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PostPosted: Fri Feb 19, 2021 2:54 pm 
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Location: Valencia, California
Hey everyone,

It's been awhile since I've posted in here, but I moved into a new place and am looking for some guidance on room treatment and/or speaker placement.

Room Dimensions:
L: 17' / 518.2cm
W: 13.75' / 419.1cm
H: 8' / 243.8cm

amroc: https://amcoustics.com/tools/amroc?l=51 ... .8&r60=0.2

Treatment: I have yet to fully build out the front and back wall, I'd like to figure out a plan of action so I don't build something that's useless or makes things worse.
Front: 24" superchunks of RW safe n sound in corners, 9" RW safe n sound 4'x8' with 1/8' hardboard facing listening position.
Sides: 6" OC703 2'x8', QRD-11 diffuser, 4" OC703 2'x8'
Back: 12" deep membranes in corners (2" OC703 > 2"RW60 > 1.5" Air > 4" OC703 > 2.5" Air > Rigid Back. Sealed.), QRD-11 diffuser, 12" deep membrane (0.5lb MLV > 4" Air > 4" RW60 > 4" Air > Rigid Back. Sealed.)
Ceiling: 6" OC703 4'x8' cloud between speakers and listening position, 6" OC703 2'x6' 3/4 from front wall.


Speaker Placement: Speakers are Barefoot MM27 with -2db on the subs.
I've tried a few placements
#1. Speakers against the front wall - 40hz and below spikes, 125hz null still exists
#2. Speakers 30" from front wall - 40hz and below is more balanced, 125hz null still exists
#3. Speakers 48" from front wall - all bass is a mess, tons of nulls, 125 null still exists
#4. Speakers 16" from front wall - 40hz and below is balanced, 125hz null improved about 5db.

I tried lowering speaker height from 48" to 45" to see if it was ceiling/floor bounce.
I also tried putting treatment in between the listening position and the speaker to see if it was floor bounce, it improved the null by about 3db.
I ran a 125hz sine wave and went around the room with an SPL meter, there are no reasonable listening areas where it's improved, only spikes in the modal areas such as side walls and 2-side corners.

Questions:
- Is it a collection of room modes causing my issue?
- What treatment will fix the 125hz null?
- Any other general recommendations on improving my room?


Budget & Goal
Everything I've built thus far is extremely modular, I'm open to moving anything around, adding/changing/removing treatment I've already built.
I'm open to building hangers, membranes, slat resonators, VPRs, and more permanent structures as well.
Budget $2-$3k.
Goal: +/- 5db

Link to .mdat file: https://www.dropbox.com/s/5mch5gutii5t2 ... .mdat?dl=0

Let me know what you guys think and what questions you have.
Thank you!


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Last edited by mattsal on Sat Feb 20, 2021 2:28 am, edited 1 time in total.

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PostPosted: Fri Feb 19, 2021 11:13 pm 
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Joined: Wed Oct 14, 2020 10:33 am
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Location: Chicago Suburbs
Carl Tatz discusses 125 nulls on a podcast or youtube video. Says that ALL speakers "on stands" have this.
I believe he was blaming stands. I believe he uses 2 front corner subs timed to remove this.
I apologize I do not have the details.


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PostPosted: Sat Feb 20, 2021 1:24 am 
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maybe Carl is assuming that everyone has 125hz nulls because all stands and rooms have the same dimensions and positioning? i think we're way past that in terms of knowledge.

of course rooms can have similar issues - many rooms are a "standard" height (96" in US, 2400mm elsewhere) and most (fixed) stands likely place the speaker at a "standard" height, etc. but then speaker positions, SBIR, subwoofer placement, listening position, room mode excitation levels, tangential and oblique modes creeping in, and more etc all determine the response.

as a note: i've not seen where adding more corner traps (even the cheap ones) has done anything other than improve a room, if someone has seen a room with corner absorption causing a problem, it would be interesting to study.

simply adding a pair of subwoofers is not always a proper answer - so best to work within the room you are working in...

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PostPosted: Sat Feb 20, 2021 9:57 am 
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Location: Valencia, California
Yes, not really looking for active solutions.
I'm not sure what the AVAA's could do for me, but not prepared to spend $5k+.

I'm wondering if slot resonators on the front and side walls with alternating slot widths to be effective between 100-200hz could be a good option?


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PostPosted: Sun Feb 21, 2021 2:29 am 
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the resonators could do the job, however, for 100-200 plain old absorption (POA) should work, and then adding some scattering/diffusing/etc on top of the absorbers to balance HF reflections can be aesthetically correct.

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PostPosted: Sun Feb 21, 2021 5:22 am 
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Quote:
9" RW safe n sound 4'x8' with 1/8' hardboard facing listening position.


