If it is possible to slow the beam using dense materials, maybe it is possible to focus the effects of this longitudinal wave using a lens. Assuming that the wave fronts, transverse to its direction of travel, remain continuous lines,
it is possible to focus the space beam using a dense matter concave lens.
And, using total internal refraction,
due to the dense space layer just above the vibrating diaphragm, a free standing virtual image from a screen image.
Maybe...
