Proceedings of Meetings on Acoustics

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1 Lee et a. Proceedings of Meetings on Acoustics Voume 19, ICA 2013 Montrea Montrea, Canada 2-7 June 2013 Underwater Acoustics Session 2pUWa: Ocean Ambient Noise 2pUWa13. Modeing of underwater piing noise mitigation using an array of soft spheres in the ocean Keunhwa Lee, Kyungmin Baik and Woojae Seong* *Corresponding author's address: Dept. of Ocean Eng., Seou Nationa Univ., Seou, , Seou, Korea, Repubic of Korea, The ocean noise generated by marine piing affects severey fish, other marine ife, and fishery activities. Accordingy, a few kind of noise mitigation system are presented. Among them, the noise mitigation system using soft scatterers such as air bubbes or rubber spheres is reported to show higher noise reduction than the cassica cofferdam system composed of mass-absorbing materias. In this proceeding, a numerica scheme is deveoped to mode and design the noise mitigation system using an array of soft spheres. This scheme is originay based on sef-consistent equation of Zhen Ye for mutipe scattering [Z. Ye and A. Avarez, Phys. Rev. Lett. 80, 3503 (1998)]. We generaize the origina sef-consistent equation for the oceanic waveguide using the waveguide green function. This generaized sef-consistent equation is usefu to mode the noise propagation through an array of soft spheres in the ocean and assess the abiity of the noise mitigation system. The effect of the oceanic waveguide on the noise reduction is studied and the vaidity of effective medium approach for a bubby ayer is aso anayzed numericay. Pubished by the Acoustica Society of America through the American Institute of Physics 2013 Acoustica Society of America [DOI: / ] Received 22 Jan 2013; pubished 2 Jun 2013 Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 1

2 Lee et a. INTRODUCTION The underwater noise generated from the pie driving at the ocean wind turbine construction site beongs to a successivey intermittent noise to have the broad bandwidth of 10 Hz to 3 khz, and its sound exposure eve (SEL) is usuay higher than the disturbance eve of marine mammas of 140 db re 1 Pa 1-2. Thus, a serious threat to marine ife and fishery activities is posed by the underwater piing noise, which often causes ega conficts among stakehoders. The mitigation of underwater piing noise has been worked by the direct reduction of noise source eve or the modification of the noise transmission paths. The former is made from the enhancement of hammer housing system and the atter uses the encosure and the barrier such as air bubbe curtain, encapsuated gas bubbe curtain, pie seeve, and encosed cofferdam 2-3. Among them, it is known that the screen of soft bubbe array has the superior cost performance ratio. By previous experimenta studies 4-5, the bubbe screen showed the noise reduction abiity of approximatey 5-20 db near the bubbe resonance frequency. Some studies have deat with the underwater noise propagation modeing in the waveguide considering the encosure or the barrier 5-6. In the study of Lee, Wocher, and Wison 5, the encapsuated bubbe screen is regarded as an effective continuum medium based on Commander and Prosperetti mode, and aso put to the finite eement (FE) mode. Reinha and Dah 6 considered a Temporary Noise Attenuation Pie (TNAP) consisting of the two pipes, the sound absorbing materia, the bubby water ayer, and the appendage. They modeed each eement of TNAP to be an effective continuum, and the detaied finite mode of the TNAP is appied to the FE mode for the acoustic wave propagation. In this proceeding, we present new simuation study for the ow frequency acoustic wave propagation in the waveguide when the noise source is surrounded by the screen of soft spheres. This approach is based on the mutipe-scattering formuation of sef-consistent form 7-8. The origina formuation of Fody and Lax in the free-fied is generaized for the oceanic waveguide using a waveguide Green function. The couped equation can be rigorousy soved by the procedure of Ye and Avarez 7. The noise reduction screen used in the cacuation is assumed to be a cyindrica array of soft spheres (Fig. 1). FORMULATION In the free fied, a unit point source surrounded by soft spheres radiates the continuous wave with the anguar 3 frequency of. The sphere radius is a and the void fraction is 4 an/3 with the numerica density of the bubbe of n. Then, the scattered wave from the th sphere at the position of r can be composed as where N jk r r e ps( r; ) f p0( r) ps( r; m) (1) m1, m r r r is the position vector of the th f is the isotropic scattering function of the th sphere, ( ) jkr p / 0 r e r 2 2 /( / 1 ) sphere, and f a j with the with the wavenumber of k. Here it is assumed that 0 sphere radius a and the anguar resonance frequency of 0 and the damping factor of for the th sphere 7-8. By observing the hierarchy of Eq. (1), the above equation can be generaized for the waveguide environment as foows. N ps( r; ) fg0( r0, r) ps( r; m) G( r, r) (2) m1, m where Gr (, r) is the waveguide Green function satisfying the Hemhotz equation and the waveguide boundary conditions, and G0( r 0, r ) is the waveguide Green function for the incident wave with the point source position vector of r 0. When p ( r ; ) AG( r, r ) and setting r at another scatterer s position, Eq. (2) is modified as a matrix equation as foows. s Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 2

