A simple method to compute aircraft noise inside dwellings

Transcription

1 The 33 rd International Congress and Exposition on Noise Control Engineering A simple method to compute aircraft noise inside dwellings C. Brutel-Vuilmet, M. Villot and P. Jean CSTB, 24 av Joseph Fourier, 384 St-Martin-d'Heres, France [c.brutel; villot ; p.jean]@cstb.fr Abstract [351] A method to estimate sound levels inside dwellings due to aircrafts is presented. The originality of this method lies in considering the incidence angles of waves in the calculation: both the directivity of the noise source and a transmission loss depending on the incidence angle of the incoming wave are used. Indeed, airplane noise at building façades is very directional and it has been shown that the incidence angle plays an important role in sound transmission. A calculation method has been developed in the simple case of flight tracks without any diffraction phenomena, where the incident field is only composed of a direct wave and a ground reflected wave (tracks parallel to a smooth façade). Many parameters are taken into account, such as source characteristics (obtained from measured data for two types of aircraft), air absorption and geometrical attenuation, the propagation time, the ground impedance, and the characteristics of the glazing (dimensions, material, simple or double glazing ), in order to estimate the transmitted noise levels inside buildings. 1 INTRODUCTION A model to compute noise level spectra and A-weighted noise levels inside dwellings in the vicinity of airports is presented. The originality of the method lies in considering the incidence angles of waves in the different steps of the calculation. SOURCE: Measured data of sound power and directivity for two types of aircraft are used to describe accurately the noise source. Realistic flight tracks for landing or takeoff give the position of the source as a function of time. PROPAGATION: During the propagation, the sound attenuation is computed with air absorption and geometrical attenuation in normal conditions of temperature, pressure and humidity. The ground impedance gives the level of the reflected wave. Indeed, as the aircraft is far from the building, one may assume that the incident field is composed by only two plane waves corresponding to two well defined incidences: the direct wave and the ground reflected wave. TRANSMISSION: A model of transmission is used, giving the transmission loss as a function of the incidence angle for finite plate and for different materials. Indeed, it has been proved that the direction of the incoming wave is an important parameter for the transmission [1]. Three types of glazing are examined: simple, double, and laminated. 1/8

2 In this paper only simple flight tracks without any diffraction phenomena are analyzed. The building façade is planar and the track is parallel to the façade. The aircraft can take off, land, or fly at a constant altitude. (Figure 1) z straignt track y direct wave reflected wave Figure 1: Example of track, a landing In order to compute the different noise levels inside the dwelling during the flight, the time is discretised. Each instant corresponds to a defined position of the aircraft. x 2 SOUND LEVEL COMPUTATION 2.1 noise source To compute the incident field on the building, it is necessary to have some source characteristics: its spectrum, its directivity and its position as function of time. Measured data of the aircraft sound power level at one meter and the aircraft directivity have been used. These data came from the Norwegian Air Traffic Airport Management and the company SINTEF Telecom and Informatics [2,3]. In their study, flight recorder data, meteorological measured data (wind velocity and direction, temperature, humidity rate ), data from radar tracking systems, and third octave sound measurements have been the basis of an extensive analysis. The project collected accurate data for two types of aircraft: MacDouglas MD8 and Boeing B737. In particular, directivity patterns for different flight regimes (takeoff or landing) have been obtained. An example of directivity pattern is presented (Figure 2) for the B737. In order to have coherent incidence angles of incoming waves, realistic tracks have been employed. Velocity and thrust of the plane are obtained from INM ("Integrated Noise Model") database to describe takeoffs or landings. 2/8

