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PostPosted: Mon Jun 04, 2018 11:39 pm 
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Location: Sacramento, CA
I'm not doing a flush/soffit style. That'd be nice, but not now. I have small speakers I really like: NHTPro M-00 with the subwoofer S-00. I'm about to move into my new cottage control room. It's small. 146sf. 10'6" wide and 14' in length. I understand the equidistance from the side walls. But what rules should I apply for the distance from the wall in back of them?

Also subwoofer placement. I had previously operated under the conception that sub location wasn't really important. Those bass waves take so long to develop and low frequencies aren't directional. Something like that. But if I'm applying the mirror speaker reflection DIY rule, do I also apply it to the sub? I was planning on placing the sub closer to the back wall than the pair and equidistant between them.

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PostPosted: Tue Jun 05, 2018 1:34 am 
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understand the equidistance from the side walls. But what rules should I apply for the distance from the wall in back of them?
Your room is small, so the ONLY option you have is to place your speakers tight up against the front wall, with only a 4" gap between the speaker and the wall. That gap is for the 4" thick panel of OC-703 that you will need there, to help deal with the SBIR issue, as well as other problems.

The issue is this: At low frequencies, your speaker is omnidirectional: It sounds out sound pretty much equally in all directions. The sound waves that head out BACKWARDS from your speaker will hit the wall behind, bounce back towards you, and interfere with the other part of the same wave that is heading directly towards you. So there's a delay in time between the instant that the direct wave hits you, and the instant that the "bounced" wave hit you: the "bounced" wave gets there later, since it took a longer path. So, since you have two copies of the same wave reaching your ears, but they are out-of-phase with each other, and delayed, there will be interference patterns going on. Basically, there will be a massively huge dip in your frequency response at the frequency where the wavelength matches the bounce distance. If the difference in distance-traveled is large, then the frequency will be very low, and practically impossible to treat. If you make the distance shorter, then the frequency will be higher, and can be treated. The higher it is, the easier it is to treat, and the less intrusive it is in any case.

In a large room, it is possible to place the speakers very far away from the wall, such that the lowest dip is actually below the bottom end of the audible spectrum, but the room has to be pretty big to do that! For all other rooms, the only other option is to place them against the front wall, with just enough space to put in the absorption that you need to treat the problem. With typical studio monitors, that places the SBIR issue up in the lower end of the mid-range, where it isn't too noticeable anyway, and where 4" of OC-703 does a decent job of dealing with it.

Quote:
Also subwoofer placement. I had previously operated under the conception that sub location wasn't really important.
That's a common misconception, but in reality, sub placement and setup is VERY important! . . .

Quote:
Those bass waves take so long to develop and low frequencies aren't directional.
Welllllll.... sort of! The bass waves themselves might not be too directional, but the effect of moving the speaker around, and even changing the orientation of the speaker, can actually have a pretty big impact on the overall acoustic response of the room. It is surprising that even a couple of inches difference in sub position can have a noticeable effect on bass response in the room.

Yes, the waves are long, and for this exact reason it is critical to position the sub correctly. For high frequencies, where the wavelengths are just a few inches, even moving your head a little to the left or right, forward or backward, can move your ears through several cycles of the wave. But for lows, moving your head a few inches can move your ears long the SAME wave, from close to a peak to close to a null. And for the exact same reason, subs have a "phase invert" setting on them, just in case you can't get the sub to a spot where it is in phase with your head position.

So setting up your sub properly is very important, and not easy to do.

Quote:
I was planning on placing the sub closer to the back wall than the pair and equidistant between them
I think you mean the FRONT wall, not the BACK wall! You never want to have speakers behind you (except for the surround speakers on a multi-channel system), and NEVER have the only sub behind you. The mains and the sub(s) in a 2.1 or 2.2 system are always in front of you, on the front wall.

Placing the sub equidistant between the mains is generally not a good location, as it places the three speakers in a strange relationship where some frequencies that they have in common are emphasized, and some are attenuated. For most rooms, it is usually better to have the sub off-center to the left or right, by about 10% to 30% of the room width. Maybe more.

One good way to find the best spot for your sub, is to flip things around: Set up the sub on a tall stand such that it is exactly where your head will be when you are mixing, then YOU move around the front of the room, on your knees, getting your head into every possible location where you could place the sub, while listening to pink noise, sine sweeps, or full-range music. When you find the spot where the bass response is the smoothest, that's where the sub needs to go.

Even better is to do this using the REW acoustic software (free!), and move the measurement mic around the front of the room, instead of your head. It's a lot more comfortable, and a lot more accurate! You can easily see how the response changes as you move the mic, then hunt for the best spot, and set up the sub there. Don't forget to repeat this with the "phase invert" switch flipped the other way! Very often you can't get good results with the phase switch in one setting, and only get things smooth with it flipped the other way.

Sometimes, one sub is not enough, and you need two or more.

I'm not sure if you have seen the Studio Three thread, but if not, here it is: www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=20471 With that room, after we got the mix position near perfect, we found rather uneven response at the client couch position, and could not fix it by moving the couch around (not enough room). So I added a second sub to the room, and move that one around until it filled in the nulls at the client couch without causing too much harm at the mix position. It took quite a while to fiddle with both subs, sliding them around in 1" increments, to get the best possible response at both locations. The second sub ended up directly behind the left wing of the desk, right next to the front leg! Not a spot I would have guessed, at first glance, but that's where we got the best response. Of course, we also had to use delays on the mains to compensate for the shorter distance, but it worked out fine in the end.


- Stuart -

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PostPosted: Tue Jun 05, 2018 3:33 am 
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Soundman2020 wrote:
Your room is small, so the ONLY option you have is to place your speakers tight up against the front wall, with only a 4" gap between the speaker and the wall. That gap is for the 4" thick panel of OC-703 that you will need there, to help deal with the SBIR issue, as well as other problems.


Thank you. Let get this straight. The 4" bass traps should go directly behind the speakers. How about between them as well? And forgive me, what does SBIR mean? I have an abundance of 703 and Knauf, both 2". I'm planning on building some 2 and mostly 4" panels and tri-corner traps.

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PostPosted: Tue Jun 05, 2018 7:58 am 
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Thank you. Let get this straight. The 4" bass traps should go directly behind the speakers.
4" yes, but it's not a bass trap: 4" is not thick enough to be a good bass trap. It's just acoustic absorption that goes flat on the front wall of the room (the wall you face when you are seated at the mix position), directly between the rear corner of the speaker and the wall.

Quote:
How about between them as well?
Possibly, but not yet! That's a decision that you would make further down the line, once you have the room set up properly and the basic initial treatment in place. Depending on how that works out, you might need additional treatment on the front wall: Or might not.

Quote:
what does SBIR mean
"Speaker-Boundary Interference Response". It's a technical term used to describe what happens to the sound field in the room when the sound coming directly out of the speaker meets up with a copy of itself that bounced off a wall, the floor, or the ceiling. Those are the "boundaries" of the room, and have a major effect on the overall room acoustics. So SBIR is just the way the room "responds" when the sound from the "speaker" hits a "boundary" and as a result "interferes" with itself. It's actually a very important aspect of room acoustics.

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PostPosted: Thu Jun 07, 2018 10:35 pm 
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Location: Sacramento, CA
Thank you. My understanding is I should put a gap between the panel and the wall. That it should sit away from the wall a little. Not touch. For the behind the speaker thing, should I do the same, or are you suggesting I just kind of extend the wall with a couple of absorbers by place it right up against the wall?

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