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Null at 140hz - how to figure out best place to put treatment?
Old 1 week ago
  #1
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Thread Starter
Null at 140hz - how to figure out best place to put treatment?

Hey people, I'm in need of some help. I have a dedicated listening room which I've done a fair amount of bass trapping to. Overall I get generally get a pretty good sound, however I'm bothered by a null at 140Hz. I was wondering how I would hypothetically go about figuring out where the reflection is coming from to cause this null and where best to put more bass trapping. Please note I'm purposing ignoring talking about room dimensions or changing listening positions - I'm not too interested in these variables as I am well aware of them and I'm considering them fixed.

Basically, is there a technique you can use to determine where the reflection has come from so that you can optimize bass trap placement? Do I walk around the room and look for a 140hz mode and then try and treat this spot? Is there another more scientific way? I can't hear any significant modes at ground level but the room has a relatively high ceiling so perhaps bass is collecting higher up. How can I choose the best possible spot to trap to treat this specific frequency?
Old 1 week ago
  #2
Old 1 week ago
  #3
Gear Addict
I'm following as I have a the same null at 140hz in my small composition studio.

Room is 9ft large x 14ft long ( 8 feet ceiling )

All walls and ceiling are in Giprock and the floor is hardwood Slat.

Isolation is 10 panels of 2ft x 4ft x 4inch thick of ProRox SL-920 ( former Roxul RHT30)

All panel are 3 inch spaced from the wall.

Here's a scheme of my setup and panel position (Red) ( the blue rectangle is a 3 inch long hair carpet ( 5 ft x 8 ft )

Also, behind my speakers ( facing my listening position ) there is a window 5 feet wide and 2 1/2 feet high and at 4 feet height from the floor.

The measurement was took at the green arrow with REW.

I get an extreme dip at 100hz and a super peak at 140hz.

Any idea on how to solve my problem ?
Attached Thumbnails
Null at 140hz - how to figure out best place to put treatment?-studio.jpg   Null at 140hz - how to figure out best place to put treatment?-rew.jpg  
Old 1 week ago
  #4
Gear Guru
 
DanDan's Avatar
BRRRR

Old 1 week ago
  #5
Gear Addict
Quote:
Originally Posted by DanDan View Post
Bass Nulls are extremely common. We often sit at half height, and at half width, what could possibly go wrong? Then we often move the speakers away from the Front Wall, because of a Hi Fi Myth. Lastly there is also the Floor Bounce....
How To Optimize Your Speaker Placement & Avoid Common Mistakes

DD
Shet....
Are you telling me I should reconfigure my speaker placement .....that mean I will need to unscrew my panel from the mounting bracket, unscrew the brackets, plaster the holes , repaint all the walls.

Oh god :(

Ok, so where should I start looking for as far as speaker placement is concerned ?

I have old JBL in my main control room and they are in walls and I dont have that sort of problems.

Where should I put back ported 6.5inchers monitors in a 9x8x14 room ?
Old 1 week ago
  #6
Gear Guru
 
DanDan's Avatar
Guessing

One would need all the details to make any kind of guess as to the optimum locations for speakers and listener. Test all viable options and you will find the certain optimum. I wouldn't worry about back ports, the speakers will probably be toed in, allowing the air to flow out and in back there.
DD
Old 1 week ago
  #7
Old 1 week ago
  #8
Here for the gear
 

Thread Starter
Quote:
Bass Nulls are extremely common. We often sit at half height, and at half width, what could possibly go wrong? Then we often move the speakers away from the Front Wall, because of a Hi Fi Myth. Lastly there is also the Floor Bounce....
I've already attempted to avoid half of the room distance and ceiling height with my placement. I'm using dipole speakers so I assume the sound radiated upwards and sideways is a lot less then a box speaker anyway. The first reflection points of the sidewalls are also treated.

