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post #1 of 11 Old 10-04-2019, 07:42 AM - Thread Starter
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Speaker Placement Help - Small "Dedicated" Home Theater

So I'll try and keep this brief - short version is I aligned my listening position with the window casement in my spare-room-turned-HT, placed the surrounds and in-ceiling atmos speakers according to Dolby angle guidelines, and then afterwards came to realize my listening position is about two inches back of dead center in the room.

I spent last night hand-wringing and reading about optimal listening position (not really sure why I didn't start from there ), but thought I would come to ya'll and get some general feedback.

I've attached a few pics of the room as it currently stands. Obviously there's no seating yet, and I'm willing to move speakers as necessary, but would love to avoid moving the atmos speakers if at all possible. They're 5 feet to the front and rear of the listening position, as I assumed a 3 foot ear height (8 foot ceilings) which made 5 foot distances a 45 degree angle. The left surround only has about 6 inches to move back on account of the bump out.

Just soliciting some general feedback and hoping to quell my anxiety about nulls a bit haha. Also, open to as many acoustical treatments as necessary - this is a single-purpose room and my wife has given me free reign to do whatever I'd like. Room will be totally painted and thick blackout curtains installed before we go live. Thanks all!

Oh the two subs are used HSU VTF3-MK3s I just picked up last night. Would like to use them both, and can move around as needed. The projector frame will be a hybrid of a false wall and a fixed frame - it's going to jut out about a foot and cover all three front speakers, but won't be a true false wall in that I'm not framing out the length of the wall. I'll just mask off the screen area then, which I'm figuring on being 100" at this point.

Any questions, ask away!

EDIT - figured dimensions would help.

Short side of room (where screen will go and LCR is currently located) - 122" (10' 2")
Long wall - 184" (15' 4")
8' ceilings
MLP currently assumed at 94" (aligned with outside edge of window casement along long wall)
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post #2 of 11 Old 10-07-2019, 08:08 AM
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Ideally seated head would be about 120" from the front wall. That still leaves you about 5' behind you to avoid rear wall buildup a bit and give a nice surround field.

Rear wall limited range limp membrane lower frequency absorbers will help or potentially diffusion but would wait til you get all set up, measured, and the rest of the room treated to decide.

Front corner bass control will be needed. Deaden the front wall pretty much 100%.

Address early side wall reflections - likely will need to be on stands given the windows.

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post #3 of 11 Old 10-07-2019, 01:00 PM - Thread Starter
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Thank you, gives me some terms to research as some of that is a foreign language to me haha.

Let me ask this - for argument’s sake let’s say I can’t move the listening position. How can I go about maximizing sound quality with room treatments and positioning of dual subs?
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post #4 of 11 Old 10-09-2019, 06:50 AM
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If you are in the center of the length, really not much you can do to address the issues caused by that position (length modes will cause severe peaks and nulls in bass response). They'll just be too low in frequency to really try to deal with without being FEET thick. You can still do the rest - you just won't fix the room length modes.

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post #5 of 11 Old 10-09-2019, 10:19 AM
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Quote:
Originally Posted by AFMan916 View Post
Thank you, gives me some terms to research as some of that is a foreign language to me haha.
If you blow across an empty bottle, you can get the air in that small chamber to resonate (make that booooh sound). If you enlarge that small chamber to the size of your room, the air in there will still resonate (of course at different frequencies than a bottle). Below is a chart of room modes (resonances) for your 15'4" room length. Note that each problem frequency is colour coded, so you can see where it peaks & nulls along the length of your room.



Ideally, you'd want the listeners' ears at 2/3rd room length from the front wall (around 5' from the back wall, as @bpape recommended), where most of the problem frequencies are roughly the same level.



Worst location would be near the midpoint of room length, where the problem frequencies are at the extremes when it comes to level: loudest peaks or deepest nulls (no moderation).

Quote:
Let me ask this - for argument’s sake let’s say I can’t move the listening position. How can I go about maximizing sound quality with room treatments and positioning of dual subs?
Treatment would need to be feet thick. However, looking at the graph, two of the problems at your listening position fall within the typical subwoofer range: null at 37Hz (blue trace) and peak at 74Hz (black trace). The good thing about the 74Hz peak is that it will be the same for all listeners (they're all the same distance from the front & back walls), so any competent room correction system should be able to flatten it out, fixing the problem for all listeners.

The room mode creating the 37Hz null can be cancelled by placing your two subs on the front and back walls. Imagine you're at a playground and you don't want a swing to move (resonate). One way to do that is to have two kids push equally hard from both sides of the swing. Likewise, applying equal sound pressure from both sides of a null will cancel its room mode (and all the peaks & nulls created by that mode).

Look at the 3rd graph in the pic below. Placing subs on opposite sides cancels the 37Hz null, leaving only the 74Hz peak (which can be flattened with EQ).


