Sound of Silence

I found this video on WIMP.COM. I figured out pretty quickly that the loudspeaker must be vibrating in sync with the camera. Digital camera's are much faster than film cameras, which is why you can see the individual drops of water, rather than just a blur (same reason that in digital pictures of prop driven aircraft 
the propellers appear to be stopped, whereas in pictures taken with film cameras, the propellers appear as a blur).

But if the speaker is vibrating (it must be, he even turns up the volume!) why can't we see that, and more importantly, why can't we hear it? We can hear the water running, why can't we hear the loudspeaker? So I poked around a bit until I found this explanation:

. . . the camera is shooting at 25 frames per second, which is exactly the same rate that the tube carrying the water is being vibrated by the speaker. 

25 cycles per second is the rate for the camera AND the loudspeaker. 20 cycles per second is the lower limit of hearing for most people. I'm not sure YouTube will even transmit a frequency that low, and even if it did, I am not sure my speakers could produce it. If it was turned down low, you might not even notice it if you were in the same room.

We can't see the speaker vibrating for the same reason that the water droplets appear to be standing still: the camera is shooting frames at the same rate as the speaker is vibrating, so each time the camera takes a pic, the speaker is in exactly the same position as it was the last time.

If you changed the frequency of the sound (or the camera) just slightly, the speaker would appear to move slowly in and out, and the water droplets would appear to be falling, or even rising back up.