I may just have to drag a trap around the room and see whether the null improves at any point. Can two speakers create a notable interference just from their two sound waves interacting (assuming classic listening triangle arrangement)?
Old 1 week ago
  #9
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DanDan's Avatar
Front Wall

Hi John, Dipoles..... how far from the Front Wall?
Thanks Henrik, I sent a message to nms about the dead links.
DD
Old 1 week ago
  #10
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Thread Starter
Quote:
Hi John, Dipoles..... how far from the Front Wall?
About 6 feet away. I have put absorption on the wall behind them too to see if this way the issue.
Old 1 week ago
  #11
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DanDan's Avatar
Area

I think you would have to place absorption covering the whole Front Wall to see any effect. If a wall reflection is the cause, moving the speaker towards it should change the frequency of that null. Similarly moving the listener position etc. should help locate the source or sources.... If it were modal playing a sine wave should light up a pattern of peaks and nulls. Don't forget vertical!
DD
Old 1 week ago
  #12
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Thread Starter
Thanks DD!

Your post is very helpful. Just to clarify, so moving the speakers toward a wall should make the frequency of the null change at my given listening position (if the reflection is caused by a wall you're moving closer or further away from)?

Could you explain what the advantage of moving the listening position is for determining the cause / direction of a reflection, I don't quite follow that one sorry.

Finally how do you use the pattern of peaks and nulls? Do you use them to find what axis the reflection is coming from? (e.g. if they run horizontal, vertical, oblique etc)
Old 1 week ago
  #13
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DanDan's Avatar
Location

Quote:
Your post is very helpful. Just to clarify, so moving the speakers toward a wall should make the frequency of the null change at my given listening position (if the reflection is caused by a wall you're moving closer or further away from)?
With regular speakers pretty much touching the Front Wall, the SBIR Null caused by the Front Wall will have a higher frequency. As the speakers are moved away from the FW the difference between the direct and reflection paths will increase, lowering the frequency of the dip.

Quote:
Could you explain what the advantage of moving the listening position is for determining the cause / direction of a reflection, I don't quite follow that one sorry.
Your situation is not so simple. The dip could be caused by the Back Wall reflection. In which case moving the speakers towards that would raise the frequency. The Dipole compounds the complexity I guess.

Quote:
Finally how do you use the pattern of peaks and nulls? Do you use them to find what axis the reflection is coming from? (e.g. if they run horizontal, vertical, oblique etc)
Different matter, and easier to figure. Let's take just the lowest strongest mode. If you play a Sine Wave at exactly that frequency, you will probably find the modal pattern. e.g. Loud peak at the BW, Null at half length, another loud peak at the FW.

DD
Old 5 days ago
  #14
Gear Head
 

Hello, i had exactly the same null at 100hz and bump around 80hz after finishing my broadband shell at my studio. after testing a ton of speaker and console placement, when i had the best compromise, i had the same bump/dip (always try to define and test the best placement, it can cure a lot, speaker placement is primordial, and then you can go for tuned treatement)

i made 2 big helmholtz traps tuned to the bump frequency, then i used my measurement mic with REW and the SPL meter tool included, played a sinewave of the bump frequency and then scanned the room completely. the spots where the sound at this given frequency was the loudest are where there is most pressure for this wavelenght, hence where you should put your tuned traps. i only put 2 big 80hz tuned traps and the bump and dip where completely gone in my case.

you can check all my measurements and stuff on this thread :

New CR from the ground up. Measurements + timelapse included. Need some advice
Old 5 days ago
  #15
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Thread Starter
Thanks, that looks like a good thread and congratulations on your build!

I have one last potentially stupid question for people. Can you put a bass trap in a null? I assume the null is two waves that are out of phase meet, ergo wouldn't nulls be legitimate spots to trap, or am I missing something here?
Old 5 days ago
  #16
Gear Guru
 
DanDan's Avatar
Big Smart Question

That is a HUGE question. Fibre traps should work mostly on a velocity rather than pressure basis. In pressure nulls, e.g. Room centre, velocity should be max. Surely an ideal place to place a big fluffy seat absorber? It's not that simple though. Traps moved away from boundaries are only say 1/3 as effective. BBC R&D have great research on such matters. Note that Tangential Modes hit the boundaries typically mid wall, perhaps a great place for fibre trapping. But again, all modes end up in the corners which have LF magnifying effects per se........

