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Predicting noise from aircraft operated on the ground
Applied Acoustics 64 (2003) 941–953 www.elsevier.com/locate/apacoust
Predicting noise from aircraft operated on the ground Oleksander Zaporozhetsa, Vadim Tokareva, Keith Attenboroughb,* a
National Aviation University (NAU), 1, Cosmonaut Komarov Prospect, Kyiv 03058, Ukraine b Department of Engineering, University of Hull, Hull, HU6 7RX, UK Received 1 March 2003; received in revised form 7 April 2003; accepted 14 April 2003
Abstract Averaged data for sound levels due to aircraft engine testing out to a distance of 3 km obtained under low wind speed conditions are presented. Predictions of standard analytical approximations are compared with these averaged data. The measured average sound levels are shown to be consistent with the predicted influence of ground effect including impedance discontinuities. There is a noticeable influence of directionality also. # 2003 Elsevier Ltd. All rights reserved. Keywords: Aircraft noise; Engine test; Ground effect
1. Introduction Although air-to-ground propagation is a very important to the impact of aircraft noise, there is interest also in ground-to-ground propagation. As the name suggests, ground-to-ground propagation is relevant to the assessment of noise from aircraft on the ground, for example during engine run-ups or as a result of taxiing. In these examples, the height of noise source is simply the height of engine installation on the aircraft. Ground-to-ground propagation is relevant also to predicting noise from aircraft engine testing facilities, for which the source height may be between 4 and 5 m. It is to be expected that ground reflection interference and diffraction effects are more evident in ground-to-ground propagation than for air-to-ground propagation [1–7]. * Corresponding author. Tel.: +44-1482-466-600; fax: +44-1482-466-600. E-mail address: [email protected] (K. Attenborough). 0003-682X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0003-682X(03)00064-1
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There are several empirical schemes for predicting ground effect for aircraft sources near to the ground. ISO 9613/2 [8] provides a general calculation method. The SAE 1751 standard [9] predicts the ‘lateral attenuation’ used widely in aircraft noise calculations (see Fig. 1), for example in FAA INM (both version 5.2 and the latest version 6.0) and in the current Ukrainian method [10] (realized in software ISOBELL’a). Another well-known approximation is contained in the UK method for aircraft noise assessment [11]. Results of calculations using the SAE/ICAO and UK methods along the sound propagation path, at horizontal ranges between 0 and 600 m, and for grazing angles between 0 and 15 differ by between 3 and +1.5 dB. All of the prediction schemes are based on data averaged over a great number of sources and ambient conditions. Some of them are recommended for use with particular noise indices. For example, the SAE/ICAO approximation is used for both SEL and EPNL calculations without any reference to the particular source spectrum and directivity pattern. This is in spite of the fact that each index is defined by a different frequency weighting function. Moreover, the type of local ground cover is neglected. Strong ground effects have been observed in relatively-short range acoustic measurements around engine testing facilities. In Fig. 2, data [12] are compared with predictions for a monopole noise source over hard ground [2,13]. Clearly these data are consistent with predictions for ground effect over relatively hard ground.
2. Long range data from engine testing Many noise measurements have been made in the former Soviet Union during aircraft engine run-ups with the aim of defining noise contours in the vicinity of airports. Data are available for a large number of aircraft types. The directivity results have been used for flyover level assessment. Noise contours (directivity patterns) have been obtained for a range of power settings during several summer days under near to calm weather conditions (wind speed < 5 m/s, temperature between 20
Fig. 1. Predictions of lateral attenuation as a function of grazing angle according to SAE/ICAO.
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Fig. 2. Comparison between predictions and measurements [12] at an engine testing facility with a concrete ground surface. The receiver height is 1.5 m, the assumed point source height is 4.5 m and the horizontal distance between the engine and microphone is 50 m. The prediction has assumed an impedance for concrete given by a semi-empirical one-parameter model [13] with effective flow resistivity =20 000 kPa s m2.
and 25 C). That these data are consistent with nearly acoustically-neutral conditions is demonstrated later. Although the measurements were intended only for assessing overall sound levels, the results are useful for a preliminary analysis of ground effect on noise from aircraft engine run-ups. Fig. 3 shows A-weighted sound pressure levels as a function of distance (on a logarithmic axis) and direction from a low by-pass engine installed on the Il’ushin-86 aircraft, obtained at different times but for the same power setting and ambient conditions. Between 7 and 10 measurements were made at every point under consideration (in accordance with ICAO Annex 16 requirements) and the results have been averaged to obtain the contours for each type of aircraft investigated. These data suggest that the directivity pattern is an important characteristic of aircraft engine noise and that it can have a significant influence on the overall sound pressure. Fig. 4(a) shows the variation in spectra with direction observed in the IL86 run up tests at a reference distance of 1 m. Fig. 4(b) shows computations of overall A-weighted levels LA, PNL and unweighted levels L as a function of direction, normalised to a distance of 1 m.
