May 11, 2021 / Overture

The Science of Building Quieter Aircraft: Acoustics 101

Aircraft today are dramatically quieter than 50 years ago, but they still produce considerable noise. Here’s how that’s changing.

When you think of airports, what sound comes to mind? Whether it’s roaring engines, tires touching down or aircraft gear raising, each is considered aircraft environmental noise.

It might surprise you that today, aircraft are dramatically quieter than 50 years ago. A number of advancements have each helped to reduce aircraft noise, including:

  1. The introduction of modern turbojets and turbofan engines
  2. The retirement of older and noisier aircraft
  3. New, enhanced noise regulations
  4. A range of new technologies

But, what factors make aircraft noisy in the first place? We asked Boom’s acoustic engineering team to shed light on the science of sound and advancements in reducing aircraft noise.

First, what is acoustics?

Acoustics is the study of mechanical wave behavior in fluid and solid media. When waves propagate — or broadcast — through a gas like air and people hear those waves, we think of it as “noise” or “sound.” When those waves propagate on solids like an aircraft component or engine component, we think of it as “vibration.”

What is noise?

Noise is an unwanted sound. The perception of sound determines whether it is considered desirable or displeasing. Many factors play a role in the distinction between sound versus noise, including amplitude, duration and time, as well as the place we experience it and the frequency content of the sound. For example, listening to music at a high volume concert could be considered appealing, but hearing that same music in the middle of the night when trying to sleep would be a nuisance.

How do acoustic engineers study noise?

Acoustic engineers identify what makes noise, how it travels, what makes it quieter or louder, and how people and animals experience it from various locations. They measure and evaluate noise based on three perspectives:

  1. The source: what makes the noise
  2. Engineers study the physics behind noise generating mechanisms.
  3. The receiver: who experiences the noise
  4. Engineers study and describe how people react to or perceive a noise and typically relate it to both qualitative and quantitative measures
  5. The path between the source and the receiver
  6. Engineers help predict or test the possible paths that sound and vibration travel
Acoustics engineers use spectrogram charts (like the one above) to look at the time history of the frequency content of an acoustic source or distribution of acoustic sources. This simulated example depicts an aircraft flying at subsonic speed several hundred feet over a microphone mounted at 1.2 meters above soft ground.

What is aircraft environmental noise?

Aircraft environmental noise is the noise generated by aircraft during flight, as well as during takeoff, landing and taxiing on the runway. It’s primarily generated by engines but also includes airframe noise sources like landing gear.

What affects how we experience noise from airplanes?

In addition to the type of aircraft and its source characteristics, the way we experience aircraft noise can vary depending on objective elements like:

  • Atmospheric conditions
  • Relative proximity of the aircraft to the listener
  • The local terrain between the aircraft and the listener
  • Whether the person is indoors or outdoors
  • Whether aircraft are arriving or departing

How does the industry study aircraft noise?

Since the 1960s and 1970s, manufacturers, research institutions, and government agencies have made huge investments in computational resources and testing facilities to help advance our understanding of aircraft noise. There is also ongoing research that began years ago to study the human response to aircraft noise. This increase in research isn’t just limited to subsonic noise. It also includes supersonic noise and sonic boom measurements and predictions.

Are airplanes quieter today than in the 1970s?

New aircraft generate considerably less noise than their predecessors. Aircraft engines today are significantly quieter, thanks to dramatic advancements in aircraft engine design. Examples of engine design improvements that reduce noise include high bypass ratio turbofan, acoustic liner technology and the use of mixers and chevrons. Changes in aircraft operations, including where aircraft are routed, and improvements in noise abatement procedures, also play a role in reducing noise.

Apart from its engines, how else does an aircraft make noise?

Aircraft also produce noise from the airframe, specifically from the landing gear and high lift devices, as well as the cavities that remain open during landing. These categories of noise sources are the same for both subsonic and supersonic aircraft. The most notable noise source that is unique to supersonic aircraft is the sonic boom, and that doesn’t happen during takeoff and landing at the airport.

While there aren’t any supersonic commercial airliners in operation today, it’s worth noting that Concorde only reached supersonic speeds at a specific altitude over the ocean.

Acoustics engineers use notional illustrations like the one above to vary several key takeoff parameters from possible aircraft design configurations to determine which combinations result in more or less favorable noise reduction. This type of data brings to light the various changes or trades made by a design team due to noise considerations. Engineers calculate numerous instances of possible design aspects to understand the noise implications for their design decisions.

Is the sonic boom regulated?

Currently, there are no international standards for how loud a sonic boom can be over land or oceans.

In the U.S., Federal Aviation Administration regulations do not permit civil supersonic flight — and the subsequent sonic boom — over land. Civil supersonic flight is only allowed over the ocean. The European Aviation Safety Agency has the same restrictions.

There is a process for obtaining permission for civil supersonic flight over land in the U.S for purposes such as testing. With appropriate approvals, supersonic flight is permitted in special restricted flight corridors such as California’s Edwards Air Force Base and Mojave Air & Space Port.

What common challenges do acoustic engineers face in aircraft design?

The most common challenge for acoustic engineers is understanding the necessary requirements for other sections of the aircraft, such as environmental control systems and propulsion systems, and how acoustics impacts all decisions during aircraft design.

Ultimately, the best acoustic outcomes are achieved by good design and incorporating requirements very early on in the design process. Trying to achieve low noise — and significant reductions in aircraft environmental noise — as an afterthought is typically not as effective as making noise a priority from the start.

The closer you are to treating noise at the source, the more effective the solution will be.

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