z
	x

Observe that:

• The leading edge of the x-shaped region of disturbed flow extends outward from the source at the group velocity.
• The lines of constant phase run parallel to these x-shaped arms.
• The phase speed is perpendicular to the group velocity in a sense such that the phase lines appear to propagate toward a horizontal line passing through the location of the source.

Zoom in to view fluid-parcel motions in the vicinity of the upper right arm.