The Serial output on an Arduino is quite slow. By printing your output, you will slow down the readings so much that it will appear to be nonsense.
You should collect a series of readings into a C array of floats in RAM, stop recording, and then log that array of values.
Note that you don't have a lot of RAM to work with on most Arduinos, so you will only be able to record a small number of values. A Mega 2560 has a lot more memory than others (8KB). It's still not a lot, but better than other boards. One of the ARM based Arduinos will have both a much faster processor and a lot more RAM.