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The autonomous assessment of aircraft sound power and position

Final Report Summary - SOUNDTRACK (The autonomous assessment of aircraft sound power and position)

The SOUNDTRACK project aimed at improving the available tools for airport noise assessment and management. These tools are prediction models and airport noise monitoring systems. The idea is to help bridge the gap between the current capabilities of such tools and the arising needs and requirements of the airport noise managers. The current features of prediction models and airport noise monitoring systems allow to predict and measure respectively airport long term average noise levels. However, it is necessary to be able to predict and measure the acoustic impact of individual flights to design noise abatement operational procedures affecting not only flight paths but also the aircraft flight configuration (i.e. slats, flaps, landing gear) and power settings.

At that aim, within the project SOUNDTRACK a new concept of aircraft noise monitoring terminal (NMT) has been developed and experimentally tested. This NMT provides background and ground reflected noise attenuation with respect to a conventional NMT integrated by a single microphone; automatically identifies aircraft sound events based on the direction of arrival (DOA) of the sound rather than on the sound pressure level or radar data; provides an indicator of the quality of the sound pressure level measurement, i.e. if it is possibly disturbed by other non targeted sources. This NMT consists of an array of nine microphones distributed in a 3D geometry with a diameter of about 1 m. The placement of the compact array must guarantee that all ground borne noise and ground reflections of the aircraft sound impinge on it from below, e.g. a roof top terrace.
The NMT conceptually consists of three main modules: the DOA estimation module, the directional monitoring module and the event identification module. The output of the DOA estimation module is a time series of aircraft noise DOA estimates. A theoretical description and evaluation of this module has been published in: M. Genescà, U. P. Svensson, G. Taraldsen, Estimation of aircraft sound direction of arrival using directional-microphone arrays, Acta Acustica united with Acustica 101 (1) (2015) 113–123.
Also, an experimental assessment of the performance of this module has been published in: M. Genescà, U. P. Svensson, G. Taraldsen, Estimation of aircraft angular coordinates using a directional-microphone array - an experimental study, Journal of the Acoustical Society of America 137 (4) (2015) 1914-1922.

The output of the directional monitoring module is a time series equivalent con-tinuous sound pressure level measurements. In the presence of a target aircraft, the influence of the ground borne sources and ground reflections is reduced in comparison with a single microphone measurement. The output of the event identification module is a time stamp marking the beginning and the end of each aircraft noise event along with a flag indicating the quality of the measurement. A publication describing and testing these two modules will be shortly submitted to a peer-reviewed journal.
The technical and socio-economic impacts of the project are the following:
• The data provided by this advanced NMT could be the base for the cal-culation of reliable aircraft spectral sound power data of individual flights which is required to extend the capabilities of the prediction models beyond the calculation of long term average. Current NMT do not provide appropriate data due to the influence of the background noise of the measurement site. If at least two of these NMT are used, the position of the aircraft could be estimated by triangulation. Once the aircraft position and the associated sound pressure levels at a set of NMT are available, the aircraft sound power spectral data could be calculated back propagating to the source. This is relevant to developers of aircraft noise prediction models and policy makers since it provides a new source of valuable information.
• This NMT empowers airports without radar and external (to the airport) entities or administrations to monitor the airport noise. This is because the NMT estimates DOA of the aircraft sound and uses this data for automatic aircraft event detection purposes so that non-acoustical data such as radar or flight plan information is not required anymore. This empowers civil society to make an independent assessment of the noise situation around the airports.
• This NMT could be used to penalize an airline when a particular flight exceeds the legal noise limits thanks to the reduction of the background noise influence, and also to the information provided on the quality of the measurement. This gives policy makers a new tool to describe noise limit enforcement strategies.
• This NMT could help to better understand the relationship between aircraft noise and annoyance in high background noise areas. In such locations background noise masks the aircraft noise and often the acoustic impact of the airport is underestimated because the aircraft noise events are not detected. As a consequence, the measured data does not correlate with the level of an-noyance of the residents. This NMT bases the aircraft noise events detection on the direction of arrival (DOA) of the sound and not on the sound pressure level and therefore can provide a more realistic assessment of the aircraft noise contribution in such areas. This would help researchers, airport noise managers and administrations to better understand the complaints of the civil society affected by aircraft noise.

Current contact details:

Peter Svensson (Scientist in charge)
Department of Electronics and Telecommunications
NTNU – Norwegian University of Science and Technology
NO-7491 Trondheim
+47 73 59 05 46

Meritxell Genescà Francitorra (Researcher)
Acoustics/Noise Control
Überlandstrasse 129
CH-8600 Dübendorf
+41 58 765 6036