3. Comparisons of predictions with data The data shown in Figs. 3 and 4 may be compared with various predictions for ground effect. Fig. 5 presents the data for overall A-weighted levels due to an IL-86 aircraft engine in the direction of maximum jet noise generation ( ffi 140 ). As in Fig. 3, each data point is an average of between 7 and 10 measurements. Also shown are predictions for levels due to a point source with the same sound power spectrum allowing for wave front spreading (Div), for spreading and absorption (Div+abs)
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Fig. 3. Noise directivity patterns around an IL-86 aircraft at ranges up to 3 km.
and for various sorts of ground cover (concrete, soil and grass). The ground effect has been obtained by using the standard expression for a point source over an impedance ground, a widely-used one parameter impedance model [2] and assuming effective flow resistivities of 20 000, 300, and 2000 Pa s m2 respectively for concrete, grass and soil. Up to distances of between 500 and 700 m from the engine, the data suggest attenuation rates near to the ‘‘concrete’’ prediction. Beyond 700 m the measured attenuation rate is nearer to the ‘‘soil’’ or between the ‘‘soil’’ and ‘‘grass’’ predictions. These results are consistent with the fact that the run-ups took place over the concrete surface of an apron and further away (i.e. between 500 and 700 m in various directions) the ground surface was ‘soil’ and/or ‘grass’. The effects of discontinuous impedance are considered further in Section 4.
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Fig. 4. (a) SPL Spectra, normalised to a distance of 1 m from an IL-86 engine test, as a function of angle (theta) (forward of the aircraft=0 ) (b) Directivity patterns around IL-86 engine at maximum power mode (defined at reference distance R0=1 m). The circular grid contours are at intervals of 10 dB varying from 80 dB at the inner circle to 150 dB at the outer circle.
Table 1 shows that not only ground covering but also direction are predicted to influence the attenuation rates for overall sound pressure levels. The differences between levels with and without surfaces can be as little as 1 dB and as much as +10.45 dB due to the spectrum variations in different directions. For the purposes of more detailed analyses all the data have been expressed in the form of A-weighted sound level differences with respect to a reference distance of R=100 m. Fig. 6 and Table 2 show the data and predictions for A-weighted sound level differences along the direction of maximum jet noise from an IL-86 aircraft engine.
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Fig. 5. Measured sound levels for noise propagating from Il-86 aircraft’s engine in the direction of maximum jet noise generation and predictions for levels due to a point source at the engine centre height in free space and over various forms of ground cover.
Table 1 Predicted A-weighted levels and attenuation due to ground effect () in various directions of noise propagationa Direction Divergence and With grass covered With concrete covered without-with, ( ) grass dBA absorption (dBA) surface (dBA) surface (dBA)
without-with, concrete (dBA)
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0
5.95 6.1 6.4 8.2 9.2 10.45 9.1 7.1 7.25 6.4 4.3 4.6 3.4 1.4 0.1 1.0
a
89.4 90.2 90.9 93.9 97.4 100.8 97.9 92.5 93.9 94.0 94.5 95.3 96.4 97.7 97.2 95.8
82.2 83.1 83.6 85.2 87.7 90.6 87.9 83.4 84.8 85.4 86.4 87.4 88.9 91.9 93.6 93.7
83.5 84.1 84.5 85.7 87.7 90.3 88.1 85.4 86.6 87.7 89.1 90.6 93.0 96.3 97.3 96.8
7.2 7.15 7.4 8.75 9.7 10.1 10 9 9 8.6 8 7.8 7.5 5.8 3.6 2.1
Predictions assume an Il’jushin-86 engine and a propagation distance of 500 m.
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Fig. 6. Measured and predicted sound level differences re 100 m for noise propagating from Il-86 aircraft’s engine in the direction of maximum jet noise generation ( 40 from exhaust axis).
