For those of you wondering how this is happening, YouTube user Necraduq left a semi-helpful explanation:
Explanation: When the frequency of the speaker is the same with the frequency of the video frames (frames/sec) the camera actually captures the same frame everytime and thus the same water shape.
You can see the change 1 hz makes when filming (if filming is kept at the same rate 24fps) as this translates into a ‘forward movement’.
Imagine a clock with only a seconds indicator. If you would close your eyes and open them precisely once every 60 seconds you would think the indicator is not moving.
Can anyone explain it better? Leave a comment below.