Bass traps and diffusers

[RoadrunnerOZ]

Thanks - I’m familiar with the math and physics of the resonator, I have just assumed that a room like that size would need a handful of these to make a dent in the modal pressure. I like the idea of some “pedestals” in corners that can double as a resonator and lamp table. Hanging them is a good way to save floor space, too. Great info!

I’ve seen them even used as stands for speakers as well - open face of tube to the bottom, speaker on cushioned pads on timber plates on the closed top of the tube, with 3 strong L braced angle shelf brackets as feet bolted to the tube periphery, with the bottom of the L facing out for stability - this was in a room very tight for space and the speakers almost in the corners of the room front.

 

[Spencerleehorton]

As I said earlier in the thread my panels use 30kg/m3 rockwool at 200mm deep inside 1000x600mm plywood frames.
I have built an internal frame which has a cotton dust sheet stretched over it and inserted inside the main frame.
The rockwool is covered with a very thin layer of clingfilm to stop the fibres getting airbourne and two slats on the back hold the rock wool in and a hessian back over the whole rockwool.

my rockwool is 2 layers of 4"

 

[Spencerleehorton]

I will try and move monitors wider again and take another measurement, also as the top side panels don’t seem to have done anything I will move them to see if I can track down a better place for them

 

[RoadrunnerOZ]

Understand your panel construction - basically a sheet of Rockwool inside a frame. Low density Rockwool without a hard backing of some mass to soak up energy won’t absorb much LF. Hard against a wall it will work but not hanging in space as the Rockwool is free to move. Absorption relies on converting the resistance to movement of the air trying to pass through the fibre to heat, but if there is no backplate there is no real resistance only the mass of the Rockwool which is not much.
For mobile panels on casters I have made them with a centre plate of 19mm MDF mounted on rubber blocks in a timber frame and surfaced either side with 25mm 96Kg/m3 compressed glass board - the heavy mass of the centre board absorbs a lot of energy.
For wall panels I use an MDF backplate in a frame with 50mm of compressed glass fibre board on the face - to mount I use twin timber rails screwed to the walls running around the room which have a 45° angle edged top (the angle goes up and out) and identical rails with the same spacing apart on the back of the panels but with the angled edge facing down (the angle goes down and out). To mount on the wall the angled rails on the panel are simply placed over the angled topped rails on the wall. This causes the weight of the panel to force it in against the wall and lock it into place but allows the movement of the panels anywhere in the room by simply lifting them off the rails and repositioning.

 

[Spencerleehorton]

ok so to simplify, i need to put backs on all panels for them to have some effect right?
whats thickness?

 

[Spencerleehorton]

moving monitors wider gets better results than closer together, wider it just seems that the null is at 118hz, when closer together it seems its 83hz and 118hz.
so far moving the panels around doesnt make much difference.

 

[RoadrunnerOZ]

ok so to simplify, i need to put backs on all panels for them to have some effect right?
whats thickness?

If the panels are to go up against a wall they can be hard backed as long as the gap between is small, if angled in a corner they should have absorbent material on both sides with a centre hard plate. For against the wall if hard fixed I use 12mm or 16mm MDF (particleboard) as the wall provides mass but if not hard fixed and say rubber mounted I’ll use 19mm flooring particleboard - for free standing double sided I use 19mm (or even 22mm for drum isolators) flooring particleboard. I’ve found that flooring particleboard is the cheapest - here we have yellow tongue which is 19mm and red tongue is 22mm, blue tongue is 25mm - different thicknesses are for different floor span joist spacing (the tongue is a plastic strip that fits into slots on the edges to flush join the boards and colour indicates thickness of board).
I use this for wall panelling in studio construction as well, instead of heavy plasterboard, if I’m going to have heavy shelving, video monitor or speaker mounts on walls.

 

[RoadrunnerOZ]

moving monitors wider gets better results than closer together, wider it just seems that the null is at 118hz, when closer together it seems its 83hz and 118hz.
so far moving the panels around doesnt make much difference.

The 83Hz is due to the length down the room. The angling of the monitors is critical as well - if they point inward too far you get a false image the same as if they point straight down the room. If I recall correctly your monitors are designed for 60°.

 

[Spencerleehorton]

I have some left over plasterboard, would this be too hard to put in panels?
I have loads of slats left which I could put in the back of the panels, the slats are pretty thin though and you were saying 18mm.
Strange that Jesco didn’t mention any of this?

 

[Spencerleehorton]

I’m still very confused on how to reduce the 118hz null, nothing seems to have reduced it at all?

 

[Electrobumps]

This will give you some ballpark figures on what you can expect with absorption.

Porous Absorber Calculator.

 

[Electrobumps]

I’m still very confused on how to reduce the 118hz null, nothing seems to have reduced it at all?

Put the oscillator on and walk around the room at all heights and places and see if you can hear where the frequencies are loudest. If you can't get your head behind the corner traps I would run a room response test with your mic behind them and see what that tells you. If you have a build-up of those freq's in the corners then your corner traps are not absorbing enough.

