Jet aircraft generate a considerable amount of noise, contributing to noise pollution near airports. In modern high-bypass turbofan engines, the fan mounted afore the jet engine core is very large. The bigger the fan in comparison to the jet core, the more effective the bypass air is in enveloping the jet exhaust at the rear of the engine, reducing noise. The larger turbines needed to spin the large fan slow the jet exhaust, which also reduces noise. A hush kit produces a similar effect, using several modifications to the existing engine. Primarily, a device called a multilobe exhaust mixer on the rear of the engine mixes the exhaust gases of the jet core with the surrounding air and the small amount of bypass air available. Similar systems are also employed on many modern high-bypass turbofan engines as standard equipment to further reduce noise. Most kits also make further modifications to the exhaust via acoustically treated tailpipes, revised inlet nacelles and guide vanes, all of which reduce forward propagating high-pitched noise caused by the small high speed fan.
Modern aircraft equipped with high-bypass turbofan engines are able to comply with contemporary aviation noise abatement laws and ICAO regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as the Boeing 727 and 737-200, Douglas DC-8 and DC-9, and Tupolev Tu-154. They are also deployed on smaller business and personal jets which are too small for large high-bypass turbofans.
While hush kits effectively reduce noise emissions from older aircraft, noise cannot always be reduced to the level of modern planes at a reasonable cost. In 1999 this has led to a dispute between the United States and the European Union. The EU's proposed noise ordinances effectively prevented the use of hush-kit outfitted aircraft in Europe, reducing the value of the mostly American used airplanes so equipped and hurting the profits of American hush kit manufacturers. EU Regulation 925/99 was passed over US threats to ban the Concorde, but was superseded and effectively repealed by EU Directive No. 2002/30/EC issued March 26, 2002.