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Author Topic: huge nul at 70hz and 250hz  (Read 5306 times)

brett

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huge nul at 70hz and 250hz
« on: November 21, 2008, 05:39:53 pm »

I shot my room and have identified a large annode from 50 to 100hz that peaks down almost to zero at 70hz. Music sound punchy on the bottom but I have the hardest time hearing the thud of the kick. I just feel the bottom of it and hear the top. There is another null at 250 about -20db that is making dialing in bass guitar impossible. and a null of -10 db at 850hz. Over 1k is within 6db.

My room is 13x11x8. I am 5 feet from the front wall listening position. I have a cloud 4" thick, the front wall is treated with only 4 panels 2x4x4" thick. I know these need to be thicker and I need to address the multi wall corners not just the sides and ceiling and floor. I have 12 panels in there now.

My question is do I need to first complete the rest of the room so all corners are treated. that will take about 24 panels. Then reshoot? Would it help to do all the front wall panels at 6" instead of four or is that 2" futile at 70h and 250hz? I trying to figure out if a tuned enclosured will be needed. I could build the mix desk and position up on a tuned trap. Just an idea.  

Any direction. thanks
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Thomas Jouanjean

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Re: huge nul at 70hz and 250hz
« Reply #1 on: November 22, 2008, 06:20:24 am »

brett wrote on Fri, 21 November 2008 16:39

My question is do I need to first complete the rest of the room so all corners are treated. that will take about 24 panels. Then reshoot? Would it help to do all the front wall panels at 6" instead of four or is that 2" futile at 70h and 250hz? I trying to figure out if a tuned enclosured will be needed. I could build the mix desk and position up on a tuned trap. Just an idea.  

Any direction. thanks


I would suggest you treat your rear corners and back wall with 4" thick rockwool (with a density of over 60kg/m
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Thomas Jouanjean
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franman

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Re: huge nul at 70hz and 250hz
« Reply #2 on: November 23, 2008, 12:09:42 pm »

Yeah it seems to me you may be dangerously close to the mid point (front to rear) sitting 5' from the front wall. I agree with Thomas that you should try moving forward with the listening position to raise the SBI.... Moving both the speakers and the listening position is the single biggest thing you can do in a small room to optimize the LF response.. of course the additional deep trapping is a good thing, but 6" Rockwool isn't going to solve your 70Hz issue... If you can't get it better moving things around, then some other tuned trapping may be indicated.. let's see..

FM
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brett

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Re: huge nul at 70hz and 250hz
« Reply #3 on: November 24, 2008, 05:07:01 pm »

 Will let you know what happens once I move and get the rest of the corners in the rear and additional panels up front complete. But did want to note that the 5' listening positon is me sitting back int he cahir. that point is 38% from the front wall as 50% is 1.5' behind that. With small rooms the rule doesn't seem to work.  

Thanks
 
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Thomas Jouanjean

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Re: huge nul at 70hz and 250hz
« Reply #4 on: November 25, 2008, 03:02:02 am »

brett wrote on Mon, 24 November 2008 16:07

 Will let you know what happens once I move and get the rest of the corners in the rear and additional panels up front complete. But did want to note that the 5' listening positon is me sitting back int he cahir. that point is 38% from the front wall as 50% is 1.5' behind that. With small rooms the rule doesn't seem to work.  

Thanks
 


AFAIC, the 38% rule isn't one...
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Thomas Jouanjean
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brett

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Re: huge nul at 70hz and 250hz
« Reply #5 on: December 02, 2008, 03:51:45 pm »

thanks so much for the advice. Good news to report!

I moved everything in my room fwrd so my ears are only 3 ft from the front wall and the speakers are right in front of the window on the front wall. Placed them 3 ft apart, and verticle. I had them on their sides. The P11A's sound aweful on their side even with the tweeter turned. Placed the sub sideways between them mounted on paver stones on top of carpet. Moved all my 4" thick panels to the front wall corners(no treatment on back half of room. My room is now +-4 db all the way down to 300hz. there is a narrow -8db notch down at 250hz, and a wider bump of +10db @ 140hz, and a -10db at 70hz. Running the P11A full with no filter and the sub 8 is set to -20db with the filter at 100hz. From 60hz down to 20hz I am also + or - only 5db. Speaker placement and completely treating all the front/side corners made a huge difference. Also treated the area on the front wall between the speakers to prevent comb filtering that was happening as the highs bounced off the glass window.

In a small room taking advantage of the front wall amplification of bass to counter the rooms nulls made a huge difference. Over all with no tuned traps I am very happy with the results. This is better than I would have ever thought possible in a room this size 13x11x8 with only roxul panels.  The big window up front helps and there are gaps on all the walls as this is a loft. I am sure that lets a lot of energy escape out into the rest of my place.

Now I need to firm up my speaker stands and I am sure the bass will get even better. They wobble a bit right now.


QUESTION?

Will treating the back half of the room improve things any? possibly make the sweet spot bigger?
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Thomas Jouanjean

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Re: huge nul at 70hz and 250hz
« Reply #6 on: December 02, 2008, 05:37:46 pm »

brett wrote on Tue, 02 December 2008 14:51

thanks so much for the advice. Good news to report!


Excellent!




brett wrote on Tue, 02 December 2008 14:51


QUESTION?

Will treating the back half of the room improve things any?


It will likely make a huge improvement.

