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Chapter 10: Waves
1.

Wave Pulse

When you clap your hands, a disturbance is created in the air which travels at 343m/s and which our ears experience as sound. Note that it isn't the air that moves at 343m/s— this would be much faster than the fastest hurricane!— but the disturbance in the air. This is called a pulse.

Pulses exist in a material (called a medium) made up of individual components (atoms, molecules, etc) that act as oscillators: they have an equilibrium point where they like to be, and if they are moved away from that equilibrium point then they feel a force to return them to that restoring force. Neighboring oscillators are connected together, so when one is displaced, that displaces the next one, and so on.

A wave pulse is called transverse if the medium is disturbed in a direction that is perpendicular to the motion of the wave. In a longitudinal wave pulse, on the other hand, the medium moves back and forth in the same direction as the motion of the wave.

The speed of the pulse doesn't depend (much) on any properties of the pulse itself, but it depends entirely on the medium the pulse is travelling through. If the medium is "stiff", and really likes being in the shape it's in, then the speed will tend to be higher because the restoring forces are higher. If the medium is dense, on the other hand, then the oscillators will have more inertia and take longer to oscillate, and the speed of the pulse will be smaller.

Example

Sound pulses (which are longitudinal pulses) travel in air at 343m/s, which is pretty fast, but they travel much faster in solids: in steel, the speed is almost 6000m/s! Air is very "squishy", and while it does want to return to its equilibrium position it's not in any particularly hurry to do it. Steel, on the other hand, is stiffer, with a definite shape and crystalline structure that its atoms want to be in, and when you distort it with a pulse those atoms are going to snap back into place quickly.