The configuration is the same as that shown in Fig. 13.21
for a piston in an open baffle except that the velocity on the rear of the piston is zero. We can achieve this boundary condition by superposition of fields (or Gutin concept) whereby we combine the field of a piston in an open finite baffle with that of a piston in an infinite baffle. The former has negative velocity
on its rear surface, whereas the latter, if treated as a “breathing” disk in free space, has positive velocity
on its rear ...