3 Lee et a. A where X { fg( r, r) ( m) or -1 ( m)} m m 1 X Y (3), Y fg 0( r0, r), and A is a coumn vector composing of A. From Eq. (3), the scattered ampitudes of A are cacuated, and then the tota wave fied can be obtained as the sum of the incident wave fied and the scattered wave fied, N pr ( ) G( r, r ) AGr (, r ) In the above equation, the waveguide Green function is obtained by the combination of the image method and the norma mode method 9. These methods give the exact soution in the idea waveguide with the pressure reeased surface and the hard bottom even for near fied. (4) FIGURE 1. Arrangement of soft spheres in the numerica experiment. The circe symbo indicates soft spheres and the asterisk symbo means the position of unit point source. FIGURE 2. Comparison of the noise reduction oss as a function of ka for the free-fied, the haf-space, and the idea waveguide. (a) r = 3.9 m, (b) r = 3.9 km. NUMERICAL EXAMPLES Numerica experiments are performed respectivey in the free fied, the haf-space, and the idea waveguide. The water depth of the waveguide is 10 m. This depth is same to that of the wind turbine construction site ocated in Yeow sea, southwest off Korea Peninsuar. The water sound speed is 1500 m/s and the water density is 1000 kg/m 3. The unit point source is ocated at the depth of 5 m beow the ocean surface. The cyindrica array of soft sphere has the configuration of the center radius of 1.25 m and the height of 9.23 m. The tota number of spheres is 130. A Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 3

4 Lee et a. spheres have same size and are reguary arranged on the surface of transparent cyinder as shown in Fig. 1. The sphere radius is 5 cm and the void fraction is approximatey The resonant frequency of singe sphere is obtained using Minnaert formua 10, and then ka at the resonant frequency is The damping factor of the scattering function is set to be 0.01 for a frequencies. Note that the vaue of damping factor in these exampes is arbitrariy chosen since our objective focuses on the observation of the mutipe-scattering effect and the waveguide effect. For the reaistic modeing of soft sphere, the use of the improved physica modes such as Church mode 11 is recommended. In the present study, the noise reduction oss (NR oss, db) is defined that NR = SL TL Lp, where SL is the source eve, TL is the transmission oss of the noise source, and Lp is the sound pressure eve (SPL) at the receiver. In a exampes, the SL is set to be 0 db. Fig. 2 shows the noise reduction oss as a function of ka. The resuts of Fig. 2 are respectivey measured at the range of 3.9 m and 3.9 km and the depth of 5 m. Here, the noise reduction ceary happens at the range between and 0.25 in ka. The highest noise reduction occurs that ka = We note that this critica point can be changed, dependent on the sphere arrangement. Some peaks at the eary part of curve are ceary different from the singe resonant frequency of sphere. These peaks are the resuts of the coective motion of spheres and the cutoff phenomenon in the waveguide. In Eq. (2), if we eiminate the mutipe-scattering terms and consider ony singe scattering term, the noise reduction oss gets totay different. Fig. 3 exhibits the resut of singe scattering approximation at the range of 3.9 m. Compared to Fig. 2, Fig. 3 shows no acoustic ocaization. The singe peak of Fig. 3 is attributed to the singe resonant frequency. -60 r = 3.9 m -50 Noise Reduction Leve (db) ka FIGURE 3. Noise reduction oss obtained from singe scattering terms as a function of ka. The reguar spikes shown in Fig. 2 (a) occur due to the transition of evanescent mode to propagation mode for increasing the frequency. In the reaistic oceanic waveguide, such eccentric trends may be weak due to energy penetration through ocean bottom. In Fig. 2 (b), the noise reduction curves of the free fied and the haf-space become same whie the idea waveguide case shows more compex fuctuation due to the highy osciation of horizonta phase term. Aso, it is observed that the energy inhabitation occurs beow ka = This frequency coincides with the first cutoff frequency of this waveguide. FIGURE 4. Sound pressure eve with and without the bubbe screen as a function of the range for three frequencies of 75, 150, and 300 Hz. (a) 75 Hz, (b) 150 Hz, (c) 300 Hz. Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 4

