I had a horrible slap echo between parallel walls in my living room. Made a 4' x 8' x 4" frame, glued 4-2' x 4' x 3" panels of acoustic egg crate foam from PE and covered it with burlap (Very cheap.). Hung it on one wall. Works like a dream.
Click on photo.
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Regards,
Dave
I did the same thing with a flat surface that is right behind and to the right of my listening position. The acoustic foam worked great.