#1906720 - 03/10/08 12:44 PM
Acoustic interference, room nulls, please help
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ricotrex
Senior Member
Registered: 09/26/06
Posts: 32
Loc: Santa Clarita, California
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Hi Ethan,
I am very frustrated with a specific problem I'm having in my room. I am having severe cancelations between 450 Hz and 600 Hz. I have read and understand the invaluable information in your Acoustic FAQ and have followed all the advice contained therein.
I currently have twenty, 6" Johns Manville I/S 300, burlap wrapped bass traps in my very tiny room. The low frequency response in my room is excellent by the way!
I have treated my side wall first reflection points using 2" OC 703 panels. On the ceiling above my head, I am using two of the previously mentioned 6" JM I/S 300 burlap wrapped panels.
My listening position is 38% of my room length (13.5'). On my back wall I have made a wall of six, 6" JM I/S 300 burlap wrapped panels. This covers 48 square feet of the back wall behind me. This is almost the entire wall!
Some of the things I have done to try to solve this problem myself are:
1. Increase the absorbtion on the side wall first reflection points from 2" thick to 8" thick. This did not help. I re-measured and still had no significant changes
2. I moved the speakers out into the room a little more and once again had little to no significant change.
3. I placed a 6" JM I/S 300 burlap wrapped panel on my desk just below my speakers to try to eliminate any early reflections I might be experiencing from my workstation desk itself. Once again, this did little or nothing to help.
One thing I noticed that changed things was the distance that the speakers were placed from one another. I did several measurements this way and got different results. The null shifted from one frequency to another depending on the distance that the speakers and microphone were from one another.
This first result is with the speakers and mic forming an equilateral triangle of 34 and 1/4"
This next measurement is with the speakers and microphone forming an equilateral triangle at 40"
The null shifts. I am at a loss and not sure what else I can do?! Please help anyone! Any other advice anyone can offer other than the things I have already tried would be greatly appreciated! Thanks
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#1906743 - 03/10/08 01:23 PM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Rod Gervais
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Registered: 10/08/03
Posts: 478
Loc: Central Village, CT
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What is the spacing between the flat treatments on the wall and the wall itelf -
Same question with the ceiling cloud....... (distance from back to ceiling)
Right now you have nothing to handle SBIR (which can occur because of the relarionship of speaker location to walls in close proximity to them - which would explain why the frequency changes as you move the speakers. SBIR can cause some major dips.
Try covering the reflection points that exist using the mirror on the front wall and let us know what happens........
It shouldn't take you long to just throw some temps up to see what that does for the problem.
Sincerely,
Rod
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#1906814 - 03/10/08 03:20 PM
Re: Acoustic interference, room nulls, please help
[Re: Rod Gervais]
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ricotrex
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Registered: 09/26/06
Posts: 32
Loc: Santa Clarita, California
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Hi Rod,
Thanks for your reply. To answer your questions:
(What is the spacing between the flat treatments on the wall and the wall itelf -
Same question with the ceiling cloud....... (distance from back to ceiling)
There is a 4" air gap between the side wall absorbers and the wall itself.
My ceiling is vaulted so the air gap changes but at the biggest point there is probably a 1.5' air gap and at the smallest there is probably a 6" air gap. My cloud is 4' by 4' square and is 6" thick JM I/S 300 burlap wrapped panels.
(Try covering the reflection points that exist using the mirror on the front wall and let us know what happens........)
I have experimented with putting 6" panels on the front wall behind my speakers with no improvement.
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#1907375 - 03/11/08 10:56 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Ethan Winer
MP Hall of Fame Member
Registered: 06/12/00
Posts: 5299
Loc: New Milford, CT, USA
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I am having severe cancelations between 450 Hz and 600 Hz.
Wow, no kidding. That null is so severe I have to wonder if something is outright broken. First, I suggest measuring each speaker separately (with the microphone in the middle left/right at the listening position) and see if both speakers have the same null. The second test I'd do is put the measuring microphone a foot in front of one speaker, play through that one speaker only, then move it back slowly in 6 to 12 inch increments to see how the response changes.
Let us know what happens.
--Ethan
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#1908051 - 03/12/08 10:04 AM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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ricotrex
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Registered: 09/26/06
Posts: 32
Loc: Santa Clarita, California
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Hey Ethan,
I took the measurements as you instructed.
This is the left speaker only with the mic in the middle at listening position:
This is the right speaker only with the mic in the middle at listening position:
This is the left speaker only 12" away.
