What is the difference between acoustics and aeroacoustics?

 

The answer is…

Aeroacoustics is a subset of the field of acoustics. The end...

But before you go, it’s not quite so simple as it may seem, and it is much more interesting.

According to the Oxford English Dictionary, Acoustics is the branch of physics concerned with the properties of sound, and aeroacoustics is a subcategory. However, while aeroacoustics definitely falls under that umbrella, it also encompasses aerodynamics, which contributes to the conditions that cause the sound; and therefore, it gets a little complicated.

Noise in aeroacoustics involves both traditional aspects (like source path contributions, transmissivity, dampening, etc.) and aerodynamic contributions.

Aerodynamics is the study of how an object affects the medium (gasses, in this case) through which it moves or how the gasses or fluids move around the object. In day-to-day life for most, this is typically air.

And this is what makes aeroacoustics so interesting. It is the by-products of the aerodynamic forces themselves that make up much of the noise in aeroacoustics—in other words, it is the motion of the object through the medium itself that instigates the noise events in and around the object. For example, external air pressure fluctuation can cause a deflection in the surface material itself, turning it into a diaphragm, which if it’s thin enough, will generate noise. Or airflow across an edge creates vortices that can result in a whistling sound. This whistle can then be heard outside the structure and propagate inside—where the structure might dampen or amplify the sound, which involves the transmissivity of the materials.  Or it could result in purely structure-borne sound to be heard in odd places around the structure. This noise could then be masked by other noise depending on the various source path contributions.

And the key to understanding how to mitigate or entirely remove that noise is understanding aerodynamics and how that disruption in the airflow can be reduced—for example, by altering the structure to remove an edge or directing the flow such that the turbulence occurs at a different point in the flow. This is complicated enough when only dealing with a motor vehicle or aircraft noise, but altering the design can have a great effect on the lift and drag aspects of aerodynamics.

It is true that aeroacoustics is “just” a subset of acoustics, but it is becoming more and more intertwined with aerodynamics. And with the continued move towards greener energy sources and electric vehicles the relationship only gets stronger. Aircraft and vehicles have more stringent noise requirements, and in the case of the much quieter electric cars, any aeroacoustic noise is more prominent due to the lack of the engine and transmission masking noise. And often, solving that problem aerodynamically will impact the drag coefficient, decrease drag and improve battery life.

So, sure. Aeroacoustics is a subset—but so much more.

 
 

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Thursday, 2. December 2021

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