Helmholtz resonator and room modes

MagashMusic

New member
Hi guys!

I really hope you can help me on this one...

I have some room mode problems around 38Hz so I decided to build a helmholtz resonator to lower the peak.
What I made is a well sealed plywood box around 400 litres with a port(hole). I used mh-audios calculator for the size of the port and it is supposed to work at around 38Hz with a Q of 1.5. The resonator is placed in the corner of the room (the room is 4x4x2,5m).

The problem is: it doesn't work!

I measured the room response before and after and saw no real difference. (I used REW and ECM8000). I then tryed filling the resonator with wool, first halfways then full but it made no difference.

What am I doing wrong..? :(

Please help!!
 
You have other bass trapping in the room to start with and you added a box that's how big? Like a 1m cube or something? The metric conversion is giving me a headache...

P.S. Your square room - almost exactly like mine in size, is always going to give you a problem. Focus on where you sit is my only solution.
 
I have about 4 cubic meters of wool to deal with the midrange and mid-bass. The box is about half a cubic metre. I'm already sitting in the 38% mark of the room... :/
 
So I did a rough calculation and 4 cu.m. means you have the equivalent of 5cm covering your entire surface area, including floors and ceiling. Obviously it's not quite like that, but maybe you've overtreated the mids and highs?

I don't have quite 1 cubic meter (assuming I did the math right), and my room is far from horrible, at least compared to where it started!
 
Here are some pictures of the response, the room and the resonator.
As you can see my back wall is covered with a layer of 60cm thick wool (yes 60cm). So it alone has 4m-1(for the door)x2.5mx0,6m=4,5m3. It looks like I underestimated the amount of wool in the room, It's more like 6 cubic meters :)
Anyways, look at the responses and let me know what you think.
spl.jpg
waterfall.jpg
IMG_20180126_083905.jpg
IMG_20180126_083937.jpg
IMG_20180126_084008.jpg
 
"The resonator is placed in the corner"
How do you know THAT is the position of the peak 38Hz pressure? If you put the resonator "where the signal is not" IYSWIM! It cannot 'suck it out'?

You need to move subs about for best results I would think HRs are the same?

Dave.
 
Well it's port is placed in the corner Isn't that what matters..?
Hm... You're right, I was looking at the pressure in my listening position and not in the position of the resonator. I'll have to do some more measurements.
What about the filling of the resonator, does it have to be filled with wool, half filled or empty....?
 
Well it's port is placed in the corner Isn't that what matters..?
Hm... You're right, I was looking at the pressure in my listening position and not in the position of the resonator. I'll have to do some more measurements.
What about the filling of the resonator, does it have to be filled with wool, half filled or empty....?

Hey! I am FAR from any expert on this stuff (good on valves tha'knows) but from the reflex speaker analogy, the more you 'stuff' the box the less it works as a resonator. Got to be an optimum stuff?

Then, does the resonator need to be at maximum 'pressure' or maximum 'velocity' ?

You might have to resort to excellent headphones for the LF checking and use monitors for 'image' analysis?

Dave.
 
Hold on - is that big whitish shape over your desk all rockwool? IF so, that could be part of your problem - you don't need that much in the cloud position and it looks like you have the resonator on the floor on the front wall - maybe it needs to be on the back wall (is there rockwool trapping on it?)
 
Hm... I don't know about does it work at max pressure or velocity...? Woll and other porous materials are for max velocity and I kinda assumed that helmholtz were like panel traps for max pressure...
I use headphones but they just aren't the same as real speakers...
Here is the response measured in front of the resonators port open vs closed but the difference seams to be just 1dB...
spl 2.jpg
 
Hold on - is that big whitish shape over your desk all rockwool? IF so, that could be part of your problem - you don't need that much in the cloud position and it looks like you have the resonator on the floor on the front wall - maybe it needs to be on the back wall (is there rockwool trapping on it?)

Yeah, that is all rockwoll. Ok, maybe it is overkill but it couldn't of added bass in the room... :/
 
Nice looking room man! Shitty dimensions but looks like you have treated it quite well. Better than mine.

