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How best to tackle 138Hz standing wave
Old 17th September 2012
  #1
Here for the gear
 
🎧 10 years
How best to tackle 138Hz standing wave

Hi everyone,

I've just moved house and am setting up my control room again. In my last place I was able to tame the room with just acoustic absorption scattered sparingly across the walls (it was supposedly a cuboid room, but the house was literally sloping into the ground, which solved a lot of parallel wall problems for me!).

My new room is 11.3' x 12' and is 8' tall. After setting up what I had before I have a 6dB spike at 138Hz from where I'm sat at the desk.

Directly behind me I have space for a 4'x4' diffuser, but said space is, unfortunately, wedged between two doors and is not centred, so I think that probably rules out a large convex deflector.

Am I barking up the wrong tree with regards to a diffuser? If it is a good idea, though, I want to make it myself because budget is a serious concern. Obviously this design would be easiest to make:



If I went with this design, what would be the ideal depth to tackle this awful 138Hz spike?

Thanks for your help in advance,


David
Old 17th September 2012
  #2
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3 Reviews written
🎧 10 years
You're going to want to start with bass traps. But let's back up. Please describe how your room is set up and how you measured. Diffusers that go down to 138Hz are going to be pretty large and probably won't do the trick in your room. assuming your photo is of a 2' x 2' diffuser, it probably diffuses up over 1KHz.
Old 17th September 2012
  #3
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Jens Eklund's Avatar
 
🎧 10 years
First of all; you need to know what surfaces are related to your (presumably modal) peak. Even if you bought the most efficient absorber tuned to 138 Hz; placed in the wrong spot, and it won’t do anything useful and even if it´s the right spot, be prepared to cover some surface in order to get successful results. Oh, and yes; it´s an absorber you´re after, not a diffuser (unless used as the lid of a membrane absorber but even if so, it´s not the diffuser that addresses the sub 200-300 Hz issues. Even if it’s possible to build diffusers that scatters below 200 Hz (but they would need to be humongous), we normally don´t need diffusion below about 300-500 Hz depending on room size.
Old 17th September 2012
  #4
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🎧 5 years
You have an 8' tall room which will have modal interference close to 138hz. How high are your monitors from the floor, and how high is your listening position, from floor to ears?
Old 17th September 2012 | Show parent
  #5
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🎧 10 years
Hi there,

OK, so as you can see it's almost square, which is the first problem.

I have my desk and nearfield monitors centrally on one wall. To either side of me I have clear walls where I have placed my primary reflection absorption (which is doing the trick for reducing reverb from mix position).

From the mix position (facing forward) I shall describe the array of doors so you can see the layout and potential problems for rear-of-room treatment...

- On the wall to the right of me, almost completely to the rear is a door to a room on the right.

- On the back wall, approximately 1' away from the corner/right wall door is another door to a closet.

- On the other side of the back wall, flush to the corner, is a door to the hallway.

As you can imagine, this leaves a 4 foot space of clear wall for use, however it is off centre by 1' because of the closet door.


The image I used was simply for illustrative purposes. I meant I'd be able to make a block diffuser like that easily. It could be up to 4'x4' and it could be 1' deep.

Thanks for your help!
Old 17th September 2012 | Show parent
  #6
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🎧 10 years
Quote:
Originally Posted by OpusOfTrolls ➡️
You have an 8' tall room which will have modal interference close to 138hz. How high are your monitors from the floor, and how high is your listening position, from floor to ears?
The centre of the speaker cabinets are 46" from the ground, as are my ears...
Old 17th September 2012
  #7
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🎧 10 years
Do you think that one of these in the centre of the wall behind mix position would work to cancel the perceivable wave at mix position (which is all that is really needed at this moment in time):

ATS Acoustic Panel 24x48x4

All of my 2" panels are ATS ones and they work a treat!

Cheers,

David
Old 17th September 2012 | Show parent
  #8
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🎧 5 years
Quote:
Originally Posted by Didgeridave ➡️
The centre of the speaker cabinets are 46" from the ground, as are my ears...
And that is my 90% sure guess why you are hearing problems then. You should look into an absorber for the ceiling.
Old 17th September 2012
  #9
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🎧 10 years
Opus, thanks for your help...

Do you reckon that covering the area above the desk with these would work?

ATS B-Stock Foam Panels (Charcoal) - 24x24x4 (3PK)

I am only really concerned with reducing the resonance at the mixing position.

Thanks again
Old 17th September 2012 | Show parent
  #10
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🎧 5 years
Quote:
Originally Posted by Didgeridave ➡️
Opus, thanks for your help...

