Recording of Simple Sine Wave
Attached is a simple recording of a Slam Stick when being excited with a 100 Hz, 10g sine wave. The recording is 60 seconds long.
It can be helpful to analyze this type of recording in the frequency domain to highlight how difficult it is to get a FFT to exactly match up with your expectations. Even though the data shows a very clean 10g 100 Hz signal, the FFT and frequency domain won't be able to exactly see that.
There are two reasons for this. One, there is some mechanical and electrical noise in the system that goes into some other frequency "bins." The other reason is that because the FFT needs to produce a Fourier series across discrete frequencies. These discrete frequencies won't perfectly fall on 100 Hz as shown.