How can aircraft engines be designed to reduce noise pollution?

Noise pollution from aircraft engines is a major environmental and social concern that affects the quality of life and health of people living near airports and flight paths. It also limits the operational capacity and efficiency of the aviation industry, as noise regulations impose restrictions on flight schedules, routes, and altitudes. Therefore, reducing noise pollution from aircraft engines is a key challenge and opportunity for both engineers and policymakers. In this article, you will learn about some of the main sources and characteristics of aircraft engine noise, and how they can be designed to minimize it.

1 Noise sources

Aircraft engine noise is composed of different components, each originating from a different part of the engine and interacting with the air. The main sources of noise are fan noise, jet noise, combustion noise, and turbine noise. Fan noise is usually dominant at low frequencies and during takeoff and landing, while jet noise is usually dominant at high frequencies and during cruise and climb. Combustion noise is usually broadband and intermittent, potentially amplified by resonance effects. Turbine noise is usually tonal and harmonic, potentially affected by shock waves and blade interactions. The characteristics of these noises depend on various factors such as fan speed, jet speed, fuel type, mixture, injection, ignition, chamber geometry, pressure, size, shape, number of blades, angle and velocity of the air/gas flow.

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Azzam Elzain

CEO & Co-founder | ESCWA Entrepreneurship Award 2023 | Celebrating Handmade Treasures | Entrepreneurship Mentor | Business Consultant | Capacity Building, Economic Development
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Designing aircraft engines to reduce noise pollution involves addressing various noise sources. Engineers focus on mitigating both aerodynamic and mechanical noise generated during flight. This includes optimizing the shape of engine components to minimize air turbulence, incorporating advanced materials for quieter mechanical operation, and implementing innovative technologies like serrated trailing edges on fan blades to reduce noise. By systematically targeting these noise sources, aircraft engine designers aim to create quieter and more environmentally friendly aviation solutions.

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2 Noise characteristics

Aircraft engine noise has several characteristics that make it distinctive and challenging to measure, model, and reduce. For instance, the directionality of the noise varies depending on the angle of observation relative to the engine axis and the aircraft orientation. Additionally, aircraft engine noise covers a wide range of frequencies, from infrasound to ultrasound, with most of the noise energy concentrated in the audible range. Moreover, aircraft engine noise can reach very high levels of sound pressure, with peak sound pressure levels exceeding 140 decibels. Such high levels of noise can cause physical damage to structures and human organs, as well as psychological stress and annoyance.

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3 Noise reduction

Aircraft engine noise can be reduced by applying various design strategies and technologies that target the different sources and characteristics of noise. For example, fan design can be optimized by altering blade geometry, number, spacing, sweep, fan speed, and diameter. This can reduce the fan tip speed and noise generated by the blade passing frequency. Additionally, a shroud or duct around the fan can improve aerodynamic performance and reduce noise radiation. Jet design can be modified by changing jet shape, speed, temperature, pressure, nozzle geometry, and configuration. Decreasing jet speed and increasing jet temperature and pressure can lower Mach number and turbulent kinetic energy to reduce jet noise. Adding chevrons, lobes, or slots to the nozzle enhances mixing and spreading of the jet flow to reduce noise. Combustion design controls fuel type, mixture, injection, ignition, chamber geometry, and pressure. Using lean or alternative fuels and adjusting fuel-air ratio and injection timing stabilizes flame and minimizes pressure fluctuations to reduce combustion noise. Liners, baffles, or dampers in the chamber absorb or dissipate acoustic energy to reduce combustion noise. Turbine design optimizes blade geometry, number, spacing, sweep, turbine speed, and diameter to lower blade passing frequency and shock wave formation. Placing a silencer or diffuser after the turbine expands and slows down gas flow to reduce turbine noise.

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Bisma Nasim

Avionics Systems Engineer | Engineering Manager | MEng, BE
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AI is revolutionizing the fight against aircraft engine noise and making the way for a quieter, more sustainable aviation future. Companies like Airbus is exploring the potential of AI-powered noise monitoring systems to dynamically adjust flight paths and minimize noise impact on communities. Rolls-Royce is developing AI algorithms to analyze sensor data and detect anomalies before they become noise problems, ensuring reduced noise emissions. Additionally, Boeing is utilizing AI to optimize the production of SAF, offering a cleaner option to traditional fuel and further reducing noise output. These ongoing initiatives highlight the immense potential of AI and data analytics to create a sustainable future.

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4 Noise mitigation

Besides reducing noise at the source, noise can also be mitigated by various measures that affect the propagation and perception of sound. Flight operations can be modified to reduce noise by adjusting the flight schedules, routes, and altitudes. Additionally, noise barriers such as walls, fences, berms, or vegetation can be installed to block or deflect the noise waves from reaching the receivers. Noise insulation can also be applied to improve the soundproofing of buildings exposed to noise. Lastly, noise compensation can be offered to provide financial or non-financial benefits to the affected communities and individuals. Each method has its own advantages and disadvantages; however, they all work towards mitigating noise in different ways.

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Daniel Neufeld
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Another side of noise control is also propeller synchronisation. If two propellers are rotating out of sync on a multi-engine aircraft, there is a persistent "droning" sound that can be a source of noise pollution, as well potentially causing negative effects on the flight crew (annoyance, disorientation etc...)

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Daniel Neufeld
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Another side of noise pollution I dealt with in a previous company was its effect on passengers. The operating fleet consisted of turbo propeller aircraft featuring the majority of the passengers behind the propeller (which is noticeably louder). We experimented with an experimental insulating material that would replace the fireproofing cushions in the sidewalls, and performed test flights with a decimeter to analyze the results.

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How can aircraft engines be designed to reduce noise pollution?

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1 Noise sources

2 Noise characteristics

3 Noise reduction

4 Noise mitigation

5 Here’s what else to consider

Aviation

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