investment may be protected from attack by the fluid or gas by insert of a short collett usually of metal having one end swaged according to the various embodiments of the devices of our invention the fiber loops may extend perpendicularly from the internal wall of the conduit and remain so over the whole of their length with this arrangement in use the fluid flow may cause the fiber loops to bend over at some distance from their point of attachment so as to provide the optimum or most economical use of materials a ratio will be established involving several factors such as the amount of incident energy and the statistical data relating to the fiber loops population per unit area density or specific gravity young 39 s modulus diameter and length particularly that part of the fiber loop investment which is parallel to the direction of fluid flow effective thickness or depth from the roots when in use environment humidity and the length of axial path invested with the fibers we have also found that a gas may be released silently yet at high velocity from the end of a conduit such as a compressed air line for related physical reasons organ piping is eliminated without reduction of rheological efficiency our invention is not to be considered limited in any way to the silencing of an internal combustion engine as there are diverse areas in which the principles set forth above also apply other areas in acoustics where our technique may be applied are those where high noise level.