The simplest monopole planar source is the oscillating circular piston (or rigid disk) in an infinite baffle. The piston is assumed to be rigid so that all parts of its surface vibrate in
phase, and its velocity amplitude is independent of the mechanical or acoustic loading on its radiating surface. Remarkably, its radiation impedance was first derived by Rayleigh 
before the direct radiator loudspeaker had even been invented 
, yet it has been widely accepted as an idealized model for such when mounted in an enclosure situated near a wall or, even better, mounted directly in a wall as commonly ...