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!