Now.. GET TO WORK!  Very Happy
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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brett

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Re: huge nul at 70hz and 250hz
« Reply #7 on: December 03, 2008, 01:50:11 pm »

not ideal on the bottom but it feels usable at this point. Still want to smooth out the sub 300hz. Was looking at rpg modex plate but they are $800. Can I build a fairly shallow foot print membrane box for the front wall ceiling and get this worked out?  index.php/fa/10581/0/
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Ethan Winer

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Re: huge nul at 70hz and 250hz
« Reply #8 on: December 03, 2008, 02:07:29 pm »

brett wrote on Wed, 03 December 2008 13:50

Can I build a fairly shallow foot print membrane box for the front wall ceiling and get this worked out?


Reflections off the rear wall affect all bass frequencies, not just those related to the room dimensions. So tuned trapping there is not a great solution. You'll probably do better to cover the entire wall - and especially the wall-wall, wall-floor, and wall-ceiling corners - with fiberglass as thick as you can manage. If you can go a foot thick that will help to a pretty low frequency. Two feet thick will help to even lower.

--Ethan

brett

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Re: huge nul at 70hz and 250hz
« Reply #9 on: December 03, 2008, 06:56:38 pm »

" Reflections off the rear wall affect all bass frequencies, not just those related to the room dimensions. So tuned trapping there is not a great solution"

I said front wall ceiling? Is that what you meant too? I was planning a box from the left wall over to the right wall on the ceiling sticking out about 4' from the front wall. I want to build a membrane tuned enclosure up there. The rear wall reflections are not the cause of my 70hz and 140hz issue. The 8' ceiling is.

Would that be a bad idea? I am certainly not going to have room for 1' of insulation up front or anywhere else. I can load the corners behind the panels turning them into super chucks but that is about it.    

I read on RPG's web site, no matter how thick you go there isbn't enough air movement in the corners to absorb the energy using fiber panels at these freq. And tuned is the only way to do it. Your thoughts?

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Andres Gonzalez

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Re: huge nul at 70hz and 250hz
« Reply #10 on: December 03, 2008, 09:31:15 pm »

Sorry for a slight tangent, but......

brett, what tool did you use to take these measurements and give you a print out of the frequency response?

-Andres
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Andres Gonzalez

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Re: huge nul at 70hz and 250hz
« Reply #11 on: December 03, 2008, 10:00:27 pm »

I just read an earlier post that answered my question.

Thanks,

-Andres
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Thomas Jouanjean

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Re: huge nul at 70hz and 250hz
« Reply #12 on: December 04, 2008, 02:29:43 am »

brett wrote on Wed, 03 December 2008 17:56

I read on RPG's web site, no matter how thick you go there isbn't enough air movement in the corners to absorb the energy using fiber panels at these freq. And tuned is the only way to do it. Your thoughts?




What you need in your room is a mix of both resistance to flow treatment (rockwool) and tuned membrane systems (free membranes, not glued to the rockwool). You need to interface them with air cavities.

Start with your backwall now. This is where you really need to do something now.

Treating your back wall and corners is more important than any treatment on the front wall or ceiling.
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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Ethan Winer

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Re: huge nul at 70hz and 250hz
« Reply #13 on: December 04, 2008, 12:00:00 pm »

brett wrote on Wed, 03 December 2008 18:56

I said front wall ceiling? Is that what you meant too?


Sorry, I must have been on crack, I have no explanation for this.  Very Happy

In the grand scheme of things, it's not possible to have too much bass trapping in a room this size. You could line every single surface with 705 two feet thick and it still won't be perfectly flat. (Though you'd get pretty close!)

Quote:

I read on RPG's web site, no matter how thick you go there isbn't enough air movement in the corners to absorb the energy using fiber panels at these freq. And tuned is the only way to do it. Your thoughts?


I would not used tuned absorption in a room that size. Tuned traps are also more expensive and much more difficult to implement. The main problems you have are due to close proximity to the room boundaries. This means that small changes in placement - loudspeakers, instruments, your ears - changes the frequencies of the peaks and nulls. This is why you need broadband absorption more than tuned. The problem is not one low frequency that can be targeted, but many low frequencies.

--Ethan

brett

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Re: huge nul at 70hz and 250hz
« Reply #14 on: December 04, 2008, 03:18:07 pm »

Thomas Jouanjean wrote on Thu, 04 December 2008 07:29

brett wrote on Wed, 03 December 2008 17:56

I read on RPG's web site, no matter how thick you go there isbn't enough air movement in the corners to absorb the energy using fiber panels at these freq. And tuned is the only way to do it. Your thoughts?




What you need in your room is a mix of both resistance to flow treatment (rockwool) and tuned membrane systems (free membranes, not glued to the rockwool). You need to interface them with air cavities.

Start with your backwall now. This is where you really need to do something now.

Treating your back wall and corners is more important than any treatment on the front wall or ceiling.


My buddy said the same thing last night. Before he opened up his current place, he was mixing in a residential bedroom and he did the entire rear wall 4' thick with fiberglass. That isn't going to happen in my room. It is a loft. The rear wall is partialy open to the living room below and there is a wardrobe on the right that I need. And A murphy bed on the rear wall that I sleep on.

I'll do what I can on the rear wall. I need some instruction on membrane construction and tuning and placement. I have a general idea, but is there a calculator for tuning a membrane resonator. I found one for Helmholtz but a helmholts at 70hz is  10" deep with 2x10 slats. Would weigh a freeking ton! I would like to make four membrane boxes like the RPG modex corners and place them in the four corners on the floor. Then I plan to do the entire rear and front wall conrers in 6". That is about all I have room for.

I also can add some limp mass foil to the broadband absorbers. That may help with the 250hz null.  


Another idea is to have a drywall guy come in and do the ceiling with 1/4 plywood. Then reshoot with popcorn. The landlord would never know and the entire ceiling would be absorbant down to 70hz. I could load the cavities with ridgid glass or double or triple pack it with compressed r30 batts. Any thoughts on this?
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