Measurement System for Aircraft Noise

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"The use of NI-DAQmx facilitates the use of different hardware models without software modifications. "

- Daniel Murmann, Anotec Consulting

The Challenge:
Measure aircraft noise at 12 microphones, distributed at both sides of a runway in a line with an approximate length of 1500 m. The microphones have to be synchronized in time, also with the on-board measurement system. A centralized control is required, but the runway may not be crossed with cables.

The Solution:
Develop a data acquisition system distributed over a central station and two remote stations, each with 4 channels and interconnected through a long range wireless network. Develop a Labview application for centralized control of all stations, with real-time monitoring of all channels.

Daniel Murmann - Anotec Consulting

In the framework of the European “IMAGINE” project an advanced aircraft noise prediction model has been developed. In order to validate this model, Anotec performed flight tests at Ocaña (Toledo-Spain) airfield. These measurements consisted of measuring noise from a highly instrumented aircraft under well controlled conditions, similar to those required for noise certification under ICAO standards.
 System design
The system design was conditioned by the following requirements:
 Both time and budget available for the flight tests were limited, thus requiring maximum efficiency (minimized flight hours with maximum guarantee of success)
The model validation required simultaneous measurement of the noise received at various distances from the aircraft, with a maximum of about 1200 m from the runway on one side and 500 m on the other side, making it impossible to use cables from a single point
The first condition required a centralized control, whilst the second indicated the need for a distributed configuration. The majority of existing products available on the market do not have the required functionality and those which comply are too expensive. Therefore it was decided to develop the system in-house with NI LabVIEW. The system developed combines both conditions satisfactorily.
The system comprises:
- 1 central station with an industrial PC with board NI PCI-4472 and 4 microphones, a GPS clock and a weather station (“CGS”)
- 2 remote stations with laptop with a NI USB-9233 module and 4 microphones (“RGS”)
All PCs used Microsoft Windows XP professional.
Both remote stations were connected with the central station through a long range wireless network. In this manner the need for cables over 1 km long was eliminated, reducing it to only 200 m for each station. The ICP supply provided by the NI boards supports this distance easily.
The GPS clock installed in CGS synchronizes the central PC, which then synchronizes the remote stations by means of NTP (Network Time Protocol). Together with the clock of the differential GPS installed in the aircraft, all systems are synchronized to within milliseconds.
The Application
The application developed is the same for all 3 stations, offering different modes for operation as CGS or RGS.
For each test run (overflight) the following measurement cycle is defined:
-       CGS→RGS: initiate measurement (run number, configuration, etc)
-       CGS→RGS: start measurement (start recording to hard disk (in ascii and wav files))
-       RGS→CGS: send third octave spectra and overall level (Leq) to central, each 500 ms
-       CGS→RGS: stop measurement (stop recording to hard disk)
-       RGS→CGS: send ascii files to central
For monitoring purposes the test director, located in CGS, has a screen which shows both spectra and overall level of all channels in real-time. This is important in order to identify the time at which the noise has dropped sufficiently to stop the measurement. In addition controls are available to indicate network status and signal quality (overload).
Data acquisition is performed through NI-DAQmx and with the LabVIEW Sound and Vibration Toolkit the signal of each channel is analyzed in third octave bands each 500 ms and overall level in dB(A). The use of NI-DAQmx facilitates the use of different hardware models without software modifications. The data acquisition at 24 bits provides a dynamic range of over 100 dB, sufficient to cover the whole range of noise levels to be measured, thus eliminating the need for additional gain settings.
Communication between the stations in monitoring mode (during the measurement) is performed with datasocket, since in this case a network oriented connection is not preferred. However, file transfer to CGS after the measurement is done through TCP/IP since in this case a secure communication, as provided by a network oriented connection, is of interest.

 Advantages of the developed system:
In the past Anotec has performed various flight tests, until now with a system, based on sound level meters and DAT recorders. With teh development of the new system, described above, the following advantages have been obtained:
-   reduction of the number of people required, due to the centralized control of all stations
-   higher signal quality, using high quality, 24 bits data acquisition boards
-   reduction of measurement time (flight hours), due to minimization of the run duration by means of real-time monitoring of all stations
-   scalability of the system, using stations according to need, without geographical limitations
-   risk reduction (less cables, less human errors, guaranteed signal quality due to real time monitoring)
-   reduction of the time required for final analysis, since all data is readily available on hard disk
-   low cost compared to similar noise measurement systems available on the market
-   application to noise certification tests according to ICAO without significant modifications


Author Information:
Daniel Murmann
Anotec Consulting

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