Quick question: is the diaphragm used in a compression normally a concave shape as the image suggests? Providing a narrow focused wave of sound that is then widened by the throat and expanded by the horn?
No there are variation on the theme. Though the essence, is a vibrating diaphragm in a high impedance chamber and a restriction called the throat, leading to an expanding structure.
In essence all good loading is acoustic transformation, that converts small vibrations at high pressure to large air displacement at low pressure.
Pipes and horns, especially the latter doe this most effectively.
In a pipe pressure and displacement are 180 degrees out of phase. So in a closed pipe the closed end where the speaker is has an antinode of pressure and a node of displacement. At the opening or port, there is an antinode of displacement of air and a node of pressure. In this way small cone movements will cause large air displacements.
In horns this effect is maximized with huge pressure change at the diaphragm and huge air movement at the mouth. In addition horns are by nature low Q and non resonant.
Pipes require some damping to lower Q, but can still have useful port output.
Reflex enclosures have enormous internal pressure at tuning with large air displacement from the port. The driver decouples abruptly below tuning. These enclosures only tolerate partial damping and reproduction is resonant otherwise port output is killed.
In a sealed, open baffle or IB system there is no acoustic transformation and reproduction is highly inefficient and entirely dependent on cone displacement.
