‍DALLAS — Standing beneath a busy airport flight path these days, it's easy to overlook how noisy commercial aviation once was. Aircraft noise has been one of the environmental challenges of aviation for decades, not only for travelers, but more particularly for communities living near airports.

Noise affects public health, property values, and overall acceptance of air transport expansion. Over the past six decades, engineering innovation and regulatory pressure have converged to reduce the noise in the air significantly.

During the 1960s, early commercial jets, such as the Boeing 707 and Douglas DC-8, produced noise levels exceeding 105 decibels during takeoff. Present-day widebodies, such as the A350 and 787, are currently 50% quieter, with takeoff noise typically below 85 decibels. Even narrowbodies like the A320neo benefit from similar decreases.

These reductions are guided by global standards of noise, like ICAO Chapter 14, and supported by national airport regulations. Every decibel of progress is the result of decades of engineering: chevron nozzles, high-bypass turbofans, and aerodynamic refinements all contribute to reducing noise at the expense of efficiency.

While emissions today take center stage in discussions of sustainability, the quieting of aircraft is the most significant, if lesser-celebrated, success of aviation.

Understanding Aircraft Noise: Where Does It Come from?

One source alone does not make an aircraft's characteristic roar, but it is an aggregation of several mechanisms generating noise collectively. To fully comprehend how engineers succeeded in minimizing this effect, it's first necessary to understand where the noise originates in aircraft.

1. Jet Engine Noise

The majority of noise heard by passengers and observers during takeoff comes from the engines. Inside these complicated machines, three separate sources of noise are in operation:

• Core Noise: This is a result of the burning process within the engine core. Though required to produce thrust, controlled fuel and air explosion emits strong, high-pitched sound waves.
• Fan Noise: Especially audible at lower power settings such as taxiing or approach, fan noise is caused by the enormous blades in the front of the engine sucking in air. A lot of this air bypasses the engine core and passes around it.
• Jet Exhaust Mixing Noise: The most significant contributor during takeoff. It occurs when the high-speed exhaust gases from the core turbulently mix with the lower-speed bypass air and the ambient atmosphere to produce the characteristic jet roar.

2. Aerodynamic Noise

In addition to the engines, airflow past the aircraft structure is also a source of noise, especially during lower speeds when landing or taking off. Components like the landing gear, flaps, and slats disrupt airflow, creating turbulence and generating additional noise. Wingtip vortices are even included, though their contribution to noise is usually secondary compared to the engines.

3. Noise in Systems 

Aircraft are equipped with devices that emit sound, primarily during takeoff and landing. During engine shutdown, vital systems are powered by the Auxiliary Power Unit (APU), an external turbine engine located at the tail end of the fuselage. 'Packs,' also known as Environmental Control Systems (ECS), emit a constant whine to regulate cabin climate and pressure.

Engine Design Innovations towards Reducing Noise

While it's helpful to understand where noise comes from, it's the fixing of these through engineering innovation that makes real strides. The past three decades have witnessed significant technological breakthroughs in efforts to reduce the noise of commercial jets.

1. High Bypass Ratio Engines

Arguably, the most significant development has been the industry's adoption of high-bypass-ratio turbofans. Old jetliners, such as the Boeing 707, had engines with bypass ratios lower than 1:1. Newer airliners currently employ engines with bypass ratios greater than 10:1. Essentially, the larger amount of slower-moving bypass air envelops the hotter, faster core exhaust, damping the chaotic mixing process and significantly reducing noise.

Evidence of this development includes the evolution from GE90 on the Boeing 777 to GEnx on the 787, and the Rolls-Royce Trent XWB on the A350. Every advancing engine generation has a larger fan diameter and better noise qualities.

2. Acoustic Liners

Within the nacelles (engine enclosures), engineers have installed acoustic liners, most commonly in the form of honeycomb constructions. These specifically tuned surfaces of the liners soak up specific sound frequencies, most notably the fan blade sound that prevails during approach and taxiing. The CFM LEAP engines of the A320neo and 737 MAX families incorporate widespread application of these liners, making the aircraft quieter to operate.

3. Chevron Nozzles

Visually, one of the most dramatic sound-reducing innovations is the installation of chevron nozzles, which feature serrated rear edges on engine nacelles or exhaust cones. By allowing the bypass air and hot core exhaust to mix more harmoniously, the serrations reduce noise and turbulence.
These chevron designs are found in aircraft, including the Embraer E2 family, the Boeing 787, and the 737 MAX. Their effectiveness is most noticeable during takeoff, when they can cut background noise by several decibels without sacrificing engine efficiency.

 These advancements are only the beginning of a more comprehensive shift in aircraft noise control that strikes a balance between quiet operations and the efficiency requirements of modern commercial aviation.

Reducing Airframe Noise: Silencing the Aerodynamics

A large portion of the noise on approach originates from the aircraft itself, whereas engine noise predominates after takeoff. Although less significant than engine noise, airframe noise still contributes substantially to the overall noise environment surrounding airports. To combat this aerodynamic challenge, designers have devised several solutions.

These developments are merely the beginning of a broader revolution in airplane noise control that will strike a balance between the efficiency demands of contemporary commercial aviation and quieter flight.

1. Landing gear covers and fairings

Landing gear is one of the primary sources of aerodynamic noise. Low-frequency rumbling results as air travels over exposed struts, wheels, and hydraulic lines on approach. Current aircraft now feature aerodynamic fairings over these complex pieces of equipment to reduce this sound. A fine example is the Airbus A380, which features huge main landing gear sets with novel fairings.

