ceiling absorption problem

[kimve]

Dear forum members, dear acousticians,

I have this problem with the absorption of a metal perforared ceiling that I want to share and discuss.

The ceiling's mounted in a rectangle room, I attached a drawing of the room where you can see dimensions and used materials.

I noticed there's a high-frequency reverberation in the room so I did a reverberation time measurement. I used an omni-directional speaker emitting pink noise and a Bruel and Kjaer handheld sound meter.
The high-frequency reverberation is clearly visible on the graph so I made a model of the room with ray-tracing software. You can find a comparison in attachment.

Here's the problem:

Making use of the lab measurement result of the ceiling for my model, there has to be sufficient broadband absorption. This is not the case in the actual room.
The lab measurement result's also in attachment. Here, the ceiling tiles are measured with a non-woven acoustic fabric on top and a air plenum of 200mm. No mineral wool was used.
Comparing the model's result with my measurement, keeping al the other materials acoustic properties the same, you can see the change in absorption of the ceiling on the last picture in attachment.

I think the lack of broadband absorption has something to do with the very big plenum (1,7m).
Does someone have experience with very big plenums and their acoustic behaviour?

Thanks!

[bert stoltenborg]

An absorber in this configuration is going to behave like a membrane.

[kimve]

Thank you Bert. That explains a lot.

I suppose reducing the plenum height will help to achieve the desired Helmholtz? But not really a practical solution...
Other solutions to change the behaviour of the ceiling?

The easy way is to add some mid to high frequency absorbers in the room.

[Bob]

IF the false ceiling can take the weight, increasing from 30mm to 8" (200 mm) of fluffy fiberglass pink on top of it could damp the membrane's resonance measurably. Another 8" (200mm + this 200mm) could be placed on at the real/hard ceiling, for a 13 times increase in absorption depth.

Perhaps increasing it from 30mm to 150mm is the change point. (150mm the minimum worth experimenting with. 60mm would have some effect. 100 would have more of an effect. But 150mm is what I'd try. 200mm and 400mm are even better all the way down to 60 hz. No point in going past 400mm) I used ChrisW's "Porous Absorber Calculator V1.52 XL2002.xls" at 16,500 ralys/m, with 1700mm air gap and 150mm absorber thickness, to model this.

[kimve]

Thanks for your input Bob! If I understand well you used the 'porous absorber' calculation to see the resonances, ignoring the high frequency absorption of a classic porous absorber?

Before posting this I suggested to increase the absorber thickness to 80mm. The 2kHz reverberation is still audible but I did not measure it yet.
I'm planning to measure in January to see the difference! If there is, increasing the absorber thickness to 150mm would be an option.

I'll keep you guys posted.

[Bob]

[Yannick]

The problem is that the 30 mm of rockwool is wrapped in PE foil, as indicated in the drawing.
If this is really the case, that would render the rockwool completely reflective above 1000 Hz, plus there is no plenum above the perforated ceiling, as the wrapped rockwool is resting directly on the ceiling tiles.

This would render the perforation completely ineffective at higher f, as witnessed by the measurements.

Removing the foil would have a dramatic effect.

[kimve]

The problem is that the 30 mm of rockwool is wrapped in PE foil, as indicated in the drawing.
If this is really the case, that would render the rockwool completely reflective above 1000 Hz, plus there is no plenum above the perforated ceiling, as the wrapped rockwool is resting directly on the ceiling tiles.

This would render the perforation completely ineffective at higher f, as witnessed by the measurements.

Removing the foil would have a dramatic effect.

The PE foil is acoustic 'transparant. The absorption of the wrapped rockwool is almost identical to a normal rockwool sheet. The reason for the wrapping is air extraction via the plenum.