A hush house is large enough to accommodate an entire crewed or uncrewed aircraft. Some facilities are also equipped to test additional capabilities such as weight and balance, night vision and low lighting, water intrusion, heat soaking, and wind evaluation.[1]
Jet engines can be run while installed in the aircraft, which must be restrained by holdback devices to resist the engine thrust. Engines removed from an airplane can be tested while held in place by thrust frames.
The air intake and exhaust systems of indoor engine test cells and hush houses are designed to block the transmission of noise, while optimizing the engine air flows. The engine exhaust, after having been thoroughly mixed with cooling air, is generally discharged through a vertical stack. The gas path incorporates acoustic damping panels (often containing fibrous insulation protected from gas stream erosion by metal mesh) to reduce the sound energy of the gas stream and attenuate the noise transmitted to the surrounding outdoor area.
Because the engine exhaust flow is "augmented" with a relatively large flow of cooling air induced by a Venturi effect into the exhaust silencing system, the exhaust muffler of an indoor test facility is generally referred to as an augmenter tube, although the term "detuner" is commonly used in the UK.[citation needed]
Some outdoor run-up facilities used to test aircraft engines (installed or uninstalled) may also be outfitted with noise control structures, called Ground Run-Up Enclosures.[2][3][4]
