A Theoretical Study on the Sound Transmission Loss of the Double Wall with Decompressed Air Space

Sound transmission through walls varies with the frequency of the sound and the mass and stiffness of the construction. An effective way of obtaining a large amount of sound insulation for a given weight per unit area is by means of a double wall construction. For simplicity, consider a double wall made of two single panels, each of which alone would have the mass law transmission loss characteristic. Place these two panels m_1 and m_2 parallel to each other at a distance d apart, with only air in space between. Assume that the panels are very large and that they are clamped around the edges in such a way that there is no flanking. The air space between the two panels behaves like a simple spring. The stiffness of this spring is inversely proportional to the spacing. The two panels are simply two masses, one on each end of the spring. The transmission loss of the double wall increases as the separation of the two parts of the double wall increase, provided the distance between them is less than a wave length, and the combined transmission loss of two panels should be the sum of the transmission losses for the separate panels when the distance between them is much greather than the wave length of the transmitted sound. Sound is not transmitted in vacuum space. If the pressure of the space between walls is very small, it is possible for the transmission loss to become even greater than the value as discussed above. In this paper, the authors studied on the transmission loss of the double wall with decompressed air space and the theoretical equations of TL were investigated.