science

Jet noise

Jet noise is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Jet noise rather than just read about it. In short: In aeroacoustics, jet noise is the field 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 (100 kHz and higher).

Key takeaways

Jet noise belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Jet noise to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Jet noise from memory before moving on to harder problems.

Reference excerpt

In aeroacoustics, jet noise is the field 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 (100 kHz and higher). Jet noise is also responsible for some of the loudest sounds ever produced by mankind.

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; 360 km/h; 225 mph) are "jet mixing noise" and, for supersonic flow, shock associated noise. 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 five diameters from the nozzle. The high-frequency components of the sound are mainly 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. Aircraft noise is also sometimes called jet noise when emanating from jet aircraft, regardless of the mechanism of noise production.

See also Aircraft noise pollution Lighthill's eighth power law QTOL Stealth aircraft

References

Works cited

Howe, M.S. (1998). Acoustics of Fluid-Structure Interactions. Cambridge: Cambridge University Press. pp. 149–156. ISBN 978-0-521-63320-8. Aviation and the Environment: Noise, Hearing before the Subcommittee on Aviation of the Committee on Transportation and Infrastructure. Pennsylvania, U.S.: U.S. Government Printing Office. 2007. pp. 149–153. Khavaran, Abbas. (2012). Acoustic Investigation of Jet Mixing Noise in Dual Stream Nozzles. Cleveland, OH: National Aeronautics and Space Administration, Glenn Research Center.

Worked examples

Example 1 — a first encounter with Jet noise

Start with the simplest possible case. Write down what Jet noise claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Jet noise before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Jet noise ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Jet noise

In research: Jet noise appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Jet noise in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Jet noise is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aircraft noise, Fluid dynamics, Fluid dynamics stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Jet noise outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Jet noise in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Jet noise means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Jet noise out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Jet noise in simple terms?

Why does Jet noise matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Jet noise?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Jet noise.