Bass traps and diffusers

[Spencerleehorton]

Ok so let’s explore the frequency targeted pressure absorber at 120hz, from what I’ve read here so far I would build a 110hz, 120hz, 130hz.
I also my need 83hz.
How exactly would I go about building these ?

 

[kags]

Here is a starting point- you will have to find the appropriate weight wood. I use 1/4” plywood and weigh the panel myself

http://www.mh-audio.nl/Acoustics/PResonator.html
I would recommend 32 square feet, so a full sheet of plywood, which can be cut to make 2 or 4 panels. The sides and back should be airtight and solid and if you put some insulation inside (that doesn’t touch the front panel) you widen the bandwidth. If you shoot for 115 or 120 hz you can be off a bit and still be ok. It’s more critical for lower freqs where 10 hz is a big difference.

 

[Tubetec]

Hmmm I searched Caruso Iso-bond ,
Don Audio came top of the hit list ,

I found this link also ,which might be of interest
https://www.audiosciencereview.com/forum/index.php?threads/caruso-iso-bond-vs-rockwool.48067/

 

[Spencerleehorton]

I have some 18mm thick plywood racks which I could repurpose, I’ll measure them, but I reckon I could make at least 4 nice panels out of them.

 

[john12ax7]

I don’t see how lightweight glass fibre would be an improvement on heavier Rockwool.

An effective absorber matches to the impedance of air AND wavelength of sound. A 4" dense panel is better than 4" lightweight because it's impedance is better. But neither is deep enough in proportion to wavelengths of very low frequencies. Once you go 12"+ the impedance of the denser material is far too high and it becomes reflective, the lighter material is then far superior.

 

[Spencerleehorton]

I get it, less dense material with greater depth will reduce velocity of waveform much better than dense material that’s not as deep or at least there is a point where both don’t work, probably a nice visual graph which explains this somewhere!!
Think I’m gonna have to look at the panel sound absorber website on my MacBook rather than the iPhone as it doesn’t seem to work on iPhone?

 

[Spencerleehorton]

yeah works fine on macbook:

well the panel sound absorbers seem dependant on weight and the wood i have i think will weight more than 1kg, if i built a 200x100x50cm panel it would have to weigh 1kg to hit 120hz or 100hz

 

[john12ax7]

Two main takeaways to consider for a porous absorber

1) The material density (really its gas flow resistivity) will determine its ideal thickness.

2) Overall depth, or distance from a rigid boundary (like a wall), will determine how low in frequency it goes.

As a practical result of this you can extrapolate things like a 12" lightweight absorber is similar to a 4" dense absorber with 8" airgap.

 

[Spencerleehorton]

but then if i use the calculate panel depth and put in my panel size 60x100 it calculates the depth at 7.5cm?

 

[Spencerleehorton]

sorry john12ax7 im just trying to be specific and work out what i need to build and we keep seeming to go off pist.
its totally pointless me building something if im going to build it wrong, my problem frequencies are 83hz and 118hz, so i need to stick to trying to control these.
i do still need to setup my monitors at correct degrees (60degrees) and position them correctly in the room as it makes a pretty big difference in the plots but that said these frequencies still seem to be the problem whereever i move them, so to me moving the speakers could be done once i have started to notice a difference in removing the nulls.

 

[Spencerleehorton]

also the panel absorber doesnt explain what material the thin panel is?

 

[john12ax7]

Sorry for the veer, was just trying clarity some things regarding how porous absorbers work.

Those panel type absorbers are described in Everest's Master Handbook of Acoustics. Never built them personally, but have heard trying to get the frequency right can be tricky. Others probably have more insights.

The VPR seems easier though I haven't built that either. It's basically something like a steel sheet against basotect.

 

[Spencerleehorton]

well my next move as i said is to get frequency generator on and find the problem areas, this needs to happen then i can decide on how im going to tackle it.
ive hopefully got enough bits here to in these areas to try and make some impact, however little impact as long as its something then its the next step.
panel sound absorbers doesnt sound like the correct term to me if they resonate, resonate panel absorbers i would have thought was bettter!!!

 

[RoadrunnerOZ]

An effective absorber matches to the impedance of air AND wavelength of sound. A 4" dense panel is better than 4" lightweight because it's impedance is better. But neither is deep enough in proportion to wavelengths of very low frequencies. Once you go 12"+ the impedance of the denser material is far too high and it becomes reflective, the lighter material is then far superior.

Correct, but when space is your enemy you use traps or tuned absorbers. Surface treatment would require soft depth of 0.75M on each side wall for across the room to be truly effective.

 

[Spencerleehorton]

could i make a helmholtz for the 120hz and the 83hz?

would this possibly be a good option?

and again would i just place it at the high pressure zone (loudest) point of where i hear those frequencies?

 

[Spencerleehorton]

my back wall is an ideal place to put any of these bits or front wall behind mixing desk?

 

[RoadrunnerOZ]

also the panel absorber doesnt explain what material the thin panel is?

Likely ply - as long as it’s thin. 18mm ply would be too heavy I would think.

i do still need to setup my monitors at correct degrees (60degrees) and position them correctly in the room as it makes a pretty big difference in the plots but that said these frequencies still seem to be the problem whereever i move them, so to me moving the speakers could be done once i have started to notice a difference in removing the nulls.

You’ll be better off by positioning the speakers first as high pressure points may shift - you may not notice it at the seating position but the antinodes will move with speaker pointing/placement and they’re what you’re trying to find.

could i make a helmholtz for the 120hz and the 83hz?

would this possibly be a good option?

and again would i just place it at the high pressure zone (loudest) point of where i hear those frequencies?

I put a link in an earlier post on a calculator for these. Here it is again. They’re easy to make - you just have to fiddle with the calc to get a practical solution.
Calculate Perforated Helmholtz Panel Absorber
They should be placed at the peak spots.

 

[kags]

also the panel absorber doesnt explain what material the thin panel is?

Material can be wood, plastic, metal, really anything that is flexible. Wood is the easiest to work with. You could also make a membrane absorber where the front is mass-loaded vinyl, but it’s a little trickier to get it right.

 

[kags]

yeah works fine on macbook:

well the panel sound absorbers seem dependant on weight and the wood i have i think will weight more than 1kg, if i built a 200x100x50cm panel it would have to weigh 1kg to hit 120hz or 100hz

The panel will weigh more like 5 to 7 kg. Play with the depth to get the freq you want. It won’t be deep! Tuned absorbers tend to be used for probs under 100 Hz because porous absorbers work well above that.

 

[kags]

Likely ply - as long as it’s thin. 18mm ply would be too heavy I would think.

You’ll be better off by positioning the speakers first as high pressure points may shift - you may not notice it at the seating position but the antinodes will move with speaker pointing/placement and they’re what you’re trying to find.

I put a link in an earlier post on a calculator for these. Here it is again. They’re easy to make - you just have to fiddle with the calc to get a practical solution.
Calculate Perforated Helmholtz Panel Absorber
They should be placed at the peak spots.

Slat absorbers can work well and are simpler than perforated panels to get right.