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ELI5 Yahaaa

Pushing in time

Why Does One Note Rattle the Window?

Everything has a natural rhythm, and a sound matching it adds a push on every single swing.

Everything has anatural rhythmA matching note pushesin timeEach push adds to thelastThe movement builds up
Push a swing at the right moment and it climbs.

  1. Step 1 of 5

    Every object has a rhythm it prefers.

    1 Tap a glass —it rings2 Tap a window —a lower note3 Every objecthas its own

    Tap a glass, a window or a ruler over a desk edge and it vibrates at its own particular rate, decided by its size and stiffness.

  2. Step 2 of 5

    A swing shows why timing matters so much.

    Pushing at random You cancel your ownworkPushing in rhythm It builds every time

    Push a swing at random moments and you fight it as often as you help. Push exactly as it starts moving away and it climbs.

  3. Step 3 of 5

    A matching sound pushes on every wave.

    A note at thematching rate Every push landsin time

    Sound is a series of pressure pushes. If those arrive at the object's own rhythm, each one adds to whatever movement is already there.

  4. Step 4 of 5

    Small pushes build into large movement.

    One wavebarely moves itFifty waves in timeclearly rattlingHundredsloud and obvious

    No single sound wave does much. Hundreds arriving in perfect time accumulate into a vibration large enough to hear and feel.

  5. Step 5 of 5

    Nearby notes do almost nothing.

    The matching note — hugeeffectA note slightly off —almost nothing

    Change the pitch slightly and the pushes drift out of step, cancelling as often as they help, which is why only one note rattles.

    Soldiers break step crossing bridges, because a marching rhythm matching the bridge's own could build dangerous movement.

The short version

Resonance happens when a sound matches an object's natural rhythm, so every wave pushes in time and small pushes build into big movement.

Try it yourself

Hum slowly up through your range near a window pane. At one particular pitch it will buzz noticeably, and nowhere else.

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