Modal testing

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Modal testing is a form of vibration testing of an object where fundamental frequencies of the object under test is determined.

There are several ways to do modal testing. The most widely used are Impact Hammer modal testing and Shaker Modal testing. Image:[1] Impact Hammer Modal Testing When an object is subjected to the strike, it is excited. It will vibrate according to the bandwidth of the impulse. While it vibrates, the object will behave in such a way that some of the frequencies will not be responded at all (attenuated) and some frequencies will be amplified. Some of the frequencies will be amplified in such a way that the only limiting factor is the energy available for the vibration. This is called resonance.

These frequencies where the object resonates are known as the fundamental frequencies of that particular object or the modal frequencies.

Shaker Modal Testing

Another method is by using an instrument called shaker. Shakers are the devices that excites the structure according to the amplified input signal. Several input signals are available for modal testing, but the sine sweep and random vibration profiles are by far the most commonly used signals.

Shaker armature is attached to the body to be tested by way of piano wire (pulling force) or stinger (Pushing force). When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies. These frequencies are measured as modal frequencies.

When the impact hammer or the shaker stinger is fitted with the device called a load cell, one can measure the excitation signal. The response signal is acquired through an accelerometer fitted to the object. Comparing these two signals in the frequency domain (By calculating the transfer function) gives one the idea how the object moves or vibrates at modal frequencies. If adequate measurement points are available, mode shapes of the object can be evaluated from the transfer functions.