«These are not simple tubes,» El - Naggar said. (sciencedaily.com)
A pouch sling is a simple tube of fabric worn over one shoulder like a sash and used much like a ring sling but without the ability to adjust the size of the sling each time it is used. (babywearinginternational.org)
The sound waves enter the ear canal, a simple tube (but one which amplifies sounds that are between 3 and 12 kHz). (sciencedaily.com)