These additions enhance airflow over the equipment without compromising braking and structural stability, resulting in significantly reduced noise during final approaches and descents.

2. Advanced Designs for Flaps and Slats

To create lift at lower speeds during descent, flaps and slats need to be extended. They extend into the air stream, creating more turbulence and increasing noise levels.

In response to this, manufacturers have introduced more advanced aerodynamic shapes as well as carefully designed gaps inside the systems. These minimize the size and energy of the vortices, constraining the high-frequency noise commonly associated with airplane descents. The smoother, more laminar airflow over these elongated surfaces results in quieter and improved aerodynamic performance.

3. Noise-Reducing Modifications to Existing Aircraft

In addition to novel aircraft design, retrofit programs offer airlines an economical means to reduce noise from their existing fleet. The installation of additions, such as Airbus Sharklets and Boeing Split Scimitar winglets, is primarily aimed at reducing fuel consumption by minimizing drag. Such wingtip devices also serve to streamline the flow and decrease aerodynamic noise during flight. Overall, these airframe technologies demonstrate the industry's commitment to quieter skies.

Noise Assessment, Control, and Adherence

Aircraft noise reduction is a primary worldwide responsibility that is closely regulated; it is not merely an engineering problem. To enable citizens to enjoy calmer skies, the aviation industry operates within strict frameworks, ranging from international standards to local airport regulations.

• ICAO Noise Certification Standards

Regulation of aircraft noise is primarily led by the International Civil Aviation Organization (ICAO). ICAO has developed increasingly demanding standards over the decades, from Chapter 2 in the 1970s to the current Chapter 14 requirements for new jetliners.

For manufacturers, Chapter 14 compliance involves conforming to strict limits on incoming noise during takeoff, sideline, and approach. Jets such as the Airbus A350 and Boeing 787 are built with future-proof standards in mind, consistently operating multiple decibels below minimum standards. Such global standards indicate quieter plane designs while harmonizing aviation expansion with neighborhood noise issues.

• Noise Contours Around Airports

Airports use Noise Contour Maps to monitor and forecast the geographic range of airplane noise. Maps are also used to determine which neighborhoods are most affected, informing both city planning and residential zoning ordinances.

Operational methods, such as Continuous Descent Approaches (CDAs), enable aircraft to descend more slowly, thereby decreasing engine power and aerodynamic noise during approach. Airports like Amsterdam Schiphol have pioneered the introduction of CDA, achieving staggering reductions in perceived city noise.

• Local Curfews and Restrictions on the Airport

Every airport has its own curfew and noise ordinances. London Heathrow (LHR) has strict nighttime curfews, whereas Frankfurt (FRA) prohibits scheduled takeoffs and landings between 11 p.m. and 5 a.m.

Incentive schemes also induce airlines to fly quieter fleets. Airports generally offer landing fee reductions or prime time slots for aircraft that are significantly quieter than required by standards, rewarding operators who want to reduce their environmental footprint.

Future Technologies: Towards Even Quieter Skies

Despite advances in technology, engineers continue to innovate aircraft noise reduction, especially with the advent of new propulsion technologies.

• Open Rotor Engines and Acoustic Challenges

Open-rotor engines, like those seen in tests by SAFRAN, hold out the potential for significant fuel savings. But exposed propeller-like blades bring new acoustic challenges. They aim to design blades and enclosures that can mitigate tonal noise while maintaining aerodynamic efficiency.

• Hybrid-Electric and Electric Aircraft

Electric propulsion has tremendous promise for quieter flying, particularly at low speeds. Designs such as Eviation's Alice and Rolls-Royce's electric demonstrators both promise to decrease cabin and community noise by removing sources of sound attributable to combustion. Scaling such designs up for large commercial applications, though, is in its preliminary stages of development.

• Urban Air Mobility (UAM) Noise Considerations

Drone taxis and electric vertical takeoff and landing (eVTOL) aircraft are nearing commercial viability, where noise issues become increasingly important. Urban areas can reflect sound, making quiet-sounding cars particularly disturbing in concentrated neighborhoods.

Groups like NASA (and its X-57 initiative) and firms like Joby Aviation and Lilium are investing significant research dollars in acoustics to ensure that urban air mobility aligns with community health.

As regulation becomes stricter and innovation gathers pace, the aviation sector remains firmly committed to providing quieter skies for generations to come.

Conclusion: Engineering the Quiet Future of Aviation

The path to quieter skies is a testament to aviation's most significant engineering milestones. Today's aircraft, such as the A350 and 787, are fundamentally quieter than jetliners from the early days, thanks to advances in engine design, aerodynamics, and noise control measures.

But development doesn't end with quieter takeoffs and landings. New issues arise as the sector adopts hybrid-electric aircraft and anticipates the emergence of urban air mobility (UAM). Such technologies reduce emissions and operational noise, but require additional research to address new sources of acoustic impact. 

Regulatory policies, such as ICAO Chapter 14, encourage manufacturers to continue developing designs, and airports adopt innovative operating methods to minimize community impact.

Ultimately, quieter skies are a continuous pursuit. With each decibel taken out, it is a validation of thoughtful design, diligent testing, and international cooperation. The future generation of quieter flight will be constructed by the relentless innovation that is redefining aviation today.