ME458 Lab 7 8/17/2000 page 1
ME 458 Noise Control
Laboratory #7
Layered Damping Materials
Objectives:
1. Perform basic vibration measurements,
2. Investigate layered damping materials,
3. Determine loss factor using free vibration decay
4. Determine damping loss factor using an impact hammer and FFT analysis
5. Predict natural frequency of a simple beam
Background: Thin sheet metal structures are notorious for radiating lots of noise.
Engine valve covers and oil pans are good examples. An effective technique for
minimizing noise radiation (under certain conditions) is to add an elastomeric damping
layer to the structure. In this experiment, we will measure the loss factor of three
cantilevered beams with different damping treatments. Damping of the beams is
achieved via two different methods: free layer damping and constrained layer
damping.
Equipment: Each beam is equipped with a piezoelectric accelerometer mounted near
the tip of the beam to measure its vibration. The accelerometer output is measured with
an oscilloscope and/or a frequency analyzer.
Procedure:
1) Determine the resonance frequency of the three beams (undamped, free layer
damping, and constrained layer damping) using the impact hammer. Plot the
frequency response of the beams. Calculate the natural frequency of the beam from
its dimensions (using the NASA tables).
2) Determine the damping ratio and loss factor using the Q factor method (width of the
peak between half power points). Look up the appropriate formulas in your ME54
textbook.
3) Determine the critical damping ratio and the loss factor using the log decrement
method for each beam. Look up the appropriate formulas in your ME54 textbook.
Comment on the effectiveness of the damping methods. Compare the measured
vibration values to the expected attenuation at resonance (change in sound or vibration
level due to addition of damping), calculated by:
b
L
η
η
log20=∆
where: η = loss factor after treatment ηb = loss factor before treatment
∆L = change in level (Insertion Loss) (dB)