Table 2 Measured level-distance relationships in various directions from a prop-turbine engine during run-upa ( )
50 60 70 80 90 100 a
A-weighted Sound level LA (dBA) 100
95
90
85
80
75
70
65
60
55
165 170 170 170 170 165
250 260 270 270 260 255
350 350 350 350 350 340
500 500 500 490 475 450
675 670 650 640 625 600
930 930 930 915 900 870
1190 1200 1220 1220 1225 1200
1580 1600 1615 1640 1630 1590
2050 2070 2100 2150 2120 2050
2500 2500 2600 2670 2680 2600
The table shows distances at which particular levels were measured in a given direction.
The equivalent data and predictions in the direction of maximum fan noise generation, from the IL-86 aircraft engine, and in the direction of maximum propeller noise generation from a turbo-prop engine are presented in Figs. 7 and 8 respectively. Again these data presented represent averages of between seven and 10 measurements at every point under consideration. It is noticeable that the rate of attenuation shown by the averaged data in the fan noise direction differs significantly from that in the jet noise direction. The lower attenuation rate, noticeable in the averaged data for jet noise and propellor noise up to 500 m, does not occur in the averaged fan noise data. Consequently the attenuation rate for fan noise is more
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Fig. 7. Measured and predicted A-weighted sound level differences re 100 m for noise propagating from an Il-86 aircraft engine in the direction of maximum fan noise generation ( 50 from axis of engine inlet).
Fig. 8. A-weighted sound level differences re 100 m for noise propagating from an AI-24 turbo-prop engine (on an An-24 aircraft) in the direction of maximum propeller noise generation.
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or less logarithmic with distance out to 1 km. Fig. 7 shows level differences predicted for vertical dipole sources [6] as well as for monopole sources. Out to 500 m, the averaged data are somewhat closer to predictions for a dipole source above concrete than those for a monopole source above concrete. However, despite the prediction that propagation from a vertical dipole will have substantially different attenuation rates to propagation from a monopole, there is little to suggest that the different form of the measured variation of the level differences in the maximum fan noise direction can be attributed to the greater multipole component of the source in this direction. The averaged data in both the maximum jet noise and the maximum propeller noise directions indicate a change from one rate of attenuation to another at distances greater than 500 m. In the next section these rates are fitted empirically and it is argued that the differences result from change in ground impedance with range.
4. Effects of impedance discontinuities Fig. 9 shows the determination of two different empirical attenuation rates and a transition interval from the averaged data for the noise from a propellor (An-24) engine in the 80 direction. Visual best-fit lines to data at ranges up to 500 m and ranges beyond 1500 m are used to indicate the different attenuation rates. A third order polynomial fit to these data is shown also. Table 3 and Fig. 10 show the associated averaged ground effects deduced from the data. Figs. 9 and 10 for both overall levels and ground effect suggest the same transition interval. A possible explanation follows from the predicted effects of an impedance discontinuity in the ground surface. Around airports it is a common that the nature of the ground surface changes with distance, for example from concrete to grass. This means that the sound from the engines propagates along surfaces with
Fig. 9. Measured A-weighted sound levels as a function of distance from a propellor engine along the direction at angle of 80 from the engine inlet. The intersection of two best-fit lines defines the two different attenuation rates and their transition interval. A third order polynomial fit to the averaged data is shown also.
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Table 3 Averaged ground effect (AT), i.e. the change in A-weighted levels re free field, derived from the data in Table 5 by correcting the level differences for wavefront spreading and absorption Distance (m)
170.0
270.0
350.0
490.0
640.0
915.0
1220.
1640.
2150.
2670.
AT (dBA)
1.24
3.14
4.80
6.92
8.77
11.27
13.30
15.46
18.20
19.0
Fig. 10. Averaged ground effect (AT) for noise propagation from a propellor engine (corresponding to Fig. 9) showing two visual best fit lines to different distance ranges and a third order polynomial fit to the averaged data.
changing impedance. Since the ground impedance values at the measurements sites are not available, notional values for (non-porous) ‘‘concrete’’ and ‘‘grass’’ have been used. The influence of impedance discontinuity in the reflecting surface on ground effect has been predicted using the semi-empirical approach of De Jong [14], which assumes that the discontinuity introduces diffraction similar to that resulting from a 180 wedge. The predictions of the dependence of sound levels on the distance between the source (modeled by the power spectrum for Il’ushin-86 engine) and the assumed line of change in impedance, assuming a concrete-to-grass impedance discontinuity, are presented in Fig. 11. The predicted transition interval between attenuation rates around the discontinuity point is of the same character as indicated by the averaged measurements of aircraft engine noise (Figs. 9 and 10).
