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PostPosted: Mon Apr 27, 2020 6:40 am 
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Hi there,

I did some research on bass traps today and would like to check with you experts if some my assumptions are correct or not:

As far as I can see there are mainly three options to build a bass trap in a corner:

1) filling up the whole corner with triangle shaped mineral wool / fiberglass (super chunk)
2) use a thick (e.g. 4 inches) slab of the same material straddling the room corner
3) use two slabs to form a cuboid from floor to ceiling

- Comparing design 1) and 2), design 1) needs much more material, but I assume there is no real benefit if I want to absorb the low frequencies (say below 300 Hz). This is because it’s mainly the
distance of the material to the wall that allows to absorb the lower frequencies. Is that correct?

- Design 3) is the most effective one, because the panels have the maximum distance over
their whole width whereas the panels in design 2) just have maximum distance in the middle of the panel getting less to each end (due to the triangular shape). Is that correct?

Please, correct me where I'm wrong! Thanks a lot for your help!


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PostPosted: Mon Apr 27, 2020 7:39 am 
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All of those methods are velocity based absorbers, or porous absorbers, they all need to be fairly large in order to work down to what I consider the bass region (below 100hz). In theory a depth of 1/4 wavelength of the lowest frequency you'd like to attenuate is required, however in practice it is found that between 5-10% of the wavelength works very well. You can also place the absorber out from the boundary at the 1/4 wavelength (or between 5-10%)

For those types of absorbers you're generally better off using low density insulation with low gas flow resistivity, at great thickness - roughly up to about 18" thick. if you pass that thickness then it's best to include some air gaps within the traps.

You can experiment with different thicknesses and GFR values here:

http://www.acousticmodelling.com/porous.php

Anything below 0.6 should really be disregarded as it is not effective absorption in my opinion.

What are the frequencies you would like to absorb?

There are also waveguide absorbers or "hangers" which are popular here, they are generally more effective than full fill of insulation at great thicknesses, but they should be quite large and big.

There are also pressure based absorbers such as membrane absorbers which can be made thinner, but have a narrower Q of frequencies which are absorbed. Or Helmholtz resonators which work by resonance.

Paul


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PostPosted: Mon Apr 27, 2020 5:55 pm 
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Hi Paul,

thanks a lot for your reply, that makes all sense.

I want to absorb broadband, but also as much bass as I can (at least down to 80 Hz, the lower the better). And I'm also looking for a
comparably cheap and easy to build design, that's why I'm looking at those three methods.

Paulus87 wrote:
All of those methods are velocity based absorbers, or porous absorbers, they all need to be fairly large in order to work down to what I consider the bass region (below 100hz). In theory a depth of 1/4 wavelength of the lowest frequency you'd like to attenuate is required, however in practice it is found that between 5-10% of the wavelength works very well. You can also place the absorber out from the boundary at the 1/4 wavelength (or between 5-10%)
Paul


That's why I assume that design 3) is the most effective at low frequencies. Simply because it's the one with most absorbing material at the maximum distance from the walls (depending on how big a decide to build them). Is that correct?

Also, I wonder if it's necessary (or even an advantage) to completely fill those cuboids with absorbing material compared to just using e.g. a 5" boards at the outside?
I assume the inner filling doesn't really affect the low frequencies (since their velocity is too low that close to the walls). So it would mainly
absorb the high frequencies, which should be already absorbed in the first couple of inches of the whole structure. Wouldn't that be sufficient, so I could save material and cost without diminishing the impact of the absorber?
Or wouldn't such a completely filled cuboid even over-absorb the high frequencies compared to the lows and therefore be disadvantageous?

I'm probably overthinking it, but I might like to understand. So, thanks again for all your help!


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PostPosted: Mon Apr 27, 2020 7:11 pm 
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angelo wrote:
Hi Paul,

thanks a lot for your reply, that makes all sense.

I want to absorb broadband, but also as much bass as I can (at least down to 80 Hz, the lower the better). And I'm also looking for a
comparably cheap and easy to build design, that's why I'm looking at those three methods.

Paulus87 wrote:
All of those methods are velocity based absorbers, or porous absorbers, they all need to be fairly large in order to work down to what I consider the bass region (below 100hz). In theory a depth of 1/4 wavelength of the lowest frequency you'd like to attenuate is required, however in practice it is found that between 5-10% of the wavelength works very well. You can also place the absorber out from the boundary at the 1/4 wavelength (or between 5-10%)
Paul


That's why I assume that design 3) is the most effective at low frequencies. Simply because it's the one with most absorbing material at the maximum distance from the walls (depending on how big a decide to build them). Is that correct?

