|Damping Structure-Borne Sound
Structure of metal, wood, concrete and so on generally have very low internal losses. In other words, their ability to convert vibrational energy into thermal energy is poor. This means that resonant vibrations that may occur when a structure is subjected to vibration and structure-borne noise, are of high amplitude, and this usually also results in the radiation of considerable air-borne noise.
The internal losses in a material or structure are commonly represented by the loss factor h, which is a measure of how much of the vibrational energy is converted into heat. For structures made up of several layers, the combined loss hcomb is used.
The loss factor is a property of materials in just the same way as the modulus of elasticity and the density. The highest realistically possible figure for the loss factor is around 1.0. For sheet metal structures, it is usually between 0.001 and 0.01; in other words, the internal losses are negligible. However, the loss factor can be improved by using sound and vibration - damping materials.
Fig.1. Graph of loss factor against frequency for
Paragon’s MPM .020/.004/.020 + 20?C (68?F).
The most common methods for this are extensional layers and sandwich construction. MPM panels are an example of sandwich construction and can, in optimum conditions, achieve loss factor figures above 0.5.
Results as good as this are seldom obtained with extensional layers. This is because these undergo elongation/compression when subjected to bending waves, whereas the inner layer of MPM panels is subjected to shear. Deformation in shear converts larger amounts of energy than elongation, giving higher energy losses, and this results in a higher loss factor.
Unlike for extensional damping,thick constructions can be given high loss factors using MPM panels.
Fig. 2. When subjected to bending wave vibrations, extenional layers undergo
elongation/compression; he inner layer of MPM panels is subject to shear.
The properties of all damping materials are dependent to a greater or lesser extent on temperature and frequency. Fig. 3 shows the temperature-dependence of the loss factor at 200 and 1000 Hz for Paragon’s MPM .020/.004/.020. Fig.1 shows the loss factor as a function of frequency at +20?C (68?F) for the same panel.
Fig. 3. Loss factors of MPM .020/.004/.020 at 200 and 1000 Hz