More air traffic requires more noise reduction, explains Lars Winberg
In the spring of 2010, Eyjafjallajökull (a 1,651m tall volcano on the southern coast of Iceland) erupted and spewed a vast amount of ash into the sky. The resulting ash cloud obscured vision and hung around in the atmosphere for six days. This caused enormous disruptions to air travel across northern and western Europe, with fears of the ash fouling engines and causing failures. To compound the issue, the disruptions coincided with the beginning of Easter holidays, where there was usually an increase in travel.
However, there is, as they say, a silver lining in every cloud. In those areas such as the greater Rhine-Main-Area, which is home to Frankfurt Airport, these volcanic eruptions eliminated more than the increased Easter-vacation traffic – usually the cause of an increased level of aviation noise. During this event, the environmental noises consisting of aircraft turbine sounds suddenly ceased and local residents living under the flight paths were suddenly able to do something unusual. They could open their windows during the day. 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.
Concern over the noise resulting from the growing demands of worldwide transportation of goods and passengers has led to stricter public standards concerning aircraft noise. In fact, since 1960 aircraft noise pollution for comparably sized aircraft has been reduced by about 30dB.[1] However, the reaction of the people mentioned above shows that there is a long way to go – especially with an increase in the volume of flights, larger aircraft and lager engines.
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.
Of course, the main source of aircraft noise is generated by the engine, but it is not the only source. There is a considerable amount of aeroacoustic noise as well. Aircraft produce noise from the airflow around the aircraft fuselage and control surfaces, and that noise 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.
During prototype design, individual pieces or scaled models are tested in a wind tunnel to determine the aerodynamic noise. Purposes of aeroacoustic measurements span from quantifying the sound power generated by flow over a structure, to sound-source localisation using an array, to quantifying the turbulent stresses to which a structure is subjected.
Specialised Testing
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. DLR is a leading research institute in the field of aerodynamics and aeroacoustics of aeroplanes and aerothermodynamics of space vehicles, it is acting as a link between basic research at the 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.
One prominent way to determine the position and the strength of sound sources radiated by an aircraft model in the wind tunnel are measurements with phased microphone arrays. This is done by using a large number of microphones to identify the sound sources by the different travel time from the sources to the different microphones and the phase difference.[2]
Aerodynamic noise caused by the air flow over the model is characterised 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 applicable. The noise map is usually calculated for one frequency or one frequency band and gives information about the source distribution in this 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 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.
For testing, DLR uses different arrays starting with 100 up to 500 or even 1,000 microphones. At the same time, DLR uses the microphones on the array in inflight tests. Due to the need to renew microphones, DLR decided to equip the array with GRAS Ultra-thin Precision (UTP) 48LX-1 microphones. This microphone is characterised by an extremely low-profile (1mm), flat design, a frequency range up to 70kHz and a fairing enabling easy placement and repositioning. The microphone’s small profile and mounting options result in a negligible effect on measurements in the boundary layer and it can be easily placed in previously untenable locations, such as on a window. 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 set-up and relocation capabilities combined with the data quality needed by high-tech institutions offer new possibilities in recording aerodynamic noise.
REFERENCES:
2. https://www.dlr.de/as/en/desktopdefault.aspx/tabid-183/251_read-2736/