3 27 9 Figure 2: Directivity pattern for B737, landing, 5Hz (Power level in db) 2.2 Propagation The model of propagation is simple. Atmospheric absorption, geometrical attenuation, and time of propagation are taken into account; but local acoustical variables such as temperature profiles, wind gradient, humidity effects are omitted. The impedance of the ground is obtained from the empirical Delany & Bazley model. The incident sound level due to the direct wave is given by: 1 3 Lpinc = Lpi + log att D. (1) D² Lpi : source pressure level at one meter, with the directivity correction (db) D: distance between the aircraft and the façade (m) att : attenuation coefficient (db / km), from standart ISO The sound level on the façade is the result of the addition of direct and reflected waves, summed energetically and assumed uncorrelated. In the singular case of a total reflection, it has been verified that the level on the façade can be approximated by the level due to the direct wave plus 3dB. The incident power Wi is given by: θp : incidence angle (see Figure 3) Sp : surface area z 18 Lp inc Wi ( p) Sp = cos θ (2) y θp φp x Figure 3: Definition of the incidence angles θp and φp 3/8

4 The spectrum of the incident noise level (Figure 4) shows that low frequencies are dominant. 8 Incident noise level spectrum for two aircraft Spectre du bruit incident sur la paroi, sans pondération 7 6 pressure level (db) position aircraft xa=m, at 5m niveau maximum position aircraft xa=m, at m atterrissage Fréquence (Hz) frequency (Hz) Figure 4: Incident noise Level Spectrum 2.3 Transmission The transmission of the incident noise is described by a model taking into account the direction of the incoming wave, giving TL(θ,φ). Generally the TL is calculated or measured for a diffuse incident field. In the current work, a more detailed and accurate model is suggested. The calculation model is based on the wave approach, and can be applied to laminated panels. A spatial windowing technique enables to take into account the finite size of plane structures [4]. It has been shown that the azimuth angle φp has a very little influence on the transmission loss values, so only the elevation angle θp is considered. (See Figure 3) Figure 5 represents the transmission loss as a function of frequency for different incidences and compared to the diffuse field TL for a simple glazing (6mm thick glass) and a double glazing (8mm thick glass / mm thick air / 4mm thick glass), surface area 1.5x1.5 m². Transmission R(f) - Simple loss, vitrage Simple - paroi glazing finie 1m5x1m5 x 1m5 18 Transmission R(f) - Double loss, vitrage Double - paroi glazing finie 1m5x1m5 x 1m5 TL niveau (db) en db theta= theta=3 theta=6 theta=8 theta=89 diffuse champ field diffus TL (db) niveau en db theta= theta=3 theta=6 theta=8 theta=89 diffuse champ field diffus frequency Fréquence (Hz) Figure 5: TL(f), diffuse field or depending on θ Simple glazing on the left, Double glazing on the right frequency Fréquence (Hz) 4/8

5 The interest of this model lies on its very short computation time and on results very close to TL (db) measured TL computed TL, infinite panel computed TL, 1.5x1.5 m² experimental measured values as shown for a diffuse field in Figure 6. Figure 6: Comparison between measured TL values and calculated TL values for an incident diffuse field The TL coefficient enables to compute the transmitted power level Wt inside the building: 1 Wt = log( ) Wi (3) TL( θ ) 4 Wt Lp inside = log (4) A A : equivalent absorption surface area, obtained from the reverberant time of the room 3 RESULTS 3.1 Comparison between diffuse field TL and TL(θ) The pressure level inside the building is calculated either from a diffuse field TL or from a TL depending on the direction of the incoming wave TL(θ). The comparison between the two graphs (Figure 7) shows that differences up to about 5 db are obtained when the directivity of the excitation is not included in the calculation. 5/8

