Originally Posted by DanDan
Hi Nuuk, this is intriguing.
How do you reckon a rigid backing or boundary increases the effect of an absorbent layer?
I would have thought the opposite is the case. i.e. Mounting the foam directly to the boundary removes the biggest exposed surface area, leaving only the edges.
to my understanding this is not the point. You are using a metal plate in front of the foam. Therefore the plate reflecting soundwaves, the soundwaves are exciting the plate, and/or the soundwaves are just moving around the plate and the foam, if you are not (!) using a rigid backing. By using a rigid backing, the soundwaves which are moving around the plate will be caught by the foam due to edge diffractions. I am unfortunately not able to provide a scientific explanation for this right now, without doing some more research, which I can't do right now. Maybe someone else could jump in here....
However, if you are talking about soundwaves, that are moving around the plate and the foam, then are reflected by another wall and then hit the back of the foam (not using rigid backing) you might be right. But I doubt this will be more efficient than the VPR design.
Fuchs interestingly describes a few variations to the VPR design in his book: You could either way mount it to a wall, integrate it into a wall or mount it apart from a wall using another rigid backing. However, I would assume that the best way mounting a VPR is in the corners and egdes of a room.
I also read a quote form Fuchs in another forum where he described that you will be creating a shortcut between the front and the back of the VPR if you do not mount it with a rigid backing. To me that actually makes sense.