It really depends on your preferences, though to get diffusion at 400 Hz you would have to
be sitting quite far from the diffusor to get the benefits. I would say most typical
diffusers start at usually around 800 and go till about 4k, though nested or fractal
designs can increase bandwidth on the high end.
By the way, I'm in Buenos
Aires, Argentina right now visiting my Sister - your country is beautiful!
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