19 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 THE USE OF SOUND INTENSITY FOR THE DETERMINATION OF THE ACOUSTIC TRAJECTORY OF AIRCRAFTS PACS: 43.50.Lj Moschioni, Giovanni; Saggin, Bortolino; Tarabini, Marco. Politecnico di Milano, Polo regionale di Lecco, 23900, Italy; [email protected]ABSTRACT The use of a three dimensional sound intensity probe in an aeroportual environment allows the evaluation of the “acoustic trajectory” of aircrafts. The acoustic trajectory is given by the group of points that the 3D sound intensity vector assumes during a certain event. In free-field conditions this parameter instantly identifies the position from where the sound energy comes. Experiments performed close to the Milano Malpensa airport have shown that the acoustic trajectory is nearly independent from the aircraft type and from the take-off position; consequently it can be an effective parameter for the optimization of the noise control actions or for the automatic classification of the noisy events. INTRODUCTION Current standards for the determination of the acoustic impact of airports are entirely based on sound pressure measurements. Because of the isotropic (and scalar) nature of this quantity, standards have two main intrinsic limitations: ⎯ any indication derived from sound pressure cannot be used in order to separate the contribution of different noise sources with similar spectra; ⎯ in presence of reflections the use of sound pressure may overestimate the contribution of the noise source. Consequently, if at the measurement position the influence of other noise sources is noteworthy (or if the influence of reflections is not negligible), the pressure measurement may overestimate the contribution of the airport on the surroundings. More accurate analyses can be performed leveraging on the directional information provided by the sound intensity vector [1]. Sound intensity indicates the amount and the direction of the acoustic energy flow, and consequently can be used to determine the directivity patterns of the acoustic field at a specific position. If the approximation of the monopole source can be used, in absence of obstacles or reflections the sound intensity vector provides indications for the identification of the noise source position. This paper describes the results of a measurement campaign performed in the surroundings of the Milano Malpensa airport; aim of the experiments was the investigation of the effectiveness of sound intensity in order to characterize the acoustic field close to an airport. The main idea of the proposed method is to separate the contribution of the different sound sources depending on the direction of the acoustic power flow at the – properly chosen – measurement point. In the following we will describe how a quantity, derived from the 3D sound intensity, can be used to identify the contribution of different noise sources in the aeroportual environment. EXPERIMENTAL SETUP As already mentioned, experiments have been carried out close to the Milan Malpensa airport. The measurement site is located in a strategic position, being exposed to the noise of the terminal, of the airplanes taking off from the runways and to other environmental sources. For a correct monitoring of sound intensity, the choice of the measurement point is crucial: this aspect has been addressed in [2] and consequently will be only summarized in this paper. When a generic sound source uniformly spreads energy towards a surrounding space free from obstacles, the sound propagates along radial paths; consequently sound intensity vector at a specific position identifies the source position. Spherical emission can be rarely found in practice, mainly because the propagation on spherical surfaces is usually prevented by the existence of the ground and consequently the hemispherical model is generally a more realistic
Transcript
19th INTERNATIONAL CONGRESS ON ACOUSTICS
MADRID, 2-7 SEPTEMBER 2007
THE USE OF SOUND INTENSITY FOR THE DETERMINATION OF THE ACOUSTIC TRAJECTORY OF AIRCRAFTS
PACS: 43.50.Lj Moschioni, Giovanni; Saggin, Bortolino; Tarabini, Marco. Politecnico di Milano, Polo regionale di Lecco, 23900, Italy; [email protected] ABSTRACT The use of a three dimensional sound intensity probe in an aeroportual environment allows the evaluation of the “acoustic trajectory” of aircrafts. The acoustic trajectory is given by the group of points that the 3D sound intensity vector assumes during a certain event. In free-field conditions this parameter instantly identifies the position from where the sound energy comes. Experiments performed close to the Milano Malpensa airport have shown that the acoustic trajectory is nearly independent from the aircraft type and from the take-off position; consequently it can be an effective parameter for the optimization of the noise control actions or for the automatic classification of the noisy events. INTRODUCTION Current standards for the determination of the acoustic impact of airports are entirely based on sound pressure measurements. Because of the isotropic (and scalar) nature of this quantity, standards have two main intrinsic limitations: ⎯ any indication derived from sound pressure cannot be used in order to separate the
contribution of different noise sources with similar spectra; ⎯ in presence of reflections the use of sound pressure may overestimate the contribution of
