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post #91 of 97 Old 01-18-2013, 01:56 PM
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Originally Posted by jim19611961 View Post

I included the 1/12th response for your benefit really since you made the statement the the unsmoothed response isnt what the ear actually hears. In the 1/12th response, you can see a less than 1db difference caused by the -10db 24ms return. Only under close scrutiny (the unsmoothed response) can you see what that return is doing.
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Hi Jim, as far as I know 1/6th octave is about the closest resemblance at mid and higher frequencies to our ear's ability to discern timbral colorations. The ear's loudness summing bandwidth is wider, more like the traditional 1/3rd octave. So even 1/12th smoothing is excessive smile.gif

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post #92 of 97 Old 01-18-2013, 02:11 PM
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Originally Posted by Nyal Mellor View Post

Hi Jim, as far as I know 1/6th octave is about the closest resemblance at mid and higher frequencies to our ear's ability to discern timbral colorations. The ear's loudness summing bandwidth is wider, more like the traditional 1/3rd octave. So even 1/12th smoothing is excessive smile.gif

I dont think most ears can discern a <1db difference. Given the shown difference would be even less at 1/3rd octave (practically two lines on top of one another), I chose to show the 1/12th octave results. Had the difference been more than 1db, I would have shown 1/6th or 1/3rd.

The larger point is this:

1) A -10db at 24ms specular return (aimed right at the mic) doesnt make an audible difference in terms of FR.
2) The comb filtering caused by such a return may or may not be discernible (not to my ears)

Given the current direction of the thread, and the relevancy to the OP's question on how to treat his room, knowing what reflections are harmful and which are not seems relevant to me. It would be nice to know at what delay and magnitude (an ETC) Ethan's example was representative of. I kinda assume his were early reflections (<10ms) and possibly higher magnitude (-10db relative to the direct signal) than mine. Ethan?

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post #93 of 97 Old 01-19-2013, 01:50 PM
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Originally Posted by dynfan View Post

1. I have a bunch of Auralex Mini Fusors that are 12" x 12" panels and roughly 6" in depth for the diffusor itself. The plan was to fill the cavity with OC703 and place them in array at the locations I have proposed.
Sounds like a good option, especially if mounted on the side walls flanking the seating area (I do not know where you planned to put them) and yes, do make sure to fill them so they do not resonate.

The depth relates with where the diffusion effect ceases at low frequencies. But the deeper the diffusor, the further one must be away from it to avoid undesirable side effects. I'm using 4.5" diffusors at a minimum of 40" from closest listener's ears with no problems.
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3. I have been browsing home improvement stores for items that can be used in place of commercial products. I have found some rigid fiberboard panels that is sold in 4' x 8' sheets and is in the form of a waveform. The difference between the high and low points is about 2" and I was considering cutting this material and placing it behind a fabric frame to allow the treatments to better blend in the room.
That sounds more like an absorber to me, not a diffusor.
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post #94 of 97 Old 01-20-2013, 12:00 PM
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Originally Posted by jim19611961 View Post

To make sure i am interpreting the graph right. Its the rapid succession of peaks and valleys closely together giving the indication of comb filtering, yes?

Yes.
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Do you have a before and after (treated vs not) of the same space illustrating both what a comb filtered response looks like vs one that doesnt?

To ensure identical settings I opened the same file again and exported the "with treatment" measurement from the same session, attached here.

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post #95 of 97 Old 01-20-2013, 12:17 PM
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Originally Posted by Nyal Mellor View Post

Those measurements are unsmoothed, so again misleading. You know as well as I do that we don't hear what the graph shows.

It's not misleading, and it does affect what we hear. Hang on, I'll get to that. I agree that comb filtering that's different left and right is not as damaging as when the same response reaches both ears. If you run mono music through a recording studio flanger effect (comb filter) and play that through a loudspeaker, you will absolutely hear the severely affected response. But in a room the response is different left and right, so the nulls in one ear are not likely present in the other. However - and this is the big point - such a skewed response that changes drastically with small head movements harms imaging. As you move around slightly, the sound source locations change. This does not happen when early reflections are treated fully with absorption.
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And what did you mean by 'a normal nearfield listening position'? AFAIK our normal listening position is not what you would call nearfield which I interpret as where the contribution of the direct sound to measured frequency response is greater than the reflected sounds.

Yes, exactly. The measurements I posted were taken near field, which reduces the strength of the reflections. If the microphone had been farther back in the room, the response would be even more skewed.
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Why do you think comb filtering is psychoacoustically relevant? ... But maybe I am missing something?!

Yes, what you (and some others) miss is how the changes in comb filter frequency versus position harms imaging, as explained above. I know anecdotes are not proof of anything, but here's a very short story anyway: When people visit me and hear my HT system playing stereo music, they're always surprised by the stable imaging and pinpoint phantom center. One time AlexO, a regular in the Stereophile forum, drove out from NYC to visit me. I have three front speakers, all active models with small power LEDs, and the middle speaker's light stays on while playing stereo. Alex was certain the middle speaker was playing as he listened, and he had to get up and walk right up to it before he believed me that it was not playing.

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post #96 of 97 Old 01-20-2013, 01:50 PM
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Originally Posted by Ethan Winer View Post

I agree that comb filtering that's different left and right is not as damaging as when the same response reaches both ears.

But in a room the response is different left and right, so the nulls in one ear are not likely present in the other.
As we are talking about speakers and not headphones, both ears hear both speakers. The nulls from one speaker are somewhat "filled" by the sound from the other speaker, so both ears hear sound that is less severely combed.
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post #97 of 97 Old 01-21-2013, 11:02 AM
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Originally Posted by Roger Dressler View Post

both ears hear both speakers. The nulls from one speaker are somewhat "filled" by the sound from the other speaker, so both ears hear sound that is less severely combed.

Yes, of course, and I didn't mean to imply that one ear hears only one speaker. But the response at each ear is different, regardless of the loudspeaker sources. If you cover one ear it's easier to hear the comb filtering. Regardless, the main point is the comb filtering is not only a problem for frequency response, but also a cause of image-shifting.

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