Why does a thermostat hold steady, but a microphone near its speaker screams?

A thermostat keeps your room one cozy temperature for hours without you touching it. But hold a microphone near its own speaker and — EEEEEE! — it explodes into a screech in half a second. Both are loops feeding back on themselves. So why does one stay calm and one go wild?

1What a feedback loop even is

A loop, and a little nudge

You only need two ideas. Watch each one:

A loop feeds back on itself

The result becomes part of the next try. The mic hears the room, the speaker plays it, the mic hears THAT, and round it goes. What comes out loops back into what goes in.

Everything drifts a little

Nothing sits perfectly still. A tiny breeze, a small sound, a little bump — every system gets nudged off its target now and then. The loop has to decide what to do about that wobble.

2Two ways a loop can answer a wobble

The bring-it-back loop vs the make-it-bigger loop

When the line drifts off target, a loop can do one of two opposite things:

Push back

The bring-it-back loop

Drifts up? It pushes back down. Drifts down? It pushes back up. Always toward the target. Thermostat, sweating to cool off, a balanced bike.

Pile on

The make-it-bigger loop

Drifts up? It pushes further up. Drifts down? Further down. Always away from the target. Mic into speaker, a snowball, a panic.

3Your turn — feel a push-back loop

Drag the line off target and watch it fight back

Here's one push-back loop, like a thermostat. Drag the slider to shove the line away from the dashed target — then watch the loop tug it back. The harder you shove, the harder it pulls home.

Shove the line off target: right on target
WAY BELOWTARGETWAY ABOVE

4The big test — flip the loop's direction

Same strength, same nudge. Only the direction is different.

Two loops, side by side. They have the exact same strength. The only difference: one pushes a wobble back toward the target, the other pushes it further away. You'll give each the same tiny nudge. Guess first.

Guess before you nudge

Same loop strength on both. One is a push-back loop, one is a pile-on loop. You give each the exact same tiny nudge. Which one settles down?