| MadSci Network: Other |
As sound waves pass through a medium, a certain amount of energy is dissipated by the medium. In the case of the glass, small amounts of various elements are added to the glass. The elements act to disperse the energy of the sound waves as the sound moves through the glass and dampens the sound to inaudible levels.
Metals are also used for damping sound and vibration. For example, cast irons are used in drain pipes in homes because the cast iron can absorb up to 20% of the sound. Other metals often used include lead and manganese - copper alloys.
ADVANCED
Damping comes in two forms: anelastic and hysteretic. Anelastic dampening
is caused by a slight delay between the application of a stress (load) and
the attainment of an equilibrium strain (displacement). For sound
dampening applications, the sound waves striking the material induce small
stresses in the material. The amount of anelastic dampening varies by the
frequency of the application of the stress and reaches a peak at a given
frequency that is characteristic of the material.
Hysteretic dampening is caused by the stress-strain curve of a material differing upon unloading from the loading stress-strain curve. The amount of dampening can be very high. This type of dampening is particularly of interest for large machinery and other sources of loud noise. The hysteretic dampening behavior is not dependent on the frequency of the vibration, but does depend on the peak load attained (how much hits). The actual behavior is very complex and dependent on the operating conditions.
The dampening capacity of various metals can be found in Chapter 15 of Smithells Metals Reference Book, Sixth Edition, E.A. Brandes, Ed., Butterworths, London, England, (1983). Other materials reference books also contain information for more materials. ASM International and TMS are good sources for these types of books.
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