Hush house is a generic term for an enclosed, noise suppressed, aircraft jet engine testing facility for the testing of installed or uninstalled jet engines under actual load conditions.
Jet engines and aircraft can be tested either indoors or outdoors.
- Indoor engine test cells are facilities designed for testing engines removed from an aircraft (referred to as "uninstalled engines"). The engines in such facilities are generally suspended from overhead thrust frames.
- A hush house is wide enough to accommodate an entire aircraft so that the engine can be run while installed in the aircraft. Uninstalled engines secured to thrust frames can also be tested in most hush houses. However hush houses are only large enough to house fighter aircraft with either a single engine, or twin engines closely spaced around the aircraft centerline.
- The air intake and exhaust systems of indoor test cells and hush houses are designed to optimize the engine air flows, and to discharge the cooled jet exhaust through a vertical stack. The intake and exhaust systems have silencers to reduce noise transmitted to the surrounding outdoor area.
- Outdoor run-up areas are facilities where engines are tested outdoors while mounted on thrust stands, or where engines are tested outdoors while installed in an aircraft. They may, or may not, include provisions for noise control. The photograph shows a type of outdoor test facility called a "run-up pen" that is surrounded by a barrier to reduce noise in the surrounding area. While such facilities are much louder than enclosed, indoor hush houses, they are the only practical noise suppression option for run-up testing of larger multi-engine aircraft such as airliners.
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