Absorption of structure-borne sound in building materials without and with sand-filled cavities

The loss factor of bricks and reinforced concrete has been measured as function of frequency and vibration amplitude. The absorption of flexural waves on walls, computed according to the results of these measurements, is 20 to 100 times smaller than the actual absorption of structure-borne sound in buildings, the latter being of the order of 1 to 3 db/m. This absorption has been measured by means of piezoelectric transmitters and pick-ups for structure-borne sound in the frequency range 150 to 3000 c/s.

Using hollow stones filled with sand or other granular substance for walls and floors, this absorption increased by 4 to 6 db/m for middle and high frequencies. This additional damping begins at low frequencies if the cavities are large, if the granular substance has sharp edges, or if a mixture of a hard with a soft substance is used. The acoustic impedance of a closed volume of sand has been measured by means of a vibrometer in order to give a physical explanation of the sand damping. In addition, the damping of short beams with filled cavities of different sizes and the absorption at long walls with filled cavities have been investigated, the results being in good agreement with each other.