Fig. 11. Predicted A-weighted sound levels, LA dB for varying distances between a point source and a ‘concrete-grass’ impedance discontinuity.
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Fig. 12. Residue series predictions of A-weighted level difference re 100m from aircraft operated on the ground in the presence of a sound speed gradient a (a) for negative (upward-refracting) sound speed gradients of 1107 s1 and 4107 s1 and a flow resistivity 300 kPa s m2 [17] (b) for flow resistivities of 20000 and 300 kPa s m2 [17] and a sound speed gradient of 4107 s1. The spectrum of the jet engine in the maximum noise direction [see Fig. 4(a)] has been assumed. Also shown are the averaged data for jet and propellor aircraft in the directions of maximum noise levels.
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5. Estimated sound refraction effects To explore the extent to which upward refraction effects would explain the averaged data, predictions have been made using a residue series model [16] to estimate effects of upward refraction for noise from aircraft operated on the ground. The predictions assume continuous ground impedance calculated according to a two-parameter model with the second parameter set to zero [17] and the spectrum for maximum jet engine noise [Fig. 4(a)]. Results are shown in Fig. 12. Predictions for sound speed gradients of 1107 s1 and 4107 s1 above a continuous ground with flow resistivity of 300 kPa s m2, corresponding to ‘‘grass’’, are found to straddle the data (Fig. 12a). These results indicate that the attenuation rate depends on the assumed sound speed gradient. On the other hand it should be noted that these values of sound speed gradient correspond to rather small values of wind- or temperature- gradients, consistent with very minor upward refraction i.e. nearly acoustically-neutral conditions. Moreover a change in sound speed gradient is predicted to produce a uniform rather than discontinuous change in the attenuation rate with distance. The predictions for continuous ground covers with flow resistivities of 20 000 and 300 kPa s m2 [17] respectively in the presence of a negative sound speed gradient of 4107 s1 (Fig. 12b) are similar to the results shown in Figs. 6–8. They show a strong dependence on the assumed ground impedance. These results suggest that the observed change in attenuation rates with distance is due mainly to discontinuous ground effects.
6. Conclusions There are significant differences between the magnitudes of lateral attenuation predicted for different types of ground surface as a result of frequency-dependent ground effect [15]. Using averaged data obtained at ranges of up to 3 km around engine run-ups during low wind conditions and assuming that refraction and wind effects are minimal, it has been demonstrated that the attenuation rate is influenced strongly by both direction and ground cover. The averaged data for attenuation rate in the direction of maximum fan noise radiation are noticeably different from those in the maximum jet and propellor noise directions. Polynomial fits to the averaged data for the attenuation of maximum jet and propellor engine noise as a function of distance have been obtained. It has been shown that the data for maximum jet and propellor engine noise are consistent with propagation from a point source over a discontinuous impedance ground. The transition between concrete and grass surfaces is responsible for a change in the averaged attenuation rate of ground noise with distance around airports.
Acknowledgements This work was supported in part by NATO Grant EST.CLG 974767 and by the Dora Jones Bequest to the Department of Engineering, University of Hull.
