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Hush kit
A hush kit is a retrofit modification used to reduce the noise produced by older aircraft jet engines. Hush kits are typically installed on older turbojet and low-bypass turbofan engines, as they are much louder than later high-bypass turbofan engines and often fail to meet modern limits for aircraft noise pollution.
Hush kits must be designed specifically for each engine model, and their methods to reduce noise vary.
The most common form of hush kit is a multi-lobe exhaust mixer. This device is fitted to the rear of the engine and mixes the jet core's exhaust gases with the surrounding air and a small amount of available bypass air. Modern high-bypass turbofan engines build on this principle by utilizing available bypass air to envelop the jet-core exhaust at the rear of the engine, reducing noise.
Most hush kits make further modifications to the exhaust, including acoustically treated tailpipes, revised inlet nacelles and guide vanes. They reduce the forward propagating, high-pitched noise caused by the small, high-speed fan.
This kind of high-pitched noise is much less of an issue on modern high-bypass turbofan engines as the significantly larger front fans they employ are designed to spin at much lower speeds than those found in older turbojet and low-bypass turbofan engines.
Modern aircraft equipped with mid and high-bypass turbofan engines are designed to comply with contemporary aviation noise abatement laws and ICAO regulations. Several older aircraft that are still in service (typically in a cargo capacity) have hush kits retrofitted so that they are able to conform with noise regulations needed to operate in many commercial airports. Some of the examples include:
The impact of hush kits on noise level varies between each engine and retrofit device. For example, in recent measurements conducted by the German Aerospace Center (DLR), a hush kit and other modifications reduced the flyover noise of a specialized Airbus A320 by 3 dB, which the DLR claims to correspond to a perceived noise reduction of around 30 percent for people on the ground.[1]
To illustrate the varying impact of hush kits on aircraft noise levels, the table below presents certified noise data before and after hush-kit installation for select aircraft types. These values are based on a 2012 FAA advisory circular and represent the Effective Perceived Noise Level in decibels (EPNdB) measured at takeoff.
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Hush kit
A hush kit is a retrofit modification used to reduce the noise produced by older aircraft jet engines. Hush kits are typically installed on older turbojet and low-bypass turbofan engines, as they are much louder than later high-bypass turbofan engines and often fail to meet modern limits for aircraft noise pollution.
Hush kits must be designed specifically for each engine model, and their methods to reduce noise vary.
The most common form of hush kit is a multi-lobe exhaust mixer. This device is fitted to the rear of the engine and mixes the jet core's exhaust gases with the surrounding air and a small amount of available bypass air. Modern high-bypass turbofan engines build on this principle by utilizing available bypass air to envelop the jet-core exhaust at the rear of the engine, reducing noise.
Most hush kits make further modifications to the exhaust, including acoustically treated tailpipes, revised inlet nacelles and guide vanes. They reduce the forward propagating, high-pitched noise caused by the small, high-speed fan.
This kind of high-pitched noise is much less of an issue on modern high-bypass turbofan engines as the significantly larger front fans they employ are designed to spin at much lower speeds than those found in older turbojet and low-bypass turbofan engines.
Modern aircraft equipped with mid and high-bypass turbofan engines are designed to comply with contemporary aviation noise abatement laws and ICAO regulations. Several older aircraft that are still in service (typically in a cargo capacity) have hush kits retrofitted so that they are able to conform with noise regulations needed to operate in many commercial airports. Some of the examples include:
The impact of hush kits on noise level varies between each engine and retrofit device. For example, in recent measurements conducted by the German Aerospace Center (DLR), a hush kit and other modifications reduced the flyover noise of a specialized Airbus A320 by 3 dB, which the DLR claims to correspond to a perceived noise reduction of around 30 percent for people on the ground.[1]
To illustrate the varying impact of hush kits on aircraft noise levels, the table below presents certified noise data before and after hush-kit installation for select aircraft types. These values are based on a 2012 FAA advisory circular and represent the Effective Perceived Noise Level in decibels (EPNdB) measured at takeoff.