Why Does a Snare Drum Have Wires Under Its Bottom Head?

When the batter head on top of a snare drum is struck, air and the shell transmit vibration to the thinner resonant head below. A row of metal snare wires resting against that head repeatedly contacts its surface. This turns the rounder tone of the membranes into a crisp, brief rustle containing many inharmonic frequency components. The instrument’s identity therefore comes not from the shell or either head alone, but from a coupled system of two heads and wires.

Release the side lever so that the wires move away from the bottom head, and the same stroke begins to resemble an ordinary double-headed drum: its sense of pitch becomes clearer while the sharp edge disappears. Wires that are too loose rattle for too long; wires pulled too tightly may fail to follow small vibrations, making soft playing less responsive. The aim is not the largest possible crack, but a response suited to the dynamic, and a roll in which many strokes join into a sustained yet still granular sound.

In an orchestra, a snare drum can remain audible through a dense texture not simply because it is loud, but because its steep attack and complex spectrum contrast with the more definite pitches of strings and winds. Listen past the rhythmic point of each stroke into its tail: that is where the wires turn an instant into a brief texture.

https://www.pbslearningmedia.org/resource/meet-the-snare-drum-video/percussion-omaha-symphony-virtual-instruments/
https://www.britannica.com/art/snare-drum