Above is shown a representation of a 2D waveform. This could be a wave on the surface of a pond
caused by a pebble dropped in the water, a surface wave on a drum after being hit in the middle with a
drum stick, or a analogy of a 3D sound wave caused by a tuning fork (which would be located in the center).
The black sine curves just show that if we only restrict ourselves to a 1D region of whatever we're talking
about, then we only need to talk about a sine curve, as we so often do, and not a surface. Talking about
a 2D wave using a sine curve is a lot like talking about a 3D wave using the 3D surface above.
Whatever is causing the wave, it's located in the center. You can see the waves moving away--
and the sine curves moving. Notice that the wave can be described by concentric circles radiating
away from the center. In this case they're concentric because the speed of the wave
is the same in all directions. What do you suppose the wave would look like if motion
horizontal was slower than vertical motion?
One thing to note: if this were a real wave, the amplitude would decrease as it
got further from the source. This is a consequence of energy conservation and
would happen even if there was no absorption of the wave along the way. We'll
talk about this stuff later.