空気層の分割された2重壁の透過損失

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Many double wall systems have been used as light weight sound insulators, but their transmission losses (T. L. ) are little at a lower frequency in comparison with too large at a higher frequency. It is wellknown that the T. L. of a single wall takes the mass law. But the measured T. L. of a double wall is larger than the calculated T. L. , assuming that the T. L. is maximum for a infinite double wall at any incident angle. To study the difference between measured and calculated T. L. and possibility to improve T. L. at lower frequency, the vibration level on both the walls and the sound pressure level in the airspace were measured on double plaster board walls, 0. 1, 0. 2, 0. 4 m in thickness. The difference in the measured values between maximum and minimum was more than 20da in the airspace, because the lateral standing wave was excited. The distribution of vibration on the receiving wall was correlated with that of the sound pressure in the airspace (Fig. 4). The natural frequency f_imn in the finite airspace of double wall is higher than f_imn' in the case of the rigid walls. Its relation is shown in Fig. 5. In the finite sized airspace, the sound wave must be in modes, in Eq. (4), in order to satisfy the boundary condition at the edge. Any incident wave can be developed in the mode which will satisfy the condition of orthogonarity. Assuming that the radiation impedance is equal to the infinite wall and the impedance of the wall is mass reactance only, the vibration and the sound pressure are in the same mode. The T. L. can be calculated for each mode as shown in Eq. (10) and Fig. 8. For the oblique and random incidence, the T. L. of the double wall with finite sized airspace is presented by the summation of each mode as shown in Eqs. (11) and (13), which are larger than the infinite in the frequency range from f_0 to f_1, 0 at least. The measured T. L. of double plywood walls (Fig. 13) are shown in Fig. 12. Plywood partitions in airspace increase the T. L. 4-8 dB. Agreement between measurement and theory was qualitatively obtained as in Fig. 11. As for the double wall, it was shown that an improvement in T. L. is possible by the partition in the airspace.
社団法人日本音響学会の論文
1974-04-01

空気層の分割された2重壁の透過損失

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