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References [1] Oncley PB. Review of theory and methods for prediction ground effects on aircraft noise propagation. AIAA Paper 75–538, 1975. [2] Chien CF, Soroka WW. Sound propagation along an impedance plane. J Sound Vib 1975;43(1): 9–20. [3] Chien CF, Soroka WW. A note on the calculation of sound propagation along an impedance boundary. J Sound Vib 1980;69(1):340–3. [4] Kawai T, Hidaka T, Nakajama T. Sound propagation above an impedance boundary. J Sound Vib 1982;83(1):125–38. [5] Chessel CI. Propagation of noise along a finite impedance boundary. J Acoust Soc Am 1977;62: 825–34. [6] Li KM, Taherzadeh S, Attenborough K. Sound propagation from a dipole source near an impedance plane. J Acoust Soc Am 1997;101(6):3343–52. [7] Attenborough K. Natural noise control. In: Inter-Noise 96’: Proceedings of 25th Anniversary Congress, Liverpool, 1996. p. 51–73. [8] ISO 9613/2. Acoustics—attenuation of sound during propagation outdoors: part 2 a general method of calculation. [9] Prediction method for lateral attenuation of airplane noise during take-off and landing. SAE, AIR 1751, 1981. [10] Method for computing noise levels from aircrafts around airports of civil aviation—Kyiv. KIUCA, UkrAero-project report, 1997. [11] Devis LIC. A guide to the calculation of NNI. p. 12. DORA communication 7908. 2nd ed. London: CAA; 1981. [12] Kantola RA, Outdoor jet noise facility: a unique approach. In: 2nd Aero-Acoustics Conf., Hampton, VA, March, 1975 [AIAA paper 75-530]. p. 223–45. [13] Delany ME, Bazley EN. Acoustical properties of fibrous absorbent materials. Applied Acoustics 1970;3:105–16. [14] de Jong BA, Moerkerken A, van der Toorn JD. Propagation of sound over grassland and over an earth barrier. Journal of Sound and Vibration 1983;86(1):23–46. [15] Zaporozhets O, Tokarev V, Attenborough K, Li KM. Aircraft noise model corrected by operational and propagation effects. In: Proceedings of the 2nd Forum Acusticum, Berlin, 1999. [16] Li KM, Wang Q, Attenborough K. Sound propagation over convex impedance surfaces. J Acoust Soc Am 1988;104(5):2683–91. [17] Attenborough K. Ground parameter information for propagation modelling. J Acoust Soc Am 1992; 92:418–27.
Predicting noise from aircraft operated on the ground Oleksander Zaporozhetsa, Vadim Tokareva, Keith Attenboroughb,* a
National Aviation University (NAU), 1, Cosmonaut Komarov Prospect, Kyiv 03058, Ukraine b Department of Engineering, University of Hull, Hull, HU6 7RX, UK Received 1 March 2003; received in revised form 7 April 2003; accepted 14 April 2003
Abstract Averaged data for sound levels due to aircraft engine testing out to a distance of 3 km obtained under low wind speed conditions are presented. Predictions of standard analytical approximations are compared with these averaged data. The measured average sound levels are shown to be consistent with the predicted influence of ground effect including impedance discontinuities. There is a noticeable influence of directionality also. # 2003 Elsevier Ltd. All rights reserved. Keywords: Aircraft noise; Engine test; Ground effect
1. Introduction Although air-to-ground propagation is a very important to the impact of aircraft noise, there is interest also in ground-to-ground propagation. As the name suggests, ground-to-ground propagation is relevant to the assessment of noise from aircraft on the ground, for example during engine run-ups or as a result of taxiing. In these examples, the height of noise source is simply the height of engine installation on the aircraft. Ground-to-ground propagation is relevant also to predicting noise from aircraft engine testing facilities, for which the source height may be between 4 and 5 m. It is to be expected that ground reflection interference and diffraction effects are more evident in ground-to-ground propagation than for air-to-ground propagation [1–7]. * Corresponding author. Tel.: +44-1482-466-600; fax: +44-1482-466-600. E-mail address: [email protected] (K. Attenborough). 0003-682X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0003-682X(03)00064-1
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There are several empirical schemes for predicting ground effect for aircraft sources near to the ground. ISO 9613/2 [8] provides a general calculation method. The SAE 1751 standard [9] predicts the ‘lateral attenuation’ used widely in aircraft noise calculations (see Fig. 1), for example in FAA INM (both version 5.2 and the latest version 6.0) and in the current Ukrainian method [10] (realized in software ISOBELL’a). Another well-known approximation is contained in the UK method for aircraft noise assessment [11]. Results of calculations using the SAE/ICAO and UK methods along the sound propagation path, at horizontal ranges between 0 and 600 m, and for grazing angles between 0 and 15 differ by between 3 and +1.5 dB. All of the prediction schemes are based on data averaged over a great number of sources and ambient conditions. Some of them are recommended for use with particular noise indices. For example, the SAE/ICAO approximation is used for both SEL and EPNL calculations without any reference to the particular source spectrum and directivity pattern. This is in spite of the fact that each index is defined by a different frequency weighting function. Moreover, the type of local ground cover is neglected. Strong ground effects have been observed in relatively-short range acoustic measurements around engine testing facilities. In Fig. 2, data [12] are compared with predictions for a monopole noise source over hard ground [2,13]. Clearly these data are consistent with predictions for ground effect over relatively hard ground.
