Jet noise

Introduction

Jet noise is a large section of the field of Aeroacoustics that focuses on the noise generation caused by high-velocity jets and the turbulent eddies generated by shearing flow. Such noise is known as broadband noise and extends well beyond the range of human hearing (100kHz and higher). Jet noise is also responsible for the loudest sounds ever produced by mankind (Saturn V rocket launch).^{[citation needed]}

Sources of jet noise

The primary sources of Jet Noise for a high-speed air jet(meaning when the exhaust velocity exceeds about 100 m/s) are "jet mixing noise" and, for supersonic flow, shock associated noise. Also, acoustic sources within the "jet pipe" also contribute to the noise, mainly at lower speeds, which include combustion noise and sounds produced by interactions of a turbulent stream with fans, compressors, and turbine systems.

The Jet mixing sound is created by the turbulent mixing of a jet with the ambient fluid, in most cases, air. The mixing initially occurs in an annular shear layer, which grows with the length of the nozzle. The mixing region generally fills the entire jet at four or give diameters from the nozzle. The high-frequency components of the sound are manly stationed close to the nozzle, where the dimensions of the turbulence eddies are small. Further down the jet, where the eddy size is similar to the jet diameter, is where lower frequency begins.

In Supersonic, or "choked" jets there are cells through which the flow continuously expands and contracts. Several of these "shock cells" can be seen extending up to ten jet diameters from the nozzle and are responsible for two additional components of jet noise, "screech tones" and broadband "shock associated noises". Screech is produced by a feedback mechanism in which a disturbance convecting in the shear layer generates sound as it traverses the standing system of shock waves in the jet. Even though screech is a side effect of the jet's flight, it can be suppressed by an appropriate design for a nozzle.

Effect of jet noise on communities

There have been several attempts to suppress Jet Noise in the past, some due to the effect this noise has on some people living near air bases for the military and such. In fact, according to one case where the people complained of Jet Noise was the hearing in Washington, D.C. before the Subcommittee on Aviation of the Committee on Transportation and Infrastructure House Of Representatives on October 24, 2007, the United States has spent $600 million on research to reduce aircraft noise and emissions since 1990. In that same hearing, the mayor of Arlington, Texas at the time, Mayor J. Mulder stated how he received an increasing amount of complaints from inhabitants of Arlington about the noise coming from planes.