This is the right speaker only 12" away
This is the left speaker only 18" away
This is the right speaker only 18" away
I took a few more measurements yesterday as well. I found that moving my speakers behind my workstation desk and all the way up against the front wall made the null shift even lower. Below is the result:
I then moved my speakers in front of my workstation desk and away from the front wall and the results yielded this:
Notice that the null is gone now in the low frequencies but now I have some terrible problems in the mid range and high frequencies. Not to mention that having my speakers in front of my desk is not conducive to working.
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#1908072 - 03/12/08 10:31 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Ethan Winer
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Registered: 06/12/00
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Loc: New Milford, CT, USA
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Oh geez. 
Can you stand in the back of your room, take a photo toward the front, and post that here? Then we can see what you have.
At least it's not a broken speaker. 
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#1908096 - 03/12/08 11:01 AM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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ricotrex
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Registered: 09/26/06
Posts: 32
Loc: Santa Clarita, California
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Hey Ethan here are the pics of my room. I hope you can see what's going on? I know it is hard to put things in context. I have posted two pictures one is taken showing you what I have on the floor and one is taken showing you what is on the ceiling. As you can see, my speakers are currently on blocks of auralex foam on the front part of my workstation desk. This is the last measurement I posted. It is obviously not ideal due to the relationship of my keyboard and mouse.
Here is another showing you what's on my ceiling. The panels on the side walls are 2" thick OC 703. The big broad band traps are 6" Johns Manville I/S 300. 
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#1908100 - 03/12/08 11:02 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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ricotrex
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Posts: 32
Loc: Santa Clarita, California
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My room is very narrow...8'4"
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#1908788 - 03/13/08 09:32 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Ethan Winer
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Registered: 06/12/00
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Loc: New Milford, CT, USA
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Erik,
That all looks perfect. One other question - do any of the panels other than the ones straddling corners have FRK facing? If all the non-corner traps are plain fiberglass with no facing, I'm at a loss.
Okay, one more question - is the rear wall reflective and not very far back behind you?
--Ethan
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#1908831 - 03/13/08 10:44 AM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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JohnM
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Registered: 03/13/08
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Maybe a reflection off your monitor or possibly the desk? A null at 550Hz would mean a path length difference of about 11 inches, so would have to be bouncing off something quite nearby. Could make some measurements while you place an absorber panel in various spots to see which direction the reflection is coming from. May also want to update to REW V4.11, which is more Mac-friendly.
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#1909894 - 03/15/08 12:11 AM
Re: Acoustic interference, room nulls, please help
[Re: JohnM]
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ricotrex
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Loc: Santa Clarita, California
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Hey Ethan and John,
None of my panels are FRK! And my back wall is completely covered with 6" JM I/S 300 panels.
I think John is right. I removed my desk and computer monitor and the nulls dissappeared completely. Now I guess I have to decide what to do about my work station desk. I am using a "Creation Station" desk. Is there anyone else out there using this desk and having similar problems? Perhaps we can have a discussion about workstation desks that people are using that minimize reflections?
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#1910077 - 03/15/08 10:23 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Ethan Winer
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That doesn't look like the big desks I see some people use. But it's a good warning about the potential problem from all reflections off nearby surfaces.
When I occasionally do 5.1 surround mixes, I do them in my living room theater. I put my Dell laptop on a tiny low table that's just big enough to hold the laptop, and fold the laptop's screen back all the way so it's parallel to the floor. This way the only potential reflection is off the laptop itself, and only from the center speaker.
--Ethan
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#1910082 - 03/15/08 10:30 AM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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gullfo
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Registered: 03/30/04
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Loc: Old Tappan, NJ, USA
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the desk can also act like a resonate chamber which can effectively "absorb" (or resonate...) when you don't want it too... plus the reflections can interact and cause nullification at certain frequencies etc. best to arrange your monitoring and desk to avoid reflections and ensure the desk isn't performing Helmholtz absorption or acting like a pipe organ...
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#1910164 - 03/15/08 01:43 PM
Re: Acoustic interference, room nulls, please help
[Re: gullfo]
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ricotrex
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Hey Glenn,
Thanks for that bit of insight. My sub is sitting on the bottom shelf of my desk and I do notice that my desk vibrates quite a bit. I was considering buying some Auralex MoPads and possibly the Auralex SubDude! Would you recommend against this? If so, can you offer a better solution to decouple my speakers and sub from my desk?
I have experimented and measured with different placements of my sub. Underneath my desk seems to offer the best bass response in my room. So I would rather not take it off of the bottom shelf of my desk!