2. Long range data from engine testing Many noise measurements have been made in the former Soviet Union during aircraft engine run-ups with the aim of defining noise contours in the vicinity of airports. Data are available for a large number of aircraft types. The directivity results have been used for flyover level assessment. Noise contours (directivity patterns) have been obtained for a range of power settings during several summer days under near to calm weather conditions (wind speed < 5 m/s, temperature between 20
Fig. 1. Predictions of lateral attenuation as a function of grazing angle according to SAE/ICAO.
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Fig. 2. Comparison between predictions and measurements [12] at an engine testing facility with a concrete ground surface. The receiver height is 1.5 m, the assumed point source height is 4.5 m and the horizontal distance between the engine and microphone is 50 m. The prediction has assumed an impedance for concrete given by a semi-empirical one-parameter model [13] with effective flow resistivity =20 000 kPa s m2.
and 25 C). That these data are consistent with nearly acoustically-neutral conditions is demonstrated later. Although the measurements were intended only for assessing overall sound levels, the results are useful for a preliminary analysis of ground effect on noise from aircraft engine run-ups. Fig. 3 shows A-weighted sound pressure levels as a function of distance (on a logarithmic axis) and direction from a low by-pass engine installed on the Il’ushin-86 aircraft, obtained at different times but for the same power setting and ambient conditions. Between 7 and 10 measurements were made at every point under consideration (in accordance with ICAO Annex 16 requirements) and the results have been averaged to obtain the contours for each type of aircraft investigated. These data suggest that the directivity pattern is an important characteristic of aircraft engine noise and that it can have a significant influence on the overall sound pressure. Fig. 4(a) shows the variation in spectra with direction observed in the IL86 run up tests at a reference distance of 1 m. Fig. 4(b) shows computations of overall A-weighted levels LA, PNL and unweighted levels L as a function of direction, normalised to a distance of 1 m.
3. Comparisons of predictions with data The data shown in Figs. 3 and 4 may be compared with various predictions for ground effect. Fig. 5 presents the data for overall A-weighted levels due to an IL-86 aircraft engine in the direction of maximum jet noise generation ( ffi 140 ). As in Fig. 3, each data point is an average of between 7 and 10 measurements. Also shown are predictions for levels due to a point source with the same sound power spectrum allowing for wave front spreading (Div), for spreading and absorption (Div+abs)
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Fig. 3. Noise directivity patterns around an IL-86 aircraft at ranges up to 3 km.
and for various sorts of ground cover (concrete, soil and grass). The ground effect has been obtained by using the standard expression for a point source over an impedance ground, a widely-used one parameter impedance model [2] and assuming effective flow resistivities of 20 000, 300, and 2000 Pa s m2 respectively for concrete, grass and soil. Up to distances of between 500 and 700 m from the engine, the data suggest attenuation rates near to the ‘‘concrete’’ prediction. Beyond 700 m the measured attenuation rate is nearer to the ‘‘soil’’ or between the ‘‘soil’’ and ‘‘grass’’ predictions. These results are consistent with the fact that the run-ups took place over the concrete surface of an apron and further away (i.e. between 500 and 700 m in various directions) the ground surface was ‘soil’ and/or ‘grass’. The effects of discontinuous impedance are considered further in Section 4.
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Fig. 4. (a) SPL Spectra, normalised to a distance of 1 m from an IL-86 engine test, as a function of angle (theta) (forward of the aircraft=0 ) (b) Directivity patterns around IL-86 engine at maximum power mode (defined at reference distance R0=1 m). The circular grid contours are at intervals of 10 dB varying from 80 dB at the inner circle to 150 dB at the outer circle.
Table 1 shows that not only ground covering but also direction are predicted to influence the attenuation rates for overall sound pressure levels. The differences between levels with and without surfaces can be as little as 1 dB and as much as +10.45 dB due to the spectrum variations in different directions. For the purposes of more detailed analyses all the data have been expressed in the form of A-weighted sound level differences with respect to a reference distance of R=100 m. Fig. 6 and Table 2 show the data and predictions for A-weighted sound level differences along the direction of maximum jet noise from an IL-86 aircraft engine.
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Fig. 5. Measured sound levels for noise propagating from Il-86 aircraft’s engine in the direction of maximum jet noise generation and predictions for levels due to a point source at the engine centre height in free space and over various forms of ground cover.