 

[RoadrunnerOZ]

I’m still very confused on how to reduce the 118hz null, nothing seems to have reduced it at all?

You have this due to across the room standing waves. The ceiling top corners running down the side edges may be causing your issues as may the floor ones as well as the flat walls as corners concentrate the energy. It seems your panels are net giving the resistance needed - by putting the wool hard against the wall it would give it something to work with - if there’s a gap then it’s all down to only the mass and rigidity of the wool which is very low. Inside a sealed wall with absorbent material the standing waves have to pass back and forth between the hard surfaces through the absorbent material and air thus damping the sound.
In an open room the amount of absorption at low frequencies is going to depend on the acoustic impedance of your damping material which being light needs some mass assistance in the form of a backboard it is hard up against like a wall. I think your Rockwool is just not up to the job for LF the way it’s mounted - it seems to be working for the mids and highs. It needs to be in firm contact with the hard surface.
I just completed a studio which is 10M x 4.2M with double brick walls. The surface of the whole concrete ceiling is covered in 50mm absorbent compressed fibreboard which is medium density. The walls either side and to the rear of the monitoring position are also covered with the same 50mm board. The board uses the ceiling and wall as the resistant surface as it is glued to it. Behind is also a raised floor bass trap where the client couch and rack gear goes to handle the room length resonance as well as the side to side.

This will give you some ballpark figures on what you can expect with absorption.

Porous Absorber Calculator.

35kg rockwool is about 10,000 Pa*s/m². Where at your depth of panel a glass wool can be around 2000-5000 Pa*s/m² at a certain depth it is said to have better absorption than heavier materials and I agree with this. There are loads of threads on Gearspace about this.

I've been in your situation where I've tried to target nulls and I ended up spending a lot more time and money and in my case the improvements were noticeable but nowhere near what I wanted.

In my opinion, the material you are using is not absorbing as much or as low as you need. If you swapped out your rockwool for a lower-density material even at your current thickness I think your waterfall would end up looking better than leaving the rockwool in place and trying to target your nulls.

don't bother straddling with a low-density material. You are better just putting as much as you can in without air gaps.

If you are considering spending more time and money making targeted traps. I would urge you to try a low Pa*s/m² material in the traps first.

View attachment 119939

the chart you’ve shown there is for a 2000mm (2 metre) thick absorber. It’s the opposite for a 200mm absorber

 

[totoxraymond]

Do you measure with both speakers at the same time? You should measure make measurements with each speaker separately.

 

[Electrobumps]

the chart you’ve shown there is for a 2000mm (2 metre) thick absorber. It’s the opposite for a 200mm absorber

That will be the Jet lag kicking in have deleted my mis-information. It's a good calculator to play around with to give you an idea of different materials.

 

[Electrobumps]

This will give you some ballpark figures on what you can expect with absorption.

Porous Absorber Calculator.

So around 300mmm + is where you might hear a real benefit at 180Hz over rockwool.

 

[RoadrunnerOZ]

A lot of the merit in the lower density material is that it provides a smoother damping of the mids and highs whereas the high density compressed board has a much better absorption coefficient at LF without huge depth, coupled with a backboard makes a pretty effective surface treatment. Downside of the low density is the huge thickness required for LF absorption - bad news for a small room.
For real null and peak problems the best answer is tuned traps whether Helmholtz absorbers or tube/box traps placed at the peak antinodes. To achieve long wavelength absorption if have made folded traps set under raised floors or ported into floating floor chambers which vent into adjacent chambers for length. Symmetrical either side for control room setup. Each studio I have built has had its own issues due to the constraints of the building it’s housed in - I have had to trap ceilings forward of the engineers position while trapping floors behind using raised platform dais with front face and rear floor ports for tricky control rooms with limited space for wall treatment.

 

[RoadrunnerOZ]

Put the oscillator on and walk around the room at all heights and places and see if you can hear where the frequencies are loudest. If you can't get your head behind the corner traps I would run a room response test with your mic behind them and see what that tells you. If you have a build-up of those freq's in the corners then your corner traps are not absorbing enough.

Good idea especially if you know your problem frequencies. Also pays to have a look at the response curve of the mic being used but in this case it’s pretty obvious the room dimensions are the cause - it’s just finding where the problem peaks are located.

 

[Spencerleehorton]

I will say that I could probably fit into each panel another layer of the 4” rockwool if that would help?
As it is quite compressed and there is a little space at the back of each panel.
If I did this the rockwool would then definitely make contact with the wall when it is hung on the wall.
It would then be 12” of rockwool into 8” deep panel.

 

[john12ax7]

You don't want to add backing material to a velocity based porous absorber, you lessen the effectiveness as you lose the benefit of an air gap.

But if you go for a pressure based absorber it's a different story. Essentially you want a non-rigid vibrating membrane with a lot of mass, and then the insulation material provides damping. This would be a new build, simply adding wood panel backs wouldn't achieve this.

You might want to look into VPR absorbers, fairly simple, low depth, and effective provided you get the right materials. Eric Valentine on you tube has videos documenting a studio build using these.