6 5 4 as function of the incidence angle diffuse field Sound Level db(a) theta= time (s) Figure 7: Sound Level inside dwelling, aircraft B737, simple glazing, diffuse TL or TL(θ) 3.2 Comparison of different types of glazing Three types of glazing have been examined: simple glazing (6mm thick glass); double glazing (6mm thick glass - mm thick air gap - 4mm thick glass), and laminated glazing (viscoelastic material inserted between two 4mm thick glass pans - 12mm thick air gap mm thick glass). 5 Sound Level db(a) 4 3 simple glazing double glazing multi-layered glazing time (s) Figure 8: Sound Level inside the dwelling, aircraft B737, simple, double or laminated glazing This analysis shows a surprising result (Figure 8): the maximum level inside the dwelling is higher with the double glazing than with the simple one. Indeed, for the chosen track the maximum level corresponds to a low incidence angle (near normal incidence). For low frequencies and low incidence angles the transmission model predicts that the TL of the double glazing is lower than for the simple glazing (see Figure 5). 3.3 Size of the panel Tree different panel sizes are compared on Figure 9. The two larger glazing (1m5x1m5 and 2mx3m) give very close results. And it has been shown that levels computed for an infinite panel 6/8

7 are close too. Sound levels obtained for the smaller glazing are lower. So the size of the panel seems to be an important parameter for very small panels. 5 4 Sound level db(a) 3 1m5x1m5 2mx3m.6mx.4m time (s) Figure 9: Sound Level inside dwelling for different glazing size 4 CONCLUSIONS A performing tool has been implemented to compute noise level spectra and A-weighted noise levels due to aircraft inside dwellings in the vicinity of airports. The method takes into account many of parameters: the directivity of the noise source, realistic flight tracks to describe the aircraft position, air absorption and geometrical attenuation during the propagation, ground impedance for the ground reflected wave, and a detailed transmission model. The originality of the method lies on both the use of the noise source directivity and the use of a transmission loss depending on the incidence angle, whereas generally this index is calculated or measured for an incident diffuse field. Moreover, an important interest of the method is that the variation of noise level spectra during aircraft takeoff or landing can easily be used to compute specific indicators such as Leq ou the SEL (sound exposure level). We have shown that this tool can be used to detect critical configurations or singular behaviors of materials. Indeed, when the incidence dependency being included, specific properties of buildings components could be observed. For example it has been noted that a double glazing 6--4 has poor performances at low frequencies and low incidence angle, and it is even less efficient than a simple glazing 6mm thick. In this paper, only simple configurations without any diffraction phenomena have been considered. However, more complicated cases have been studied, running a beam tracing software to compute more complex incident fields. For example the cases of buildings with balconies or tracks not parallel to the façade have been studied. Results will be presented at another congress [5]. Finally, this work has confirmed that the use of a diffuse field transmission loss (independent of the wave direction) can lead to differences up to 5 db in transmitted noise levels. So it seems necessary to take into account the dependency with the incidence angle in the transmission loss in order to 7/8

8 predict accurate and realist sound levels inside dwellings. CSTB is now working on the implementation of a new laboratory setup for measuring the TL of building elements as a function of the incidence angle, aiming to validate calculated data obtained with numerical models. ACKNOWLEDGEMENTS The authors are grateful to the Norwegian Air Traffic and Airport Management and SINTEF Telecom and informatics for giving them accurate and useful data on aircraft noise. The authors also wish to thank ADEME, the French Environmental Agency, for financially founding the project. REFERENCES [1] P. Jean, J. F. Rondeau, A simple decoupled modal calculation of sound transmission between volumes, in Acta Acustica, 88, , 2 [2] K. H. Liasjø, K. Holen and N. I. Nilsen, Aircraft sound exposure, directivity and propagation, in Proceedings of Inter-Noise2, Dearborn MI, 2 [3] H. Olsen, R. T. Randeberg, S. A. Storeheier Validation of source data and propagation algotithmes for aircraft noise prediction, in Proceedings of Inter-Noise2, Dearborn MI, 2 [4] M. Villot, C. Guigou and L. Gagliardini, Predicting the acoustical radiation of finite size multi-layered structures by applying spatial windowing on infinite structures, Journal of Sound and Vibration, (1) 245(3). [5] P. Jean, C. Brutel-Vuilmet, M. Villot, The computation of aircraft noise inside dwellings, Eleventh International Congress on Sound and Vibration, St. Petersburg, Russia, 4 8/8