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January ; 1 : 22— A general formulation of the sound radiation from fluid-loaded rectangular baffled plates with arbitrary boundary conditions has been developed by Berry et al. JASA, Vol. In this paper, an extension of this formulation to inviscid, uniform subsonic flow is considered. The analysis is based on a variational formulation for the transverse vibrations of the plate and the use of the extended, to uniformly moving media, form of the Helmholtz integral equation.
The formulation shows explicitly the effect of the flow in terms of added mass, and radiation resistance.
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Furthermore, it avoids the difficult problem of integration in the complex domain, typical of the wavenumber transform approaches to fluid-loading problems. Comparison of the acoustic radiation impedance with existing studies supports the validity of the approach. The details of the formulation and its numerical implementation is exposed and a discussion of the flow effects on the radiation impedance of a rectangular piston is presented. It is shown that subsonic mean flow increases the modal radiation resistance at low frequencies and affects added mass more strongly than it affects radiation resistance.
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The panels are excited by a 1N harmonic point load at the central node on the free edge opposite to the clamped edge and the SPL is measured at a distance of 0. With similar considerations, the SPL values are computed using the present numerical model and validated by comparing with those experimentally obtained data. The comparison of the experimental and numerical results for SPL of the vibrating flat panel with M1 and M2 material properties are shown in Figure 6 a and b , respectively. It is observed that in both the cases the present numerical values of SPL follow closely the experimental results including the occurrence of peaks and depressions up to the excitation frequency of Hz, after which slight deviation is noticed.
The reason for the deviation may be due to the limitations in applying displacement boundary condition on the structure and the absence of anechoic chamber during experimentation. The coincidence frequencies in Hz for each thickness value are Therefore, a frequency range of , Hz is considered to observe the sonic and subsonic behaviour of plates. The variation of the radiation efficiency and the radiated sound power with the excitation frequency for different values of thickness ratio is shown in Figure 7 a and b , respectively.
It can be observed that the average radiation efficiency increases with increasing thickness ratio. This leads to shifting of natural modes of vibration to lower frequencies and higher coincidence frequencies in the considered frequency range. Also, it is worthy to note that the radiation efficiency exceeds 1 close to the first coincidence frequency and asymptotically decreases to 1 after the second coincidence frequency in most of the cases. The radiated sound power increases with increasing plate thickness ratio.
A similar trend is observed for the sound pressure level in near and far-fields for modes 1,1 and 2,1 of the plates as shown in the SPL directivity plots in Figure 7 c and d , respectively. The plate has the same coincidence frequencies The average sound power level follows an increasing trend with decreasing aspect ratio with the resonance peaks shifting to lower frequencies as shown in Figure 8 b. It is also worthy to note that the radiation efficiency exceeds unity indicating that radiated acoustic power is more than the vibration energy of the plates.
The first coincidence frequency is the same equal to It is interesting to note that the first natural frequencies of the configurations are very close to the first coincidence frequency. It can clearly be observed from Fig. It is can be seen that for every value of modular ratio the curves closely follow each other over the entire frequency range with the peaks and valleys occurring at almost same values of excitation frequency.
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The radiation efficiency crosses unity around the first natural frequency for all configurations of modular ratio and approaches unity after crossing the maximum value of second coincidence frequency. It is well known that, the lay-up scheme of any laminated composite structure significantly governs the stiffness and thus affects the vibration and sound radiation behaviour greatly. The SPL is obtained at a point 1m directly above the excitation location on the plate. It is observed from Figure 10 that the SPL at the field point and the radiated sound power is greatly influenced by the lay-up scheme of the vibrating plate.
It can also be seen that the peaks occur at higher frequencies for cross-ply laminations in comparison to the angle-ply schemes. The stiffness increases as the number of constraints increases or the degree of freedom decreases and this may have a substantial effect on the sound radiation characteristic of the structure. The first and the second coincidence frequencies are The influence of the number of constraints at the support is well reflected in the results.
It is observed that, over low excitation frequency range i. In this section, a case study has been presented to compare the vibro-acoustic behaviour of the laminated flat panel of different composite materials those are widely used in their key areas of application. Four different composite materials namely, graphite-epoxy, boron-epoxy, kevlar-epoxy and glass-epoxy are considered for the present analysis and their properties are listed in Table 7.
For the computation purpose a simply supported rectangular 0. The SPL directivity pattern for modes 1,1 and 2,1 , the radiation efficiency and the radiated sound power level values are computed for various composite materials using the present scheme and shown in Figure 12 a - d , respectively. In general, the RMS values of a vibrating structure are considered to judge its suitability for a particular application.