I must admit that I've used carpet in all my studios but in the recent ones I've been
careful to not have too much treatment on the walls so that the absorption is fairly even.
My first studio had carpet on the walls and roof as well which gave me too much high
frequency absorption but the bass just sailed through the thin wooden walls.
James.
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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net