Interesting, and something I would sometimes encourage based on a great absorption boost found by a pal Jeff Hedback. His frontage was pegboard, maybe more compliant? But your narrow room is killing you. So I would be very curious to measure the response with these reversed. I am also curious about that alcove. The big area of glass is a kind of resonant panel bass trap. It is also a large aberration in your Front Wall in terms of boosting all LF by 6dB. Perhaps it is a 125Hz 'Hole'

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PostPosted: Sun Feb 21, 2021 6:36 am 
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gullfo wrote:
the resonators could do the job, however, for 100-200 plain old absorption (POA) should work, and then adding some scattering/diffusing/etc on top of the absorbers to balance HF reflections can be aesthetically correct.


I tried 4" of 703 and 4" of Isobond 035 covering the entire front window/wall with zero improvement to the null.

Not sure if more layers or more absorption on other walls would help? When I had the RW safe n sound with the absorption facing, 200-1khz was about 5db lower than everything else.


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PostPosted: Sun Feb 21, 2021 6:50 am 
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DanDan wrote:
Quote:
9" RW safe n sound 4'x8' with 1/8' hardboard facing listening position.


Interesting, and something I would sometimes encourage based on a great absorption boost found by a pal Jeff Hedback. His frontage was pegboard, maybe more compliant? But your narrow room is killing you. So I would be very curious to measure the response with these reversed. I am also curious about that alcove. The big area of glass is a kind of resonant panel bass trap. It is also a large aberration in your Front Wall in terms of boosting all LF by 6dB. Perhaps it is a 125Hz 'Hole'


At the time, it seemed to balance the midrange out more, but that was with a different speaker placement.
I tried removing that treatment from the corners completely and the 125hz null was still apparent, but I should try with the treatment reversed to see what it looks like. I'll run some new tests and post them here.

Agreed, maybe that window is really causing all of the issues.

Here are the old speaker placement test, speakers up against the front wall, with the treatment reversed RW safe n sound facing: https://www.dropbox.com/s/x4coh8uuhpal6 ... .mdat?dl=0


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PostPosted: Sun Feb 21, 2021 9:24 am 
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one thought - the omni mic should be vertical...

a "null" cannot be treated. there is nothing there. :-)

a simple test would be to turn on the REW generator @ 125 hz and move around the room with the mic (and/or your ears) to fix the exact spot of the null (mark with tape, label altitude of ears/mic) and the spots without the null (mark those with tape and also the altitude notes).

if you have the null forward and back of the listening position null, this could be the side walls or ceiling - the constant null along a center line would be indicative of this. changing altitude will determine if its the ceiling or side walls. if it does change then if could be the front or back walls if it's periodic intervals in spacing, or SBIR if its pretty much localized to the listening position.

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PostPosted: Mon Feb 22, 2021 6:18 am 
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gullfo wrote:

a "null" cannot be treated. there is nothing there. :-)



That's what I thought! But I wasn't sure.
I thought treating the octaves below maybe would help, but even starting the test at 100hz still gets the same results.

gullfo wrote:
one thought - the omni mic should be vertical...

a "null" cannot be treated. there is nothing there. :-)

a simple test would be to turn on the REW generator @ 125 hz and move around the room with the mic (and/or your ears) to fix the exact spot of the null (mark with tape, label altitude of ears/mic) and the spots without the null (mark those with tape and also the altitude notes).

if you have the null forward and back of the listening position null, this could be the side walls or ceiling - the constant null along a center line would be indicative of this. changing altitude will determine if its the ceiling or side walls. if it does change then if could be the front or back walls if it's periodic intervals in spacing, or SBIR if its pretty much localized to the listening position.


Alright, I set the mic vertical.
I played 125hz and walked around the room at different heights. It was really hard to pinpoint the exact spot of the null, it sounds like it's 5' all around the listening position in all directions, standing, sitting and in between.
The db meter changed only by 5db at most.

Here are some tests of many different movements, starting with listening position, moving back, moving forward, then the same at 72" high. Then some tests with the mic near the left and right wall and rear wall:
https://www.dropbox.com/s/cekcpn4xbnmvf ... .mdat?dl=0


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PostPosted: Mon Feb 22, 2021 10:18 am 
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so, by 5' do you mean the diameter of the circle around the listening position? or 5' in altitude around the room?

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PostPosted: Mon Feb 22, 2021 12:52 pm 
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gullfo wrote:
so, by 5' do you mean the diameter of the circle around the listening position? or 5' in altitude around the room?


Sorry yes, 5’ diameter around the listening position.


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PostPosted: Tue Feb 23, 2021 4:19 am 
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and when you went closer to the back wall? same reading at listening height?

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PostPosted: Tue Feb 23, 2021 9:42 am 
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gullfo wrote:
and when you went closer to the back wall? same reading at listening height?


Correct, just slight changes in frequency.

In the attached picture it starts with the listening position, test 2 moves back 6", then 12", and finally 18". All at listening height of 46".

There's slight improvement in 12" and 18" placement, but then creates spikes around 28hz.


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PostPosted: Tue Feb 23, 2021 12:15 pm 
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hmmm ok, but if you step outside of the 5' circle, say 2' off the back wall, same?

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