I'm looking for comparable data on 4" and 6" Alpha panels, specifically relative absorption around 100 Hz (for reducing SBIR). Unfortunately, the former was tested by Riverbank, and the latter by the U of Salford. The absorption coefficient at 100 Hz is shown as about 0.7 for the thinner panel and 0.6 for the thicker, if I'm reading the reports right. That seems backwards to me.
I have thicker material elsewhere in the room (including Soffit Traps), and in this spot, 4" panels would be less intrusive. Maybe they are enough? (I have 2" ASC panels in the location now, which I'd use in other areas.)