Acoustic noise from relatively high pressure gas flows as a result of
opening for example a bypass valve in a gas turbine engine can cause
problems. By provision of an acoustic apparatus incorporating an inner
chamber where there is a pressure reduction and then a chamber divider
surface incorporating small apertures with a wide spacing distribution an
upward shift in acoustic frequency of the gas flow is achieved beyond
normal human comprehension. An outer chamber is provided to receive the
fine gas flow jets from the apertures in the chamber divider surface for
further pressure reduction and low pressure release through wider
apertures in an exit surface.