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One of the tasks of airport management is monitoring aircraft noise in the environment of airfield. At this moment the aircraft noise is calculated by using the flight path measured by radar and the estimated sound production of the aircraft (Fanamos).
Up to now no accurate measurements are possible to evaluate the calculated aircraft noise.
Other sources of noise, such as other aircraft, road traffic, stationary sources or reflections from ground or buildings, cannot be eliminated with the single microphone system.

The expectation is that the aircraft noise will be fully determined by measurements.

• a system with a single microphone, without any directivity
  
• a system with two microphones which calculates a projected angle
  
• a system with four microphones in a spatial configuration, which calculates the horizontal (azimuth) and vertical (elevation) angles, together with the relative and eventually absolute flight path and velocity 

	Fig. 1 shows a measurement with a single microphone. The sound strength is plotted vertically as a function of time.
Figure 1; Sound intensity as a function of time measured with a monitoring system with a single microphone.

By using two microphones, the source can also be discriminated on direction. Fig. 2 shows the result of a measurement with a system that has this directional capability.

Again the sound strength is plotted vertically and the time horizontally. Added is the angle between the direction, the sound origin and the microphone base. The largest trace, from 2 to 35 sec, is due to the aircraft flying by. Parallel to this, a second trace is visible, which is the ground reflection. Both have the same angle rate. In the time interval from 40 to 50 seconds, traces with a higher angle rate are present,which are caused by road traffic.

Figure 2. Sound intensity as a function of time and angle as measured with a monitoring system with two microphones.

By extending the system to four microphones arranged in a spatial configuration, the vertical and horizontal angles are determined. With the tracking capability of the system the relative path is real-time calculated from the acoustic data.

	Fig. 3 shows the results of such a calculation with the microphone signals simulated without noise. The sound strength is plotted vertically as a function of relative, retarded coordinates with a frequency of 25 Hz. The interval between the vertical lines is 0.4 s.
Figure 3. Sound intensity as a function of relative, retarded coordinates as measured with a monitoring system with four microphones. The signals are simulated.

The interval expressed in relative coordinates decreases due to the finite sound velocity. This effect, equivalent with the Doppler frequency shift, is expressed as a function of sound and aircraft velocities and the distance between the aircraft and the microphones.
By inversion of this expression, the aircraft position and the (absolute) scale are calculated.

	Fig. 4 pictures the result of such a calculation on the measured data of Fig. 3. The error in this case is smaller then one per thousand.
Figure 4. Fitting the measured tangent as a function of time for the aircraft velocity and distance parameters on the data of Fig. 3.

	Fig. 5 shows a field measurement with the path calculated accurately. By using directional measurements and position tracking, the amount of wind and environmental noise has decreased with at least 24 dB's.
Figure 5. Field measurement with a monitor system using four microphones.

Fig. 6 shows the result of the calculation of the aircraft velocity and scale. In case the horizontal distance to the microphones is known, for example during departure or landing of the aircraft, the error of the fitted parameters will be considerably smaller. Another possibility is by combining the data of two monitor systems, which reduces also the path error considerably, makes path measurements independent from radar or other information sources and relates the acoustic data immediately to the aircraft. By such an application a sound-landscape is generated, which makes correlation with complaints and health issues possible. The parameters can then be expressed in NaX curves.

Figure 6. The fitting of the measured tangent as a function of time for the aircraft velocity and distance on the data of Fig. 5.

	Fig. 7 shows soundtracks of two aircraft coming from the same runway, but with different destinations. Monitoring with a single microphone will give in this case the same Lden or Lnight value, even when the aircraft are measured at the same moment.
Figure 7. Soundtrack of two aircraft using the same runway but with different destinations.

 
Recapitulating, an effective monitoring system is developed which measures the sound intensity accurately as a function of the relative flight path. In addition the absolute path is calculated with an error, which depends on the flight path and the environmental conditions.

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