Because gases, such as air, do not support shear stresses (to first approximation), wave motion in air is primarily longitudinal.
Most acoustic music instruments have air columns that are roughly cylindrical or conical in shape.
When analyzing the acoustic behavior of these systems, we are most often interested in one-dimensional wave motion along the principal axis of the air column.
Longitudinal wave motion along the principal axis of cylinders is planar.
Longitudinal wave motion along the principal axis of cones is spherical.