Center frequency when using audio input goes no lower than 48000Hz

[NeuroForLunch]

When sampling from my mac's built in microphone, why does the center frequency not go any lower than 48000Hz and the bandwidth no lower than 500 Hz?

I'd like to use this for analyzing sounds where the very left side is 0 Hz and it goes up from there.

The frequency measurements on top also confuse me because they are all positive even though 0Hz is essentially in the middle even though it says 48000 for the middle frequency.