Question:How do I work out the frequency of a wave.?
You must determine how many waves pass each second.
There may be less that one whole wave, but its the number of seconds that is the important part.
If 4 waves pass you every 10 seconds, then in 1 second only 0.4 waves would pass = 0.4 Hz.
Frequency = Number of whole waves / number of seconds for those waves.