the noise source. Consequently, if at the measurement position the influence of other noise sources is noteworthy (or if the influence of reflections is not negligible), the pressure measurement may overestimate the contribution of the airport on the surroundings. More accurate analyses can be performed leveraging on the directional information provided by the sound intensity vector [1]. Sound intensity indicates the amount and the direction of the acoustic energy flow, and consequently can be used to determine the directivity patterns of the acoustic field at a specific position. If the approximation of the monopole source can be used, in absence of obstacles or reflections the sound intensity vector provides indications for the identification of the noise source position. This paper describes the results of a measurement campaign performed in the surroundings of the Milano Malpensa airport; aim of the experiments was the investigation of the effectiveness of sound intensity in order to characterize the acoustic field close to an airport. The main idea of the proposed method is to separate the contribution of the different sound sources depending on the direction of the acoustic power flow at the – properly chosen – measurement point. In the following we will describe how a quantity, derived from the 3D sound intensity, can be used to identify the contribution of different noise sources in the aeroportual environment. EXPERIMENTAL SETUP As already mentioned, experiments have been carried out close to the Milan Malpensa airport. The measurement site is located in a strategic position, being exposed to the noise of the terminal, of the airplanes taking off from the runways and to other environmental sources. For a correct monitoring of sound intensity, the choice of the measurement point is crucial: this aspect has been addressed in [2] and consequently will be only summarized in this paper. When a generic sound source uniformly spreads energy towards a surrounding space free from obstacles, the sound propagates along radial paths; consequently sound intensity vector at a specific position identifies the source position. Spherical emission can be rarely found in practice, mainly because the propagation on spherical surfaces is usually prevented by the existence of the ground and consequently the hemispherical model is generally a more realistic
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approximation. In the hemispherical model a point source leans on a reflective plane and irradiates uniformly in the surrounding space. The energy that the source emits toward the surface is reflected and is added to the energy that the source “naturally” spreads upwards. Since the reflectivity of the ground is hardly addressable a priori, as evidenced in [2] data concerning the vertical plane cannot be considered as fully representative of the noise source. In addition, sound is bent in presence of wind and temperature gradients, that are always present close to the ground [3], [4], [5]. The chosen measurement point is illustrated in Figure 1: the 3D sound intensity probe has been placed on a 9 m tall pylon in a position that minimizes the reflections on the horizontal plane (there are no obstacles in a range of more than 30 m). It has been shown [2] that reflections in the vertical plane are acceptable above 500 Hz.
Terminal
RunwayHighway
Measurement Point
Runway
Figure 1.- Positioning of the experimental site and of the intensity probe The measurement chain we have selected to investigate the usefulness of sound intensity is set-up with a three dimensional sound intensity probe set-up with three mutually perpendicular p-p probes [6]. The probe bandwidth is 2500 Hz [7] . Data acquisition was performed by two personal computers equipped with properly designed virtual instruments: one instrument measures sound intensity and the other one measures sound pressure. The measurement system accomplish IEC 61672 class 1 requirements. Intensity measurements are stored only if a triggering condition is verified, in order to reduce the amount of data. THE ACOUSTIC TRAJECTORY Because of its definition, the sound intensity vector points towards a direction that is opposite with respect of the noise source position. As an example, if the sound source is on the left of the measurement point, the intensity vector points to the right and vice-versa. Consequently, the opposite value of sound intensity (i.e. the vector -Ix, -Iy -Iz) is always pointing toward the noise source; the acoustic trajectory (AT) is defined as the group of points that this vector assumes during a certain event. Because of the large variations of the sound intensity magnitude, the AT has been analyzed in decibels, according to the acoustics customs (reference 10-12 W/m2). Obviously the acoustic trajectory can be referred to the overall sound intensity level (i.e. in the frequency range measurable with the 3D probe) or to different frequency ranges (i.e. octaves or thirds of octaves). Whenever a source is steady in power and position, the intensity vector is fixed and consequently the acoustic trajectory is a point. In the generic situation of moving sources with changing power emission, the AT assumes different paths depending on the actual source trajectory, on its directional characteristics and on the velocity with respect to the measurement point. Of particular interest is the acoustic trajectory of airplanes when taking off: a typical example of the AT in the frequency range of 500 – 2500 Hz is shown in Figure 2, that also shows the location of terminal and runway. Data will be initially presented in the horizontal plane since they can be easily understood and because of the larger variability of the vertical component that will be addressed in the second part of this paper. Since the almost steady background noise comes from the terminal, the vector initially points towards the right of the figure. When the airplane takes off, the vector magnitude increases from 40 to 70 dB and the vector turns counterclockwise ; when the airplane is going away, the vector points towards the left of the figure and its magnitude starts decreasing. Finally, when the airplane is very far, the terminal background
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noise becomes more important with respect to the aircraft one, and thus the vector goes back to the original position.
