This isn't acoustic design for recording, but certainly related to a number of posts I have seen here.
I'm looking to make a dog shelter quieter for their own well-being. It gets very loud around the kennels from dogs barking because the space is basically an echo chamber made from concrete and cinderblock. While I haven't measured the acoustic spectrum, the mid-to high frequencies seem to be the loudest.
Funds are limited, so cheaper is better, and easy is certainly preferred. I would prefer to not just throw a bunch of old towels on the walls because it needs to look somewhat decent. New towels, blankets, etc. might look good enough, however. I cannot put anything on the floors or the bottom few feet of the walls because of sanitary concerns.
The most attractive option I have found so far is to hang the following on the wall directly across from the kennels (starting ~3-4 feet from the floor), and possibly the ceiling as well. The price per sq. ft. seems quite low and it's easy to install. However, it is only 4 mm thick and I'm not confident this will be nearly enough to significantly reduce the noise in the area. If placing it directly on the wall might not work, perhaps installing it a few cm away from the wall would improve absorption?