Not written by me, but vey interesting, nonetheless.

For just a few cents you can buy a candle, and for possibly a price of a pizza, you can convince a friend to come over with a mirror.
Place the candle on the top of your speaker and retreat to your listening position. Your friend should then move down each side wall away from your loudspeakers with the mirror against the wall, facing into the room. As soon as you see the candles reflection out of the corner of your eye get him or her to stop. This is the first reflection point of your loudspeaker and if he/she continues you should notice a second reflection point. So, why is this useful? Well if you have an overly live midrange or a very live room, dampening these points with acoustic treatments or a rug will improve audio focus. It is also a key for audio rack positioning. If your Hi-Fi resides on a sideboard you should really consider investing in a dedicated audio rack. Once you have, endeavour to position it along a side wall - but not in the two positions outlined above. Just imagine a box that you’re pouring water into, as you pour in the water it will slosh up against the sides and then back to the middle - this is what sound tends to do. So putting your new rack in a corner is a real no-no. Between the speakers is better, but not ideal; the side wall or out of the room is the best bet! Once sited, sit back and enjoy the music

Pretty neat, Jorge!