Do you reckon that covering the area above the desk with these would work?

ATS B-Stock Foam Panels (Charcoal) - 24x24x4 (3PK)

I am only really concerned with reducing the resonance at the mixing position.

Thanks again
These panels are not sufficient to eliminate the direct reflection, but they will dampen the resonance. IOW, the frequency response won't change much, maybe 1dB, but the resonance will clean up. Though they will do a good job at a higher frequency.

Panels such as these (http://www.atsacoustics.com/item--AT...8x4--1008.html) will do what you need it to. You would need at least 3, between monitors and listening position, on the ceiling.
Old 18th September 2012 | Show parent
  #11
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🎧 10 years
I'm confused by that...

By resonance I am referring to the 138Hz peak that I hear when mixing. You say the foam panels will dampen the resonance - by resonance do you mean the same thing as me?

If not I will go with the link you put up, though I'd rather not as this is a rented property.

Thanks so much for your help with this!
Old 18th September 2012
  #12
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🎧 5 years
The panels you showed me will not affect the SBIR that well, but they will lower the amount of resonance at 138hz, as well as eliminate any reflection above 1khz.
Old 18th September 2012
  #13
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🎧 10 years
So I would still likely be able to notice a boost at 138Hz with the foam, but the panels will make a big difference...

...oh well. Filler and paint at the end of my tenancy it is!


Thanks for your help. I'll let you know how I get on!

David
Old 18th September 2012
  #14
Gear Head
 
3 Reviews written
🎧 10 years
It's most likely the ceiling, I had the same problem freq in a room with 8 ft ceiling. I tamed it with 2 x RW5 panels in a cloud above the mix position and 1 panel across each of the ceiling/wall corners either side of the cloud.
Old 18th September 2012 | Show parent
  #15
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🎧 10 years
Thanks for your input, allawishus!

With this being a rented place, do you have any recommendations for hanging traps from the ceiling? It's a drywall ceiling.

Cheers!
Old 19th September 2012 | Show parent
  #16
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🎧 10 years
Quote:
Originally Posted by Didgeridave ➡️
Thanks for your input, allawishus!

With this being a rented place, do you have any recommendations for hanging traps from the ceiling? It's a drywall ceiling.

Cheers!
Your room will have multiple issues, not just at 138Hz.

You should consider pressure based absorption. Ideal solution = RPG Modex Plate

There is no better commercial product on the market for dealing with low frequency resonance in small rooms.

(I have no commercial interest in RPG or the product)

Sean
Old 19th September 2012 | Show parent
  #17
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3 Reviews written
🎧 10 years
Quote:
Originally Posted by Didgeridave ➡️
With this being a rented place, do you have any recommendations for hanging traps from the ceiling? It's a drywall ceiling.
Do you have a stud finder? I pre-drill the drywall, and continue making a pilot hole about 1.5" deep into the rafter. I like to go heavy duty, so I use a 1/4" bit and use 3/8" diameter eyes, 3" long. You can patch easily when you leave. I've hung heavy clouds off of those. For a truly heavy load, I get a long bit and drill all the way thru the rafter and use a threaded eye, washer and nut, drilling attic-down. Won't work with a floor above unless you want to countersink. Many variations on these themes. Drywall anchors suck, use the rafters.
Old 19th September 2012 | Show parent
  #18
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GIK Acoustics's Avatar
 
🎧 5 years
Hi David,

To air out a couple ideas that might be hard to understand, here's one of our tests we did to show the difference between broadband bass traps and foam: Testing Acoustic Foam Bass Traps

You will see the frequency response curves as well as the waterfall graphs. The waterfall graphs show a third axis that is time. As you can see from the graphs, certain frequencies will resonate and ring out longer than other notes in the room. The frequency response is only one measurement out of many to show acoustic anomalies. Waterfall graphs show resonances in the room.

Our tests show that foam hardly effects frequency response under 200 Hz nor does it do much at all for resonances under 200 Hz. So if you're looking to hit 138 Hz, I would suggest broadband bass traps.
Old 10th October 2012
  #19
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🎧 10 years
Hi everyone.

Just thought I'd check in to thank you all for your advice. I have two 4" thick bass traps hanging from the ceiling above the control station and one at the back of the room (directly behind me). They've pretty much done the trick! I have a slight bulge around the 138Hz mark still, but it is only by about 1dB, so it is MORE than manageable - just a case of getting used to it really.

I've been mixing a project today, which'll probably take a week or so to finish, but I'll let you all hear the results as and when the client gives it the go ahead.

Thanks a lot again,

David
Old 10th October 2012
  #20
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🎧 5 years
I am glad it worked out!
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