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#1910458 - 03/16/08 10:41 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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audiofreek
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Registered: 05/09/01
Posts: 505
Loc: Prince George,,CANADA
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Hi Erik,
Although hard drives are vibration isolated to reduce noise,it is not a good idea to have your sub vibrating your computer, and drives.This can cause disk tracking errors,and errors in the data stream.Most would be corrected,and you may never hear them.If you must have yor sub there, put some foam underneath it to de-couple it from your desk.
Rick
Edited by audiofreek (03/16/08 11:03 AM)
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#1910808 - 03/17/08 04:14 AM
Re: Acoustic interference, room nulls, please help
[Re: audiofreek]
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gullfo
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Registered: 03/30/04
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Loc: Old Tappan, NJ, USA
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as Rick points out the desk location can hurt your disk (which is where all your work resides - lose that and you're out of business until you replace it and restore the files) that said, that may very well be one of the worst places in the room for the sub (acoustically) so i'd definitely get it out of there and off to one side or the other - preferably to the left-front or right-front of your desk on the floor and at least 3-4 feet from the corner.
Edited by gullfo (03/17/08 04:14 AM)
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#1911008 - 03/17/08 09:52 AM
Re: Acoustic interference, room nulls, please help
[Re: gullfo]
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ricotrex
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Registered: 09/26/06
Posts: 32
Loc: Santa Clarita, California
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Thanks Rick and Glenn for your responses.
I realize the potential for damage to my hard drives. I have moved my sub to several different locations around my room and found that after testing, underneath my desk is the best place for my sub acoustically. The bass response is the flattest in that location.
I'm still curious, yet skeptical, about the Auralex MoPads, and SubDude for decoupling my speakers and Sub from my desk. Has anyone used either of these and can attest to their effectiveness?
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#1911020 - 03/17/08 10:01 AM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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gullfo
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Registered: 03/30/04
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Loc: Old Tappan, NJ, USA
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i would probably opt for the subdude but i'm quite surprised that under your desk is the best location. you sometimes have to move the subs and desk around a bit to get the right position because you're now playing with modes which can easily change in just a few inches so subwoofer placement (esp single sub) takes time to get it right.
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#1911027 - 03/17/08 10:06 AM
Re: Acoustic interference, room nulls, please help
[Re: gullfo]
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ricotrex
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Loc: Santa Clarita, California
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For the sake of thoroughness, I will move my sub again and do some more tests and post the results.
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#1911238 - 03/17/08 01:59 PM
Re: Acoustic interference, room nulls, please help
[Re: ricotrex]
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Ethan Winer
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Registered: 06/12/00
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Loc: New Milford, CT, USA
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I'm still curious, yet skeptical, about the Auralex MoPads, and SubDude for decoupling my speakers and Sub from my desk.
Here's the deal - they do work, but not everyone needs decoupling. It depends on how massive / rigid the speaker cabinet is, and how massive / rigid what you put them on is. When people ask if they should decouple their speakers my standard answer is to invite some friends over, and have them lift the speakers 1/4 inch while you listen. If you hear no change, then you won't benefit.
--Ethan
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#1911339 - 03/17/08 04:36 PM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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Speaker Boy
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Registered: 02/04/08
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Loc: Santa Barbara, California
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Why are you moving your sub? I thought your problem was a null at 400hz. This null is not coming from your subwoofer crossed over at about 80hz.
You have tons of panels try putting a panel where your keyboard is sitting and see if the null goes away. Then put it in front of your monitor and measure again. Shouldnt be too hard to find what part of your desk setup is causing that null.
Jeff
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#1912091 - 03/18/08 10:52 PM
Re: Acoustic interference, room nulls, please help
[Re: Ethan Winer]
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ricotrex
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Hey Guys,
Just to clarify:
Ethan,
"they do work, but not everyone needs decoupling."
I am concerned about decoupling for the purposes of keeping my hard drives safe from damaging vibrations, not necessarily for any sonic benefits decoupling may provide. Although, I won't complain if decoupling helps sonically too!!! 
Jeff,
"Why are you moving your sub? I thought your problem was a null at 400hz."
I am moving my sub again not to try and fix the null I'm having but rather determine if I can get it off of the bottom shelf of my workstation desk so that it won't be causing any harmful vibrations.
That being said, I have moved it again and tested and have still found that under my desk is the best place for it. I have placed my sub on top of 5" thick Auralex foam on the bottom shelf of my desk and sure enough it has significantly reduced the vibrations on my desk.
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