It isn't actually the tube that makes a typical modern shotgun
microphone microphone directional, it's the openings in it and how the sound through those openings interacts with one another and the direct sound at the
diaphragm, thus the 'interference tube' reference FMEng used. Sounds from different paths that arrive in
phase are summed, sounds that arrive 180 degrees out of
phase are canceled and those in between are, well, somewhere in between. Since wavelengths, and thus relative
phase for a given distance, vary with frequency the results of a fixed interference tube also vary with frequency, thus the lobes and nulls. It also means that getting an interference tube to be effective at lower frequencies requires a very large tube.
For speakers a Bessel
array, essentially a two dimensional
array of speakers, is a much more realistic method of obtaining a more controlled and electronically variable interaction in two axes, but lobing,
etc. are still relevant.
What surprises many people is that many 'modern' audio concepts were actually conceived from 50 to 100 years ago by some of the early audio industry greats, it is just now that the technology is allowing the concepts they developed then to be practically implemented. For example, many seem to be enamored with newfangled
line arrays, not realizing that the general concept goes back to around 1940 and that 'columnar' arrays were popular in the 1950's. It's just that today's technology allows for new approaches to and implementation of those earlier concepts.