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post #6 of 11 Old 10-09-2019, 12:25 PM - Thread Starter
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Quote:
Originally Posted by sdurani View Post
...
I'm not going to quote all of that because there's no need but I just wanted to say THANK YOU for that absolute wealth of information! Exactly what I needed.

So aside from front and rear placement of the woofers (which won't be a problem), is there anything else I can do to mitigate the other peaks/nulls I'm seeing in that graphic you posted? Or do those corrections come more down to Audyssey room correction? I believe my Marantz 6011 has Multeq XT32.

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post #7 of 11 Old 10-09-2019, 12:53 PM
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Originally Posted by AFMan916 View Post
I just wanted to say THANK YOU for that absolute wealth of information! Exactly what I needed.
You're welcome. Hope it made sense. I tried to avoid using technical terms so you wouldn't have to learn a foreign language.
Quote:
So aside from front and rear placement of the woofers (which won't be a problem), is there anything else I can do to mitigate the other peaks/nulls I'm seeing in that graphic you posted? Or do those corrections come more down to Audyssey room correction? I believe my Marantz 6011 has Multeq XT32.
Peaks can be cut down by EQ (Audyssey XT32 will do a good job of that). But EQ cannot boost the level of a null (it's a cancellation). The easiest and most effective way to deal with nulls (above the subwoofer range) is to avoid sitting in them. No joke.



Notice that all the nulls along your room length fall at even divisions (half, quarters, sixths) of room length. So it is best to place the listeners' ears at odd divisions (thirds, fifths) of room length. If you don't want to move the listeners all the way back to 2/3rd room length, then try moving them slightly forward or rearward to 2/5th or 3/5th room length. This way, the listeners won't be sitting in the null of the 111Hz length mode (red trace) that is at the midpoint of room length (your proposed listening position). Best part: it's free (doesn't cost anything to move the listeners, IF you're willing to do that).

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post #8 of 11 Old 10-09-2019, 01:20 PM - Thread Starter
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Great great stuff, thanks again man. You're an excellent teacher, this is all making much more sense to me now!

Quote:
Originally Posted by sdurani View Post
...null of the 111Hz length mode (red trace)...
I wanted to ask you, concerning that 111Hz null - what exactly does that "sound" like? I understand theoretically, in terms of a literal null at the 111Hz range, but in a real-world scenario how does that manifest as it relates to listening enjoyment? I'm not even sure what sounds are typically found in the 111Hz range. Is that dialogue? A quick Google search led me nowhere, other than some odd references to it being the "holy frequency". Is my soul at stake if I keep my listening position as-is?

All kidding aside, I'm trying to better understand the implications for, say, any given movie/gaming night.
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post #9 of 11 Old 10-09-2019, 02:25 PM
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Quote:
Originally Posted by AFMan916 View Post
I wanted to ask you, concerning that 111Hz null - what exactly does that "sound" like?
Quieter than surrounding frequencies. Nulls aren't as bad as peaks (our human hearing tends not to miss what we're not hearing). By comparison, loud peaks are more easily audible and tend to mask other details higher up in the frequency range.
Quote:
I'm not even sure what sounds are typically found in the 111Hz range. Is that dialogue?
The fundamentals of male vocals start around 100Hz. And some other stuff:

Quote:
A quick Google search led me nowhere, other than some odd references to it being the "holy frequency". Is my soul at stake if I keep my listening position as-is?
Nothing a home theatre exorcism won't fix.

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post #10 of 11 Old 10-10-2019, 06:39 AM - Thread Starter
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Originally Posted by sdurani View Post
The fundamentals of male vocals start around 100Hz. And some other stuff
So help me understand the exact range that is subject to this 111Hz null a little better. Is it literally just that frequency? Or +/- a few Hz?

Is there a "peaks and nulls" primer somewhere? A non-technical document on the basics of room acoustics? Maybe, you know, something just to poke my head into the entrance of the rabbit hole?
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post #11 of 11 Old 10-10-2019, 07:34 AM
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So help me understand the exact range that is subject to this 111Hz null a little better. Is it literally just that frequency? Or +/- a few Hz?
That frequency is a prediction based on the length of your room. But if your walls flex, then it could be a slightly different frequency because the room becomes slightly longer when bass is played at loud sound pressure levels. If your home theatre is in a concrete bunker, then this is not a problem (not much wall flexing). Don't get hung up on specific numbers, instead view them as an indication of which problems are in the subwoofer range (can be fixed by subwoofer placement) and which have to be addressed by other means.
Quote:
Is there a "peaks and nulls" primer somewhere? A non-technical document on the basics of room acoustics? Maybe, you know, something just to poke my head into the entrance of the rabbit hole?
The best book on acoustics I've ever read was 'Sound Reproduction' by Floyd Toole (who posts on this forum). Thorough and easy to understand, especially for us non-technical folks.

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