5 Lee et a. Fig. 4 pots the sound pressure eve in the idea waveguide for the range at the depth of 5m and three frequencies of 75 Hz, 150 Hz and 300 Hz. At the ower frequency where the acoustic ocaization occurs, it is cear that the screen of soft sphere reduces the sound pressure eve. Moreover, two curves in each figure show the simiar interference pattern. This is because the number of propagation modes is few and the interference structure is simpe. In case of 300 Hz, the effect of noise reduction is negigibe or worse for the range. FIGURE 5. Depth versus sound pressure eve with and without bubbe screen for three frequencies of 75, 150, and 300 Hz. (a) 75 Hz, (b) 150 Hz, (c) 300 Hz. Fig. 5 pots the sound pressure eve for the depth in the idea waveguide. In Fig. 5 (a), two curves show same behavior except the ampitude difference in SPL. In this case, ony one propagation mode exists. As the frequency increases, the depth-interference pattern of SPL varies graduay and Fig. 5 (c) ceary shows the depth-dependency of the noise reduction oss. At the depth of 1.5 m, the noise reduction oss is approximatey 4 db. But, this abiity of the bubbe screen disappears at the depth of 3.5 m with the noise reduction oss of -10 db. CONCLUSION The sef-consistent equation for mutipe scattering in the free fied is generaized for the oceanic waveguide. The derived equation is used to sove the forward wave propagation in the waveguide for the noise source surrounded by the screen of soft spheres. As the study of Ye and Avarez in the free fied, the acoustic ocaization aso occurs in the waveguide. The positions of the peaks in the NR oss are different in each case of the free fied, the haf-space and the idea waveguide, but the highest inhabitation occurs near which ka = The vaue of this magic point is cosey reated to the use of Minnaert formua. Using other formua of the singe resonant frequency such as Church mode, this vaue can be changed. In this acoustic ocaization region, the mutipe-scattering wave from the bubbe screen has out-of-phase with the noise source and the acoustic propagation energy is trapped inside the screen. The modeed resuts show the range and depth dependency of the noise reduction oss. The mode propagation in the waveguide makes the noise reduction curve more dynamic. The proposed scheme is usefu to predict the abiity of the screen damper of bubby type and can be appied to the optimization design of the screen damper. REFERENCES 1. R. Oestman, D. Bueher, J. Reyff, and R. Rodkin, Technica Guidance for Assessment and Mitigation of the Hydroacoustic Effects of Pie Driving on Fish, Caifornia Department of Transportation, Sacramento, A. Stokes, K. Cockre, J. Wison, D. Davis, and D. Warwick, Mitigation of Underwater Pie Driving Noise during Offshore Construction: Fina Report, Appied Physica Sciences Corp., Groton, GS E&C, Survey of Mitigation System of Pie Driving Noise (in Korean), GS E&C, Seou, K. Emer, J. Gattermann, C. Kuhn, B. Bruns, and J. Stahmann, Mitigation of underwater piing noise using baoons and foam eements as hydro sound dampers, Edinburgh, Juy 2-6, in proceedings of the 11 th European Conference on Underwater Acoustics, (2012). 5. K. M. Lee, M. S. Wochner, and P. S. Wison, Mitigation of ow-frequency underwater anthropogenic noise using stationary encapsuated gas bubbes, Edinburgh, Juy 2-6, in proceedings of the 11 th European Conference on Underwater Acoustics, (2012). 6. P. G. Reinha and P. H. Dah, Underwater Mach wave radiation from impact pie driving: Theory and observation, J. Acoust. Soc. Am. 130, (2011). 7. Z. Ye and A. Avarez, Acoustic ocaization in bubby iquid media, Phys. Rev. Lett. 80, (1998). Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 5

6 Lee et a. 8. Z. Ye and E. Hoskinson, Band gaps and ocaization in acoustic propagation in water with air cyinders, App. Phys. Lett. 77, (2000). 9. F. B. Jensen, W. A. Kuperman, M. B. Porter, and H. Schmidt, Computationa Ocean Acoustics (Springer, New York, 2011). 10. H. Medwin and C. S. Cay, Fundamentas of Acoustica Oceanography (Academic Press, San Diego, 1998), pp C. C. Church, The effects of an eastic soid surface ayer on the radia pusations of gas bubbes, J. Acoust. Soc. Am. 97, (1995). Proceedings of Meetings on Acoustics, Vo. 19, (2013) Page 6