Gap or No Gap for Soffit Bass Traps?

[DanDan]

Well stated points.

I have difficulty with those waterfalls as the after is 5dB Louder at 50Hz.
If things could be made more equal, perhaps the clouds performance is even better than you suggest.
Do you have the .mdat?

I am a big fan of the full cloud, drop ceiling. Typically we speak of them as good down to 50Hz, but let's extrapolate from a real test.

From Bob Golds 701, plain 4" (102mm) 16" air 1.5 pcf (24 kg/m3) 0.87

So, let's say 20", 0.5M of fibre is effectively fully absorbent at 100Hz, Lambda 3.43M.

Our system is fully effective with a combi treatment of only 1/7 Lambda, and a fibre thickness of only 1/34 Lambda.

DD

[Soundman2020]

I have difficulty with those waterfalls as the after is 5dB Louder at 50Hz.

Yep. The sub got turned up accidentally in between tests, so it is actually triggering the modes even more than it would have otherwise.

If things could be made more equal, perhaps the clouds performance is even better than you suggest.

Exactly!

So, let's say 20", 0.5M of fibre is effectively fully absorbent at 100Hz, Lambda 3.43M.

But that doesn't explain the same effect at 35 Hz... And there isn't 20" of fiber up there anyway: only about 3" on the cloud, and another 3" on the ceiling, IIRC.

Do you have the .mdat?

I have hundreds of REW tests from the corner control room build, but I'd be hard pressed to go find that exact one again! I'm sort of tight on time, as well: A couple of clients are pushing me to finish up designs for the their rooms.... Perhaps if I have a little spare time at some point, I could go looking for that MDAT. Hope you understand!

- Stuart -

[DanDan]

Sorry, typing quickly. That scenario was 4" fibre with a 16" airgap.
The point being that an airgap does not have to be 1.4 Lambda to be hugely beneficial.
We commonly consider 1" ceilings with the standard 16" air gap as 'good down to 50Hz'. I can quite imagine our 4" thick fibre plus 16" gap example having much bigger effects than your example, but then.... full area and sealed.

There are very few comparative tests down on LF trapping. We see a lot of energy wasted on failing MLV fronted traps.
While the BBC and more recently Gernot, tested and found simple hardboard to be the most effective.

While I promote the full drop ceiling, I think there is a big difference between a full 'sealed' boundary like that and the typical 4-6 trap cloud. I would certainly disagree with the likes of Eklund's recommendation of as big an air gap as possible, irrespective of total area, trap size, and gaps between them.
1:1 is the only test data I have seen regarding a sample patch.


DD