The dynamic stiffness is the frequency dependant ratio between a dynamic force and the resulting dynamic displacement. Similarly, the well-known static stiffness is the ratio between a static force and the resulting static defl ection.
For linear elastic materials the stiffness, below a certain loading, will be independent on the load. For a porous material, like Rockwool, the stiffness will be dependant on the load.
When a material is cyclically compressed, some energy is lost during each cycle. The loss factor describes the fraction of the stored energy that is converted to heat during each cycle.
A high loss factor is beneficial as the propagation of vibration is reduced. This is of interest, for example, when using Rockwool as core material for panels. The dynamic stiffness is given per unit area for a given thickness of the Rockwool.
The dynamic stiffness will normally increase when the static load on the Rockwool is increased. Determination of the dynamic stiffness is normally done following EN 29052-1 (1992) that corresponds to ISO 9052-1 (1989).
The natural frequency of the two degrees of freedom system with the Rockwool acting as the spring is measured. The stiffness of the Rockwool is then determined from the measured natural frequency.
Using ISO 9052-1 provides the dynamic stiffness at one frequency, i.e. at the natural frequency of the system. The set-up can also be used to determine the loss factor by measuring the response at several frequencies.
When the stiffness as function of frequency is needed, the procedure described in ISO 10846-3 (2002) can be used. The same mechanical set-up can also be used for measurements according to ISO 10846-3.
The dynamic stiffness is an important parameter when optimising floating floors. This is true both with respect to air- and structure-borne noise, as well as impact noise.
The dynamic stiffness is infl uenced by the density of the Rockwool and the manufacturing process.
In principle, the dynamic stiffness should be as low as possible, in order to obtain a low natural frequency of the floating floor. On the other hand, the load bearing capacity and the possibly unfavourable experience of walking on a floor that is too soft may set a lower limit for the stiffness.
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