On the arduino module, I set it to "tell" the rmp3 to getspectrumanalyser(), which will then return a frequency value to my arduino module, and if it is above a certain frequency will trigger and output to a motor.
Yes - that's the general idea but it does not return a frequency - it returns a value for the different frequencies you have specified. You can specify up to 23 different frequencies and retreive the values of them
Can this be used to trigger an action for groups of different ranges of frequencies, a low frequencies group, medium and high? Rather than just if the frequency goes above a set value?
Not quite sure what you mean. Perhaps my comment above addresses this.
You can do whatever you like with the values - You could choose just to analyse low frequencies, high frequencies etc if you wanted.
Mowcius