0
20 dB
40 dB
60 dB
80 dB
Term
inal
Runway
Figure 2.- Acoustic Trajectory of a single airplane taking off. Background noise from the
terminal. The main problem concerning this representation is that there is no trace of the time duration of the event. In order to overcome this limitation, it is possible to plot points at specific time interval. The distance between points indicates the velocity with which data have changed. As an example, the above graph can be represented with points representing the mean sound intensity vector over a period of 2 seconds. One can notice that points are widely spaced for angles close to 90 °, since the direction of the energy flow rapidly changes during the take off. Conversely, when the angle of incidence is close to 180°, points are close each other, outlining that the direction of the AT varying slowly. Such a behaviour is due to the airplane trajectory, that once left the runway turns towards the left.
0
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40 dB
60 dB
80 dB
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Runway
Figure 3.- Acoustic Trajectory of the same event of Figure 2, one point each 2 seconds.
STATISTICAL ANALYSIS The acoustic trajectory would be a meaningless factor if different aircrafts would generate different trajectories. Investigations performed on this issue have shown that the acoustic trajectory is a repeatable parameter, as evidenced in Figure 4. The plot shows 40 acoustic trajectories (representative of 40 take offs recorded in April of 2006) measured the same day using the frequency band 500-2500 Hz. Figure shows that all the take offs have a similar behavior, both in 2D (only the horizontal plane) and in 3D.
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0
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40 dB
60 dB
80 dB
Term
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Runway
Figure 4.- Acoustic trajectory in two and three dimensions of 40 different take offs. One point
each two seconds. Data dispersion is slightly larger at the end of the noisy event; such a dispersion can be endorsed to the different routes that the aircrafts have after leaving the runway and to the different directional characteristics of airplanes meant as noise sources. Data can be summarized in terms of mean value and standard deviation of the measured AT; results are shown in Figure 5.
Figure 5.- Mean acoustic trajectory of 40 take offs; red lines indicate the ± standard deviation
interval. Mean sound intensity values range from 60 to 75 dB while standard deviations are close to 5 dB when the passes “in front” of the experimental site (angles close to 90°) and reaches 9 dB for angles close to 180°. Additional investigations have been carried out to investigate the behaviour of the acoustic trajectory in the vertical plane. Such a parameter suffers a larger variability when compared to the horizontal trajectory for two main reasons: • the vertical component of sound intensity depends on many factors (temperature gradients,
wind, etc) that change also during the day; • the point where the aircraft leaves the runway changes at each takeoff and the vertical
sound intensity component is heavily affected by this factor; The AT in the vertical plane have been summarized in terms of vertical component of sound intensity as a function of the horizontal angle of incidence. When the horizontal angle of incidence is close to zero (i.e. the aircraft is still on the runway) the vertical component is close to zero. Once that the aircraft leaves the ground, the vertical sound intensity magnitude increases (normally from zero to the maximum at angles close to 90°). Larger horizontal incidence angles indicate that the airplane is far from the runway so, vertical sound intensity decreases for two reasons: • the sound intensity decreases with the source distance, • the horizontal component of the aircraft velocity is much larger than the vertical one,
consequently when the airplane is far the sound arrives from a nearly horizontal direction.
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The above aspects are particularly evident by observing Figure 6, that shows the vertical sound intensity component as a function of the horizontal angle of incidence.
