What's With the Silent Meow?
Lots of cats open their mouths and look like they’re meowing, but humans don’t hear any sound. Many cat lovers find this “silent meow” utterly endearing, or perhaps, the ultimate manipulation tool: Kitty is trying to convince his person that he’s so desperate he can’t even make a sound to indicate the depths of his angst.
The “silent meow” is actually a real meow, but the sound is above the range of human hearing. Cats can hear sounds as high-pitched at 65,000 cycles per second. Humans, on the other hand, can only hear up to about 20,000 cycles per second, so what sounds like a piercing cry to your cat may be completely inaudible to you.
http://blogs.catster.com/cat_tip_of_...l%20v2%20oct09