Page 215 - The Jet Engine

Noise suppression

16. Noise absorbing 'lining' material converts be accurately calibrated. Guide vanes are fitted to
acoustic energy into heat. The absorbent linings (fig. the lobe-type suppressor to prevent excessive losses
19-6) normally consist of a porous skin supported by by guiding the exhaust gas smoothly through the
a honeycomb backing, to provide the required lobes to atmosphere. The suppressor is a fabricated
separation between the facesheet and the solid welded structure and is manufactured from heat-
engine duct. The acoustic properties of the skin and resistant alloys.
the liner depth are carefully matched to the character
of the noise, for optimum suppression. The disad- 18. Various noise absorbing lining materials are
vantage of liners is the slight increase in weight and used on jet engines. They fall mainly within two
skin friction and hence a slight increase in fuel categories, lightweight composite materials that are
consumption. They do however, provide a very used in the lower temperature regions and fibrous-
powerful suppression technique. metallic materials that are used in the higher
temperature regions. The noise absorbing material
CONSTRUCTION AND MATERIALS
consists of a perforate metal or composite facing
17. The corrugated or lobe-type noise suppressor skin, supported by a honeycomb structure on a solid
forms the exhaust propelling nozzle and is usually a backing skin which is bonded to the parent metal of
separate assembly bolted to the jet pipe. Provision is the duct or casing. For details of manufacture of
usually made to adjust the nozzle area so that it can these materials refer to Part 22.

205