DD
Old 5 days ago
  #17
Gear Addict
Quote:
Originally Posted by Jpastor View Post
Hello, i had exactly the same null at 100hz and bump around 80hz after finishing my broadband shell at my studio. after testing a ton of speaker and console placement, when i had the best compromise, i had the same bump/dip (always try to define and test the best placement, it can cure a lot, speaker placement is primordial, and then you can go for tuned treatement)

i made 2 big helmholtz traps tuned to the bump frequency, then i used my measurement mic with REW and the SPL meter tool included, played a sinewave of the bump frequency and then scanned the room completely. the spots where the sound at this given frequency was the loudest are where there is most pressure for this wavelenght, hence where you should put your tuned traps. i only put 2 big 80hz tuned traps and the bump and dip where completely gone in my case.

you can check all my measurements and stuff on this thread :

New CR from the ground up. Measurements + timelapse included. Need some advice
Jpastor,

Can you link us to an easy step by step tutorial to build a tuned Helmholtz trap ?
Old 4 days ago
  #18
Gear Head
 

hello, actually i didn't do it with a tutorial, just made big boxes, and to find their frequency of resonance (and then modify it) i used a very simple method :

- make the wooden box, use known calculator to roughly target your frequency.
- put a speaker in front of it, sweept from 0 to 150 hz a sine wave.
- with your hand on the wood, you can feel the main resonant frequency, and also the harmonics.
- you can verify it with a microphone for example.
- then i drill untill i get the frequency i want. the more you drill, the highest the freq.

this is a lot of try and redo. you will not find the best method on the first try. you shoot, then try again, improve, measure etc... untill you get to a result and a workflow that works and suits your needs.

edit : it's important that the box is really airtight on the construction parts, i use half threaded SPAX screws. that way the wooden pieces come to an extremely tight contact when put together. if you put enough and your wood is well cut and not bent, you don't need caulk in my opinion.
best
J
Old 4 days ago
  #19
Gear Addict
Quote:
Originally Posted by Jpastor View Post
hello, actually i didn't do it with a tutorial, just made big boxes, and to find their frequency of resonance (and then modify it) i used a very simple method :

- make the wooden box, use known calculator to roughly target your frequency.
- put a speaker in front of it, sweept from 0 to 150 hz a sine wave.
- with your hand on the wood, you can feel the main resonant frequency, and also the harmonics.
- you can verify it with a microphone for example.
- then i drill untill i get the frequency i want. the more you drill, the highest the freq.

this is a lot of try and redo. you will not find the best method on the first try. you shoot, then try again, improve, measure etc... untill you get to a result and a workflow that works and suits your needs.

edit : it's important that the box is really airtight on the construction parts, i use half threaded SPAX screws. that way the wooden pieces come to an extremely tight contact when put together. if you put enough and your wood is well cut and not bent, you don't need caulk in my opinion.
best
J
Thank you very much for the tips.

How big were your boxes ?
If I was to build 2 on each front corner with simple 2inx4inx8ft screwed right in the corner of my room and put a sheet of plywood in front, would that work using your method of piercing holes until the wanted frequency ?
Old 4 days ago
  #20
Here for the gear
 

Thread Starter
Quote:
Originally Posted by DanDan View Post
That is a HUGE question. Fibre traps should work mostly on a velocity rather than pressure basis. In pressure nulls, e.g. Room centre, velocity should be max. Surely an ideal place to place a big fluffy seat absorber? It's not that simple though. Traps moved away from boundaries are only say 1/3 as effective. BBC R&D have great research on such matters. Note that Tangential Modes hit the boundaries typically mid wall, perhaps a great place for fibre trapping. But again, all modes end up in the corners which have LF magnifying effects per se........

DD
That's really interesting, thanks.
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