Ducts comprising exoskeleton and sound-absorbing structures and vehicles using such ducts

A vehicle comprising:

• an environmental control system comprising a duct, wherein the duct comprises:
• an exoskeleton structure, formed using additive manufacturing and comprising an exoskeleton inner surface, the exoskeleton inner surface having a tubular shape and defining an exoskeleton interior;
• a sound-absorbing structure, disposed within the exoskeleton interior and conforming to the exoskeleton inner surface, the sound-absorbing structure comprising a sound-absorbing inner surface defining a sound-absorbing interior;
• an internal membrane, disposed within the sound-absorbing interior and conforming to the sound-absorbing inner surface;
• an external membrane, disposed between the exoskeleton structure and the sound-absorbing structure such that the external membrane is in direct contact with the exoskeleton inner surface;
• an inner-membrane fastener, wherein:
• the inner-membrane fastener protrudes through the internal membrane and the sound-absorbing structure;
• and
• the inner-membrane fastener supports the internal membrane and the sound-absorbing structure relative to the exoskeleton structure.