Frequently Asked Questions
How do I tune an absorber for the right frequency?
The easiest way is by trial and error. Remember that the spring stiffness of the 'stalk' varies inversely with the cube of its length (see the formula in the structures data book giving the end deflection of a cantilever loaded at its tip). The absorber will work quite well even if it doesn't resonate at exactly the right frequency.
How do I measure the damping rate of my absorber?
By estimation. A simple way is to 'twang' the absorber, then count the number of oscillations before the amplitude halves. Then look at the 'step response' graphs on p6 of the mechanics data book.
How do I change the damping rate for my absorber?
The rate you've got is probably about right! It's not easy to reduce it, but it could be increased e.g. by putting a plastic sleeve on the 'stalk', or by adding a soft washer at the mount.
How do I measure modal shapes?
There's probably no need to. The bottom line is that a vibration absorber with a given mass is most effective when mounted on a floor which has a large amplitude, and is ineffective when the floor is a 'node'.
How do I find the damping rate of each floor?
That doesn't really have a meaning - each mode of vibration has its own damping rate, which can be measured by the 'half power' method used in experiment A1.
How do I adapt the matlab model to show different resonances?
It's easier to set up each resonance mode as a separate 1 degree-of-freedom model with its own effective mass, spring and damper. The behaviour of the whole building can then be obtained by adding the behaviours of the individual models (assuming the building is a linear system).
How do I model multiple vibration absorbers?
By treating each mode of resonance as a separate model as described above, and then adding the appropriate absorber, as you did in experiment A1.
© Cambridge University, Engineering Department
Last updated 12/10/2012 by hrs@eng