For this case 
, hence
(1.24) and (1.25) reduce to 
and
, which imply S = 
.
Moreover, 
and
hence from (1.31) 
.
Thus contours of S and M are straight parallel lines of
slope 
on the x, t-plane.
It follows that 
from (1.19) also depends only
on 
.
It has the same spatial form for all t or the same temporal
form for all x.
An example is a plane acoustic wave.
If it were otherwise, then sound would not be a useful
mode of communication, since receivers at different
distances from a sound source would get different messages
unless 
= constant.