The part of the human ear that vibrates is the eardrum, also known as the tympanic membrane.

Here's how it works:

1. Sound waves travel through the air and enter the ear canal.

2. The sound waves reach the eardrum and cause it to vibrate.

3. These vibrations are then transferred to the middle ear through a chain of tiny bones called the malleus, incus, and stapes.

4. The stapes vibrates against the oval window, which is a membrane covering the entrance to the inner ear.

5. The vibrations in the inner ear stimulate the hair cells in the cochlea, which send signals to the brain.

6. The brain interprets these signals as sound.

So, the eardrum is the first part of the ear to vibrate in response to sound waves.