A visual representation of nodes

[kareface]

http://www.youtube.com/watch?v=s9GBf8y0lY0

This is what is known as a Chladni Plate. What you are seeing is the nodal points created by the vibrations of the plate. Where the sand is gathering the standing waves in the material are colliding to nullify each other. The areas where the waves are moving freely, and producing the greatest vibration are the anti-nodes. As you scale the frequency up, the nodes multiply, but get smaller. In a room, when you are sitting in the node of a speaker, at the frequency of the node you'll hear a dip in the sound. This, much like the sand gathering at the places that are moving the least, is caused by the waves impacting each other from opposing directions creating a null. Simply turning the volume (or gain) up won't negate the nulls. This is one of the reasons room dimensions and speaker positioning is very critical.