It has to do with the framerate of the video. Remember the old movies where a cart would go by but it would look the wheels weren't moving, because the framerate of the film was similar to the rate that the wheels turned?
Here you've got a very low soundwave being pumped out. Your average human can hear bass sounds as low as 30 Hz, so this wave is even lower than that, a very slow wave. The wave is passing through the water and making it pulse at the same rate, 24 times a second. The video is also being recorded at 24 frames a second, so the film captures the pulses of the wave at the exact same spot each time.
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Here you've got a very low soundwave being pumped out. Your average human can hear bass sounds as low as 30 Hz, so this wave is even lower than that, a very slow wave. The wave is passing through the water and making it pulse at the same rate, 24 times a second. The video is also being recorded at 24 frames a second, so the film captures the pulses of the wave at the exact same spot each time.