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Frequency doubling only with both speakers...solution? Multi-Ef­fects Plugins
Old 15th February 2012
  #1
Gear Head
 

Thread Starter
Frequency doubling only with both speakers...solution?

Hello all,

Was listening to a reference track today trying to find good placement for my monitors (Genelece 8040s) in my room (220cm x 275cm with a ceiling of 234cm). There was a noticeable boost in volume on the bass guitar every time C# was played (139Hz I believe).

What I found is...

There is no (significant) boost with just a single speaker turned on.

When I stand up at the listening position, or stand right beside the rear or side wall, there is also no significant level change.

What would this suggest usually? Is there a phase problem? If it was a room mode wouldn't that occur with the single speaker too? Though I know from the mode calculator that I would expect a mode at this frequency, so I am a little confused why it's not happening with a single speaker.

I have bass trapping at monitor level in all four corners, but I haven't got around to hanging any panel from the ceiling yet. Possible culprit? Although having a friend hold it on the ceiling didn't seem to have any effect.

Is it just a result of being in such a small room and not enough bass trapping?
Old 15th February 2012
  #2
Gear Guru
 
DanDan's Avatar
Yes

Howdy one.
Small room not enough LF trapping yes.
Modes have location but it can be complex when several work together.
Does neither speaker cause the boom, or is it just one of them that fails to trigger it?
There are other factors apart from modes. Boundary reflections cause boosts as well as nulls.
Try carefully sweeping a sine wave to the loudest resonant point. Then walk about and see if you can detect a mode pattern. e.g. Peak null peak null peak between two boundaries. Don't forget height.

DD
Old 15th February 2012
  #3
Gear Head
 

Thread Starter
Thanks alot Dan,

Neither of the speakers, when played on their own, cause the boom. Or at least it doesn't seem so severe.

With both speakers on, when I sit in the listening position and slide my chair left to right across the lenght of the room, there is no difference, the C# is always loud.

The moment I stand up it's even and smooth, almost regardless of where I stand in the room, although at that point the tweeter of my speaker is at rib height.

There are definitely peak-null-peak-null positions as you go from floor to ceiling. So would that suggest a mode between ceiling and floor?

I am going to try a 4" trap above the listening position tonight anyway.

I was also thinking of raising my corner traps so that they are flush agaist the ceiling, covering the tri-corner, though it would mean about 3 feet of uncovered corners below the traps. My floor/wall tri-corner are all uncovered too.
Old 15th February 2012
  #4
Gear Head
 

Thread Starter
Just listening to a sine sweep up to 300Hz there...I'm actually really happy with the frequency response while I'm standing, considering my initial expectations with such a small room. I'm 6'4", it's like there is a zone at that height where there are no severe peaks or nulls, except maybe a slight increase near 300Hz.
Old 15th February 2012
  #5
Gear Guru
 
DanDan's Avatar
Height

Quote:
There are definitely peak-null-peak-null positions as you go from floor to ceiling. So would that suggest a mode between ceiling and floor?
That plus the fact that this pattern is the same throughout the room....
Yes, 2nd Height Mode. It is however close to others.
One trap overhead will do little but do include an airgap to help.
The corner traps would benefit from being in the floor or ceiling tri corner.
But Floor to Ceiling is my minimum recommendation.
If you could position your woofers in that height null, the mode will be less strongly driven and the differences will be way less. Consider the speaker upside down or even sideways, whatever it takes.

Good catch and onward. DD
Old 15th February 2012
  #6
Gear Head
 

Thread Starter
Thanks again Dan, hadn't considered flipping the speakers, will definitely try that.

Had a friend move a 4" trap around the ceiling, not much effect. I'm thinking of cutting that 4' x 2' panel into 2 or 4 chunks to put in the tri-corners. Just can't afford to buy any more treatment unfortunately.

Anyway I'll update the thread in a few days incase anybody would like to know how I get on! Thanks Dan.
Old 15th February 2012
  #7
Gear Guru
 
DanDan's Avatar
Upside Down

You are welcome. Take a look at Mission speakers. Upside down, and they have a credible reason for it. It time aligns the woofer and tweeter a little better than normal.

DD
Old 15th February 2012
  #8
Gear Guru
 
Glenn Kuras's Avatar
Depending on how bad the null/peak is I would think 2 (or 3 if you can fit it) 6" panels right above your head spaced as far down as you can will help. You could also do tuned panels but that will also take a great deal of coverage but will take up less head room. Also there is the skill of building something tuned that I would not recommend if you do not understand them.
Old 15th February 2012
  #9
JWL
Lives for gear
 
JWL's Avatar
 

A tuned panel can work, but I'd use regular broadband absorption first (ie, cover all the corners in the room, as well as possibly other strategic spots). If the narrowband resonance remains then you can give it some smackdown with a tuned panel.....
Old 15th February 2012
  #10
Gear Guru
 
Glenn Kuras's Avatar
Quote:
A tuned panel can work
Thanks, but as I said I would not recommend them if you do not have the knowledge of the build.
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