RFZ ceiling, where to place the insulation?

[sxx]

Dear all,
I'm currently building my control room (RFZ).
I planned to angle the ceiling by 12° to avoid direct reflections and flutter echo and wanted to know if it was possible to put the insulation on top of the hard surface (19mm MDF).

My understanding is that high frequencies will be reflected to the back wall which is treated and fully absorbent.
Lower frequencies will be absorbed by the rockwool that is lying on top of the MDF ceiling (option 1 on the picture).

Is it possible or should I absolutely place the ceiling's treatment in front of the hard surface (option 2)?

[Soundman2020]

I planned to angle the ceiling by 12°

Why 12°? Did you check that that is a large enough angle to actually create a true reflection free zone?

wanted to know if it was possible to put the insulation on top of the hard surface

I always do that in any case, with hard-backed clouds, but not for the purpose of dealing with reflections on the OTHER side of the panel...

My understanding is that high frequencies will be reflected to the back wall which is treated and fully absorbent.

You seem to be misunderstanding how porous absorption works. If a porous absorber has an absorption coefficient of 1.0 for a certain frequency band, that does NOT mean that it will absorb 100% of that frequency, with none at all being returned to the room. That's a common misconception, but it simply isn't true. So there might well be some "high frequencies" that return to your angled cloud panel, and are then reflected back down to your ears...

Lower frequencies will be absorbed by the rockwool that is lying on top of the MDF ceiling

And why would the lower frequencies NOT be reflected by the 19mm thick MDF panel? What law of physics would prevent that panel from reflecting low frequencies?

Is it possible or should I absolutely place the ceiling's treatment in front of the hard surface

I generally do BOTH sides of the cloud with absorption. I put thick, absorption underneath (usually 4" of fiberglass) to attenuate as much of the spectrum as possible before it gets reflected towards the rear of the room, and thick absorption on top as well, to damp any resonance or reflections that might be going on up there.

- Stuart -

[sxx]

Thanks you Soundman for your excellent advice

Did you check that that is a large enough angle to actually create a true reflection free zone?

My ceiling is pretty high: 3.6m and the room is 5m long, the tracing I did showed that there shouldn't be any direct reflection if I angle the ceiling by 12°.

If a porous absorber has an absorption coefficient of 1.0 for a certain frequency band, that does NOT mean that it will absorb 100%

I understand that, when I said fully absorbent, I meant that the back wall is 100% covered by rockwool, sorry if I wasn't clear.

I generally do BOTH sides of the cloud with absorption.

Ok, I'll do that... thank you Soundman