If you have an iPhone or an Android phone, you can get a free app that will measure SPL. It might not be quite as accurate as a dedicated RS meter, but it's "something".
Bruno, you have a lot more area on the walls than on the floor, so you might want to consider a temporary experiment, even if it is only tacking a sheet on the back wall or something. The floor represents a relatively small percentage of the surface area of your room, and the furniture is going to diffuse the sound a lot there anyway.
