What's the connection?

They are not directly related, as such. An initial guess might involve how much ash plane engines can take. While that would be on the right track, it is not the connection in this particular case. 

The volcano

When the Icelandic volcano Eyjafjallajökull erupted in 2010, the massive ash cloud disrupted air travel across northern and western Europe due to fears of ash-fouled engine failures. The connection to engine noise testing comes in because of what  Rhine-area residents suddenly realized they could do for the first time in functional memory. Residents under the prime flight paths found that they could open their windows during the Easter-vacation peak travel time. With fewer aircraft, the noise dropped to a tolerable level.  This incident provided a scenario that highlighted the stressful effects of aircraft noise, especially in areas with large airports that serve as transportation hubs for logistics and travel.

While this stress is not a new concern, it is a growing problem. The efforts so far have led to tighter public standards concerning aircraft noise, and in fact, individual aircraft noise pollution was reduced by 88%[1] (ca. 30 dB). However, the reaction of the Rhine-area residents shows that the problem is far from solved. While individual aircraft may be less polluting, the cumulative effect of increasing traffic is very much continuing and growing, especially during peak holiday travel times.

The realities of environmental noise pollution have forced the aircraft industry to build quieter aircraft,  with improved engines and better mechanical and aerodynamic properties that produce less noise. And while the main source of aircraft noise is generated by the engine, it is not the only source, particularly at low altitudes where there is a considerable amount of aeroacoustic noise. The aerodynamic or aeroacoustic (for more on acoustics/aeroacoustics) noise from the airflow around the aircraft fuselage and control surfaces increases with velocity and also increases at low altitudes due to the density of the air. Typically, aerodynamic noise is generated when the airflow must pass around an object on the aircraft (for example, the wings or landing gear) and generate turbulence.

Deutsche Luft- und Raumfahrt

These noises must be ferreted out and eliminated in the design phase. Typically,  individual pieces or scaled models are tested in a wind tunnel to determine aerodynamic noise. At the Deutsche Luft- und Raumfahrt (DLR) Institute of Aerodynamics and Flow Technology in Göttingen, Germany, experiments are performed in wind tunnels and in flight. The DLR is a leading research institute in the field of aerodynamics and aeroacoustics of airplanes and aerothermodynamics of space vehicles, it is acting as a link between basic research at universities and industrial application. They examine how future aircraft and space transportation systems can be designed and operated more efficiently, ecologically, comfortably, economically, and securely.

In their aeroacoustic testing, they define a grid on the surface of the model that covers all possible source positions. Aerodynamic noise caused by the airflow over the model is characterized by different frequencies that are recorded by a two-dimensional microphone array, then in post-processing, the data is illustrated as a noise map showing the noise source distribution. This noise map shows where design improvements may be applied. The noise map is usually calculated for one frequency or frequency band and gives information about the source distribution in that frequency range.

The results can also be integrated over an area that is defined, for example, by a structural part of the model. The resulting spectrum gives an estimation of the spectral noise distribution for this part of the model. These noise maps enable the easy identification of dominant noise sources and are an important noise reduction tool. This analysis is applied to all grid points and thereby focuses successively on all possible sound sources on the model.

UTP: A new addition to DLR's testing arsenal

The microphone array installed in the wind tunnel consists of over 100 microphones that can be aligned in the specific order of the defined grid. At the same time, the DLR uses the microphones on the array in inflight tests. when the time came to renew their microphones, the DLR decided to use GRAS Ultra-thin Precision (UTP) 48LX-1 microphones, characterized by an extremely low-profile (1 mm high) and flat design, a frequency range up to 70 kHz and a faring enabling easy placement and repositioning. The microphone’s small profile and mounting options result in a negligible effect on measurements in the boundary layer (for more on the Boundary Layer) and can be easily placed in previously untenable locations, like glass. In addition, the mounting options provide both stability and compliance with the safety regulations in aircraft development. The performance capabilities of the UTP microphones, easier setup and relocation capabilities combined with the data quality needed by high-tech institutions offer new possibilities in recording aerodynamic noise.

The connection

Even with previous successes in aircraft noise reduction, the problem of aircraft noise remains. And while humans are resilient, the problems of noise are well highlighted when the noise stops and affected populations can once again enjoy the sunlight. On the other side of the scenario, air travel will most likely continue to increase, so continued efforts to minimize air-traffic noise must also increase. This means that institutions such as DLR must continue to research and find new areas where noise can be mitigated, or better, eliminated. And this, of course, also requires the continued advancement in the tools available to them, such as the UTP microphones and the measurement opportunities that new tools and techniques enable.

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[1] https://www.fluglärm-portal.de/laerm-vermeiden/?gclid=CjwKCAjw2rmWBhB4EiwAiJ0mtTbav5TkE40T-8ot-B6-YcX_CkoVIat_eQdjmbg4hPfIyPSjf8MuphoC4mIQAvD_BwE