A microphone — typically with a hypercardioid capsule — which has a perforated tube fitted in front of the diaphragm to try and create a substantially narrowed polar pattern.
Sound waves directly on-axis pass straight down the tube to reach the diaphragm without being affected in any significant way. However, this tube also has a number of slots or holes along its length which allow off-axis sound waves to enter at different points. The resulting different path lengths to the diaphragm create phase shifts resulting in partial cancellation and thus reduced sensitivity to sounds from the side of the tube. Hence the design's name of an 'interference tube microphone'. Alternative names include 'rifle' or 'shotgun' mic, principally because in the film and TV industries the mic is often fitted to a handle, resembling a firearm.
The cancellation of off-axis sounds is rarely perfect or consistent, resulting in varying amounts of attenuation for sound sources of different frequencies and at different angles of incidence. This means that off-axis sounds, while genreally attenuated, are also substantialluy coloured , and if the angle between mic and sound source is varied the cancellation dips move up and down the spectrum to give a phaser-like colouration or whoosing effect.