Anselm holds one end of a long rope in a gym. He flicks it once, and a single hump races to the far end. Then he shakes the end steadily, and a whole train of humps streams along the rope.
One flick, one hump. Steady shaking, a steady train. The difference is worth a name on each side, and one question sorts every case.
Watch the source, not the rope. The flick disturbed the material once, then stopped.
A wave made by a source that disturbed the material once is called a pulse. The single hump is a pulse.
Now the same rope, but Anselm keeps shaking the end up and down.
The source keeps vibrating, so humps keep coming, evenly spaced, one after another.
A wave made by a source that keeps vibrating is called a periodic wave. The steady train of humps is a periodic wave.
Beata claps once in an empty hall. A short burst of sound crosses the hall. Is the sound a pulse or a periodic wave?
Ask the sorting question: did the source disturb the material once, or does it keep vibrating?
The single clap disturbed the air once, then stopped.
The source disturbed the air once, so the clap's sound is a pulse.
A smoke alarm buzzes steadily until its button is pressed. While it buzzes, is the sound a pulse or a periodic wave?
Ask the sorting question: the buzzer keeps vibrating the whole time.
A source that keeps vibrating makes a periodic wave.
The buzzing source keeps vibrating, so the alarm's sound is a periodic wave.
Anselm snaps a beach towel once, and a single hump runs along the towel. Is the towel wave a pulse or a periodic wave?
Dilys jiggles the end of a pool noodle up and down steadily, and humps stream along it one after another. Is the noodle wave a pulse or a periodic wave?
Beata hears one clap of thunder roll across the valley. She says: 'That sound is a pulse: the air was disturbed once, not over and over.' Is Beata correct?
Which of these is a periodic wave?
What decides whether a wave is a pulse or a periodic wave?
A woodpecker drums on a trunk for two seconds, taps evenly spaced, then stops. While it drums, which description fits the wave in the wood?