Broadband Absorbers (Bass Trap). High or low density?

[Glenn Kuras]

For 4" bass traps (straddling corners) you do want to use higher density but as the panel becomes thicker you can use less dense. In fact it will work better. Needless to say 705 is pretty expensive. 6" of 703 is less expensive and will work better vs the 4" of 705. BTW 4" or 703 straddling corners works pretty darn well so if your budget is low don't hesitate to go that route.

[avare]

Originally Posted by deadringer971

Hi. I'm a bit confused about what fibreglass density I should be using when building broadband absorbers.

On Ethan Winer's website he says that "Even better for low frequencies is 705-FRK, which is much more absorbent than 703 at 125 Hz and below."

705-FRK has a density of 6 pcf.

But some say that I should use low density for "bass traps".

I guess that both statements can be true. But does anyone have some tips/facts/information to add to this?

How deep are you mamkng them? For 30 cm and deeper, regular insulation filling the space is best acoustically and the cheapest.

Andre

[deadringer971]

Great, thanks. Didn't even consider the cost vs. effectiveness.

Another quick question. I need to treat my ceiling. I attached a picture that pretty much show you what it looks like.

I've got tips about using rigid fiberglass between the joist and take advantage of the gap between the support beams and top ceiling. Any clue on how much density I should go for?

[avare]

Originally Posted by deadringer971

Great, thanks. Didn't even consider the cost vs. effectiveness.

Another quick question. I need to treat my ceiling. I attached a picture that pretty much show you what it looks like.

I've got tips about using rigid fiberglass between the joist and take advantage of the gap between the support beams and top ceiling. Any clue on how much density I should go for?

How deep?

Andre

[deadringer971]

Originally Posted by avare

How deep are you mamkng them?

Probably aiming for a mixture of thinnest possible, most cost effective and most effective low-freq. absorption. I was thinking about going for a 6" rigid fiberglass, density somewhere around 40-50 kg/m3, top-to-floor. Any better ideas? This is all new to me.

Originally Posted by avare

How deep?

The picture will show you. 35 cm
EDIT: The left joist is only 27 cm. I also need to cover the fiberglass with some fabric, as I started to design in the picture. So I guess 20 cm is all I can use for gap/fibreglass.

[deadringer971]

Another question. What will give the best result? A gap in the corner of the broadband absorption or not? In this example it's 22 cm deep from front to the corner. But I guess I can make it 30 cm also.

[AwwDeOhh]

Originally Posted by deadringer971

Another question. What will give the best result? A gap in the corner of the broadband absorption or not? In this example it's 22 cm deep from front to the corner. But I guess I can make it 30 cm also.

16" face [42cm] isn't very wide at all for a corner trap.. You should think of doubling that amount if you can.
61cm would be minimum. 84cm across the corner span would be great.

[GIK Acoustics]

I agree with AwwDeOhh. You should definitely go for 60cm across for the width of the trap, it will be much more effective (which would be your 30cm from front to back suggestion you made)

With the types of materials we use for broadband absorption, the effective thickness depends on the material. For example, at 5cm thick, OC703 does a good job at speech frequencies, while 5cm of loft insulation would be lousy. However, compare that to 30cm of loft insulation vs. OC703 and the loft insulation would be more efficient at absorbing lower frequencies.

If it is easier for you to source, the price is fine, and easier to construct, 6" OC703 straddling the corner will work great. If your budget is low though, doing a thick square or triangular trap in the corner with loft insulation would work great too. Make sure if you go the loft insulation route to not compress it. I would recommend building some supports for them every couple of stacks of insulation.

[avare]

Originally Posted by deadringer971

Another question. What will give the best result? A gap in the corner of the broadband absorption or not? In this example it's 22 cm deep from front to the corner. But I guess I can make it 30 cm also.

Put posts 3 and 8 together. Fill with reguar insulation. If yo can not go deeper than 22 cm then 10 cm of 703 like material (64 kg/m³ mineral wool) or fill the space with ~ 32 kg/m³ mineral wool. Choose based on price.

Andre

[deadringer971]

Thanks!

How about low density first and on the outside higher density? See picture. I can get hold on some cheap 17 kg/m3 density insulations. Or is it necessary at all?

OC703 isn't available in Norway, but Rockwool has one they call FlexEkstrem 33 which has a density of 45 kg/m3. I believe OC703 has about 48 kg/m3. Is density the only thing that I need to think about when finding something similar to OC703?

[avare]

Originally Posted by deadringer971

Thanks!

How about low density first and on the outside higher density? See picture. I can get hold on some cheap 17 kg/m3 density insulations. Or is it necessary at all?

OC703 isn't available in Norway, but Rockwool has one they call FlexEkstrem 33 which has a density of 45 kg/m3. I believe OC703 has about 48 kg/m3. Is density the only thing that I need to think about when finding something similar to OC703?

If you are filling the space, re-read my previous post. The 17 kg/m³ is just about perfect. 703 is nominally 48 kg/m³ glass wool. 64kg/m³ IS acoustically equivalent to 703. That is why it is written in the thread.

The best for deep (greater than 30 cm) is to fill it with light mineral wool. The 17 kg/m³ is good for that.

Andre

[deadringer971]

Thanks for clearing that up for me

[deadringer971]

I was thinking about building a 60 cm wide, 30 cm deep low-end absorber. And if the insulations panel where 120x60 cm it would be easy. But as most of them are 120x57 cm :(

Will it make any big difference making a superchunk low-end absorber that is 60x28,5 cm, and between every other 10 cm insulation there's a 57x30 cm?
See picture that I attached?