Figure 6.- Vertical acoustic trajectory: on the left 40 independent takeoffs, one point each two seconds; on the right mean and standard deviation of the 40 events with a resolution of 15°.
The plots clearly show that data dispersion is larger than the one in the horizontal plane. In particular, standard deviations range from 2 dB for horizontal angles close to 0° to 11 dB for angles close to 120°. One must notice that the standard deviation is 11 dB over a mean value of 15 dB, while on the horizontal plane the standard deviation was 9 dB over a mean value of 70 dB. COMPARISON BETWEEN DIFFERENT MEASUREMENT SESSIONS Data repeatability has been investigated by analyzing different measurement sessions (one in January, the one presented in April and another one in June. Experimental results have shown that in the horizontal plane differences between the mean values are at most 4 dB. Standard deviations are nearly independent from the chosen experimental session. In the vertical planes, conversely, data means have a larger variability: differences can be endorsed to the previously evidenced reasons, i.e. to the presence of wind and temperature gradients and to changes in the ground reflectivity. On the contrary, standard deviations are still independent on the experimental sessions. Because of these reasons, the vertical component of sound intensity cannot be used to discriminate the takeoffs from other noisy events. Comparison between the different experimental sessions are shown in Figure 7.
Figure 7.- Comparison between different experimental sessions: January (blue) April (red) and
June (green). The repeatability of the acoustic trajectory in the horizontal plane also suggests the possibility of focusing the active and passive noise abatement techniques on those directions where the acoustic trajectories are larger. Parallel analyses [6] have also shown that the directional behaviour is a function of frequency: in particular, low frequencies are more important for angles close to 180°, while high frequencies are relevant close to 90°. This suggests the possibility of optimizing the position of active noise control systems (whose efficiency is satisfying at low frequencies) and passive techniques (that are more efficient at high frequencies).
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CONCLUSIONS Results of the measurement campaign have shown that the acoustic trajectory in the horizontal plane can be a meaningful parameter in order to identify the noisy events due to the aircraft takeoffs. This parameter is much more selective with respect to any other measurement based on the sound pressure level and can help determining the effective contribution of the aircrafts noise to the aeroportual environment. The main limitation of this method concerns the transducer location, since the hypothesis of free field propagation is crucial. It has also been shown that, at the chosen measurement position, the vertical component of sound intensity is averagely smaller with respect to the horizontal one but suffers from a larger variability. These preliminary results have shown that the Acoustic Trajectory seems to be a promising technique to automatically identify the takeoffs from other noisy events. In addition, data repeatability suggests the possibility of using the Acoustic Trajectory (or other quantities derived from 3D sound intensity) for the optimization of the installation of noise barriers. Bibliography [1] F. J. Fahy, Sound Intensity, 2nd ed. (Spon, London, 1995). [2] G. Moschioni, B. Saggin, M. Tarabini, “Use of intensimetric techniques for measurements of airport
noise”, INTER-NOISE 2007, 28-31 August 2007, Istanbul, Turkey. [3] Keith Attenborough, “Sound Propagation close to the Ground”, Ann. Rev. Fluid. Mech. 2002.34:51-
82. [4] Embleton TFW, “Tutorial on sound propagation outdoors” Journal of Acoustic Society of America
100 31–48, 1996 [5] Sutherland LC, Daigle GA. “Atmospheric sound propagation” In Handbook of Acoustics, ed. MJ
Crocker, pp. 305–29. New York: Wiley, 1998; [6] G. Moschioni, B. Saggin, M. Tarabini, “3D Sound Intensity Measurements: Accuracy
Enhancements with Virtual Instrument Based Technology”, VECIMS 2005 – IEEE International Conference on Virtual Environments, Human-Computer Interfaces, and Measurement Systems Giardini Naxos, Itay, 18-20 July 2005.
[7] International Organization for Standardization, “Guide to the expression of un-certainty in measurement (GUM)”, Geneve, 1995.
[8] G. Moschioni, B. Saggin, M. Tarabini, “Contribution of airports to noise in surrounding environment; identification and measurement of noise sources”, INTER-NOISE 2007, 28-31 August 2007, Istanbul, Turkey.
