Building HT with 4 concrete walls. Need to get rid of Echo. Help

[fred87nc]

I'm living in Mexico and building a mini theater for 30 people. The Four walls are concrete and it's around 9 meters long and 4.5 meters wide, with the cealing being 3.9 meters high. I got a bad echo in there just talking.

Now what is the cheapest way to get rid of the echo, I been told to put thick carpet or fabric on the walls to get rid of the noise, and also thinking of puttting carpet on the floor.