Also, I wonder if it's necessary (or even an advantage) to completely fill those cuboids with absorbing material compared to just using e.g. a 5" boards at the outside?
I assume the inner filling doesn't really affect the low frequencies (since their velocity is too low that close to the walls). So it would mainly
absorb the high frequencies, which should be already absorbed in the first couple of inches of the whole structure. Wouldn't that be sufficient, so I could save material and cost without diminishing the impact of the absorber?
Or wouldn't such a completely filled cuboid even over-absorb the high frequencies compared to the lows and therefore be disadvantageous?

I'm probably overthinking it, but I might like to understand. So, thanks again for all your help!


What insulation product will you use? And how far out from the walls will you place them?

Paul


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PostPosted: Mon Apr 27, 2020 7:18 pm 
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Not sure yet.

But something like 4" of OWC 705 in case of type 3) (just boards straddling the corner). Let's say 24" / 60cm
across the corner.

Or something like OWC 703 (lower flow resistance) in case of type 2) filling the whole corner with triangular shapes.

Thanks again!


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PostPosted: Mon Apr 27, 2020 8:10 pm 
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angelo wrote:
Not sure yet.

But something like 4" of OWC 705 in case of type 3) (just boards straddling the corner). Let's say 24" / 60cm
across the corner.

Or something like OWC 703 (lower flow resistance) in case of type 2) filling the whole corner with triangular shapes.

Thanks again!


Here's some comparisons to give you a rough idea of what you can expect from the 2 options you presented: 703 full fill (blue) & 705 straddling (red). Plus I've added a third option of light pink fluffy (i.e. cheap) attic insulation (green).

This is for a total of 24" thickness, of course in a corner the depth varies so the absorption will thin out the further out from the corner you get.

You can see that the OC705 & 703 perform quite poorly in comparison to the full fill of pink fluffy. Now this is only a software prediction and real life results will vary, but I think you'll find overall the fluffy stuff is the best stuff to use at this thickness.

If you were to build wave guide absorbers you can expect better results, but takes more time and effort. For a quick cheap solution I would just build some corner frames floor to ceiling in each corner with a shelf every 2-3' to help support the fluffy insulation, fill it and then cover it with a fabric of your choice.

This is only a start but it may be all you require. If you need to go further then let us know.

Paul


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PostPosted: Mon Apr 27, 2020 8:30 pm 
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Paul, thank you so much for all that work!

Just so that I understand correctly, is the pink fluffy curve for nothing but the whole corner filled with it?
Or is it 705 straddling plus the air gap behind filled with pink fluffy?

I'm not expert at all, but it would be hard to believe that just pink fluffy yields such a result?

Thanks again, I'm really grateful for your advice!


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PostPosted: Mon Apr 27, 2020 9:37 pm 
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angelo wrote:
Paul, thank you so much for all that work!

Just so that I understand correctly, is the pink fluffy curve for nothing but the whole corner filled with it?
Or is it 705 straddling plus the air gap behind filled with pink fluffy?

I'm not expert at all, but it would be hard to believe that just pink fluffy yields such a result?

Thanks again, I'm really grateful for your advice!


You can see exactly what is in each absorber at the bottom of the diagram.

The straddling panel has only air behind it, no pink fluffy.

Yes, the oink fluffy alone outperforms the others. Why is it hard to believe? The denser something is the more reflective it becomes... why would this not also be true of insulation? Think about it... if the denser something was the more low end it absorbed then surely leaving all your drywall/concrete walls would be better than any insulation at all since they are infinitely more dense than insulation... and yet, that’s not the case is it? Those dense walls reflect the sound back into the room which is why we need acoustic treatment in the first place.

Of course, even those dense concrete and drywall materials have absorption coefficients too, but they’re reflection coefficients are higher at certain frequencies.

Paul


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PostPosted: Mon Apr 27, 2020 11:50 pm 
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Thanks Paul, I understand.

Thanks also for pointing me to the absorber calculator, that gives me a lot of food for thought.
Playing around with it, I see that at a given thickness of 50cm a lower density gives me better results.
So e.g. Rockwool Sonorock with 6 kPa/sm^2 is probably better suited.

I also see that an air gap gives me higher absorption at lower frequencies, but the curve has more peaks
and dips. While a ful filled absorber with no air gap gives me a smoother curve.

I then tried a combination of 10cm Sonorock (6 kPa/sm^2) + 30cm Air Gap + 10 cm Termarock50 (16 kPa/sm^2).
That gives me almost the same performance, but a much smoother curve.

I could even smooth out that curve if I find some fluffy material around 2 kPa/sm^2 to fill the air gap.

So, I wonder if anybody has build an absorber like that and has experience with it. Does that approach make sense?

Another question is, how does all that translate to a triangular shape? Maybe I'm better of building a 50cm x 50cm
cuboid if can find the space.

I see there's still a lot to think about for me. So, thanks again for your help!


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