That being said I think you may just have to deal with what you have. 38Hz is more something you 'feel' so I wouldn't think it is a real issue as far as mixing is concerned. Maybe the placement of sub would help a bit. The room is going to have issues no matter what you do.

Do your mixes translate well? I am one who just gave up on measurements and just work with what I hear. Well, after basic acoustic treatments obviously. Getting anal about measurements is now a waste of time for me. My ears tell me what sounds good.
 
I have some room mode problems around 38Hz...

Going by your first measurement image, I think your real problems are the huge peak at 80Hz, and the 3 deep troughs at approximately 160, 300 and 600 Hz...and in the last measurement image, it looks like everything shifted, and the peak is now at 700Hz...plus you now have 4 deep troughs, and they too shifted.
I also agree that MASSIVE cloud thing is serious overkill, and probably not doing much good up there. Work on the corners and the back wall...and also try adjusting both your listening position, which seems to be in the center of the room...and the speaker positions, which might be better right up against the front wall or some other, shorter distance than what you have. There just isn't enough room space for your mix position or speakers to be that far from the front wall, IMO.
 
Nice looking room man! Shitty dimensions but looks like you have treated it quite well. Better than mine.

That being said I think you may just have to deal with what you have. 38Hz is more something you 'feel' so I wouldn't think it is a real issue as far as mixing is concerned. Maybe the placement of sub would help a bit. The room is going to have issues no matter what you do.

Do your mixes translate well? I am one who just gave up on measurements and just work with what I hear. Well, after basic acoustic treatments obviously. Getting anal about measurements is now a waste of time for me. My ears tell me what sounds good.

+1 ^ 38Hz is below E on a bass guitar and you could probably leave a 40Hz HPF in your monitor feed and never know the difference. The real 'meat' of rock music is around 60Hz.

Dave.
 
Going by your first measurement image, I think your real problems are the huge peak at 80Hz, and the 3 deep troughs at approximately 160, 300 and 600 Hz...and in the last measurement image, it looks like everything shifted, and the peak is now at 700Hz...plus you now have 4 deep troughs, and they too shifted.
I also agree that MASSIVE cloud thing is serious overkill, and probably not doing much good up there. Work on the corners and the back wall...and also try adjusting both your listening position, which seems to be in the center of the room...and the speaker positions, which might be better right up against the front wall or some other, shorter distance than what you have. There just isn't enough room space for your mix position or speakers to be that far from the front wall, IMO.

I can see that there are other problems than the 38Hz bump but I thought that since all the other problems are the upper harmonics of the 380 that I would fix all of them by fixing the fundamental frequency...
It shifted on the second image because it wasn't measured at the listening position but at the resonators port position.
Would I gain anything by adjusting the resonators frequency to 80Hz..?

---------- Update ----------

Btw, thank you all guys for the responses, I appreciate it!
 
Did I miss that you have a subwoofer in this room? I guess I'm wondering where that LF energy is coming from? If you have a sub, put your box right in front of it, or turn it down. Once the initial 38Hz hits any solid surface, the overtones are on their own, so trying to capture reflected fundamental isn't going to help much.
 
Did I miss that you have a subwoofer in this room? I guess I'm wondering where that LF energy is coming from? If you have a sub, put your box right in front of it, or turn it down. Once the initial 38Hz hits any solid surface, the overtones are on their own, so trying to capture reflected fundamental isn't going to help much.

Yes, I forgot to mention that I have a sub. I tried moving the sub but there was no real difference.
Is there any chance of capturing the 80Hz Peak?
 
Are you doing measurements with or without the sub?
Also, are you doing measurements at the mix position or somewhere else?
And...are you measuring one side/speaker at a time or both together?

The other thing...as I said earlier...did you consider moving your speaker toward/ and even up against the front wall...along with moving your mix position.
You appear to be sitting in the center of a small room...not really good. I know some common wisdom is to have the speakers as far away from the front wall as possible...but that would really apply IF you have the big enough space. The other options is to move them right up against that front wall, and you may see some of your nodes disappear. You have to try a variety of positions for the speakers and your mixing position...along with moving the trapping to different areas of the room and measuring.
You also have to measure the same way, the same level every tine. Have you calibrated you measurement level and speaker loudness for a single reference level?
 
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