Table 1 Predicted A-weighted levels and attenuation due to ground effect () in various directions of noise propagationa Direction Divergence and With grass covered With concrete covered without-with, ( ) grass dBA absorption (dBA) surface (dBA) surface (dBA)
without-with, concrete (dBA)
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0
5.95 6.1 6.4 8.2 9.2 10.45 9.1 7.1 7.25 6.4 4.3 4.6 3.4 1.4 0.1 1.0
a
89.4 90.2 90.9 93.9 97.4 100.8 97.9 92.5 93.9 94.0 94.5 95.3 96.4 97.7 97.2 95.8
82.2 83.1 83.6 85.2 87.7 90.6 87.9 83.4 84.8 85.4 86.4 87.4 88.9 91.9 93.6 93.7
83.5 84.1 84.5 85.7 87.7 90.3 88.1 85.4 86.6 87.7 89.1 90.6 93.0 96.3 97.3 96.8
7.2 7.15 7.4 8.75 9.7 10.1 10 9 9 8.6 8 7.8 7.5 5.8 3.6 2.1
Predictions assume an Il’jushin-86 engine and a propagation distance of 500 m.
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Fig. 6. Measured and predicted sound level differences re 100 m for noise propagating from Il-86 aircraft’s engine in the direction of maximum jet noise generation ( 40 from exhaust axis).
Table 2 Measured level-distance relationships in various directions from a prop-turbine engine during run-upa ( )
50 60 70 80 90 100 a
A-weighted Sound level LA (dBA) 100
95
90
85
80
75
70
65
60
55
165 170 170 170 170 165
250 260 270 270 260 255
350 350 350 350 350 340
500 500 500 490 475 450
675 670 650 640 625 600
930 930 930 915 900 870
1190 1200 1220 1220 1225 1200
1580 1600 1615 1640 1630 1590
2050 2070 2100 2150 2120 2050
2500 2500 2600 2670 2680 2600
The table shows distances at which particular levels were measured in a given direction.
The equivalent data and predictions in the direction of maximum fan noise generation, from the IL-86 aircraft engine, and in the direction of maximum propeller noise generation from a turbo-prop engine are presented in Figs. 7 and 8 respectively. Again these data presented represent averages of between seven and 10 measurements at every point under consideration. It is noticeable that the rate of attenuation shown by the averaged data in the fan noise direction differs significantly from that in the jet noise direction. The lower attenuation rate, noticeable in the averaged data for jet noise and propellor noise up to 500 m, does not occur in the averaged fan noise data. Consequently the attenuation rate for fan noise is more
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Fig. 7. Measured and predicted A-weighted sound level differences re 100 m for noise propagating from an Il-86 aircraft engine in the direction of maximum fan noise generation ( 50 from axis of engine inlet).
Fig. 8. A-weighted sound level differences re 100 m for noise propagating from an AI-24 turbo-prop engine (on an An-24 aircraft) in the direction of maximum propeller noise generation.
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or less logarithmic with distance out to 1 km. Fig. 7 shows level differences predicted for vertical dipole sources [6] as well as for monopole sources. Out to 500 m, the averaged data are somewhat closer to predictions for a dipole source above concrete than those for a monopole source above concrete. However, despite the prediction that propagation from a vertical dipole will have substantially different attenuation rates to propagation from a monopole, there is little to suggest that the different form of the measured variation of the level differences in the maximum fan noise direction can be attributed to the greater multipole component of the source in this direction. The averaged data in both the maximum jet noise and the maximum propeller noise directions indicate a change from one rate of attenuation to another at distances greater than 500 m. In the next section these rates are fitted empirically and it is argued that the differences result from change in ground impedance with range.
4. Effects of impedance discontinuities Fig. 9 shows the determination of two different empirical attenuation rates and a transition interval from the averaged data for the noise from a propellor (An-24) engine in the 80 direction. Visual best-fit lines to data at ranges up to 500 m and ranges beyond 1500 m are used to indicate the different attenuation rates. A third order polynomial fit to these data is shown also. Table 3 and Fig. 10 show the associated averaged ground effects deduced from the data. Figs. 9 and 10 for both overall levels and ground effect suggest the same transition interval. A possible explanation follows from the predicted effects of an impedance discontinuity in the ground surface. Around airports it is a common that the nature of the ground surface changes with distance, for example from concrete to grass. This means that the sound from the engines propagates along surfaces with
Fig. 9. Measured A-weighted sound levels as a function of distance from a propellor engine along the direction at angle of 80 from the engine inlet. The intersection of two best-fit lines defines the two different attenuation rates and their transition interval. A third order polynomial fit to the averaged data is shown also.
