home environment. So at anything below Reference, the bass (for example) will be de-emphasised perceptually. Of course, Dynamic EQ etc might compensate for this and no other boosting of the bass frequencies may be needed. DEQ will bring it, perceptually, back to flat which, as you say, is where it should be for movie mixes.  Personally, since getting the dual Submersives, and because they are so clean and amazing even at the very lowest of frequencies, I prefer a house curve which boosts the bass a little. But I do emphasise the word 'prefer' in the preceding sentence.

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post #2429 of 12737 Old 04-06-2013, 08:05 AM
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I'm intrigued how he knows which system under test he owns because the results were anonymised and AFAIK the brands under test were not named in the results.
Keith, by now anyone who's familiar with the many discussions of the test knows which RC is which. The fact that Harmon has leaked UUT identities (and not only for this test) has, rightly or wrongly, fueled a certain amount of cynicism regarding their objectivity.
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Quote:
Originally Posted by kbarnes701 View Post

I'm intrigued how he knows which system under test he owns because the results were anonymised and AFAIK the brands under test were not named in the results.
Keith, by now anyone who's familiar with the many discussions of the test knows which RC is which. The fact that Harmon has leaked UUT identities (and not only for this test) has, rightly or wrongly, fueled a certain amount of cynicism regarding their objectivity.

Thanks for the info. That answers my question then.

 

Again, call me sceptical or cynical, but I am also very wary of tests conducted by anyone with a vested interest in the outcome...

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