How do you select the reference location for a modal test? What needs to be considered? Let's discuss this to see how to think about this.

Is it really necessary to reject a double impact?

Are they really a problem?

Let's talk about this.

I know that many people will say that a double impact is totally unacceptable. It is not the optimum condition for collecting impact data for a modal test. However, under certain circumstances, it may be reasonable to accept a measurement that has resulted from a double impact. Let's discuss this problem that may arise when a double impact occurs and explain how to determine if the measurement is acceptable or not.

I ran one test with an x-excitation and can see some modes and another test with a y-excitation and see some different modes. Could I use an oblique angle instead?

Well, that's a very good question. Its one that comes up often in terms of running modal tests with a shaker excitation. Of course it is totally acceptable to run one test with a shaker at some oblique angle to the structure. But the only thing we need to be careful about is to assure that we don't select the reference point at the node of a mode.

Could you explain the difference between time domain, frequency domain and modal space? I hear it all the time but I'm not sure what's the difference.

There's a lot to explain but let's start with something simple.