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Table 3 Averaged ground effect (AT), i.e. the change in A-weighted levels re free field, derived from the data in Table 5 by correcting the level differences for wavefront spreading and absorption Distance (m)
170.0
270.0
350.0
490.0
640.0
915.0
1220.
1640.
2150.
2670.
AT (dBA)
1.24
3.14
4.80
6.92
8.77
11.27
13.30
15.46
18.20
19.0
Fig. 10. Averaged ground effect (AT) for noise propagation from a propellor engine (corresponding to Fig. 9) showing two visual best fit lines to different distance ranges and a third order polynomial fit to the averaged data.
changing impedance. Since the ground impedance values at the measurements sites are not available, notional values for (non-porous) ‘‘concrete’’ and ‘‘grass’’ have been used. The influence of impedance discontinuity in the reflecting surface on ground effect has been predicted using the semi-empirical approach of De Jong [14], which assumes that the discontinuity introduces diffraction similar to that resulting from a 180 wedge. The predictions of the dependence of sound levels on the distance between the source (modeled by the power spectrum for Il’ushin-86 engine) and the assumed line of change in impedance, assuming a concrete-to-grass impedance discontinuity, are presented in Fig. 11. The predicted transition interval between attenuation rates around the discontinuity point is of the same character as indicated by the averaged measurements of aircraft engine noise (Figs. 9 and 10).
Fig. 11. Predicted A-weighted sound levels, LA dB for varying distances between a point source and a ‘concrete-grass’ impedance discontinuity.
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Fig. 12. Residue series predictions of A-weighted level difference re 100m from aircraft operated on the ground in the presence of a sound speed gradient a (a) for negative (upward-refracting) sound speed gradients of 1107 s1 and 4107 s1 and a flow resistivity 300 kPa s m2 [17] (b) for flow resistivities of 20000 and 300 kPa s m2 [17] and a sound speed gradient of 4107 s1. The spectrum of the jet engine in the maximum noise direction [see Fig. 4(a)] has been assumed. Also shown are the averaged data for jet and propellor aircraft in the directions of maximum noise levels.
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5. Estimated sound refraction effects To explore the extent to which upward refraction effects would explain the averaged data, predictions have been made using a residue series model [16] to estimate effects of upward refraction for noise from aircraft operated on the ground. The predictions assume continuous ground impedance calculated according to a two-parameter model with the second parameter set to zero [17] and the spectrum for maximum jet engine noise [Fig. 4(a)]. Results are shown in Fig. 12. Predictions for sound speed gradients of 1107 s1 and 4107 s1 above a continuous ground with flow resistivity of 300 kPa s m2, corresponding to ‘‘grass’’, are found to straddle the data (Fig. 12a). These results indicate that the attenuation rate depends on the assumed sound speed gradient. On the other hand it should be noted that these values of sound speed gradient correspond to rather small values of wind- or temperature- gradients, consistent with very minor upward refraction i.e. nearly acoustically-neutral conditions. Moreover a change in sound speed gradient is predicted to produce a uniform rather than discontinuous change in the attenuation rate with distance. The predictions for continuous ground covers with flow resistivities of 20 000 and 300 kPa s m2 [17] respectively in the presence of a negative sound speed gradient of 4107 s1 (Fig. 12b) are similar to the results shown in Figs. 6–8. They show a strong dependence on the assumed ground impedance. These results suggest that the observed change in attenuation rates with distance is due mainly to discontinuous ground effects.
6. Conclusions There are significant differences between the magnitudes of lateral attenuation predicted for different types of ground surface as a result of frequency-dependent ground effect [15]. Using averaged data obtained at ranges of up to 3 km around engine run-ups during low wind conditions and assuming that refraction and wind effects are minimal, it has been demonstrated that the attenuation rate is influenced strongly by both direction and ground cover. The averaged data for attenuation rate in the direction of maximum fan noise radiation are noticeably different from those in the maximum jet and propellor noise directions. Polynomial fits to the averaged data for the attenuation of maximum jet and propellor engine noise as a function of distance have been obtained. It has been shown that the data for maximum jet and propellor engine noise are consistent with propagation from a point source over a discontinuous impedance ground. The transition between concrete and grass surfaces is responsible for a change in the averaged attenuation rate of ground noise with distance around airports.
Acknowledgements This work was supported in part by NATO Grant EST.CLG 974767 and by the Dora Jones Bequest to the Department of Engineering, University of Hull.
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