Multiple applets in a page

Here is a simple example of the use of TheCurves multiple times in one page. One could argue that using image files is more robust. The advantage is that with the applet, one needn’t generate the files. If you change your idea how you want the plots to look, you change the code in the page.

A moving sound source

If the sound source is not moving, the waves of sound move outward from the source in concetric circles.

If the source is emmitting a pure tone, these circles will be evenly-spaced, and any stationary listener will hear the same pure tone coming form the source (nearer listeners will perceive the tone as being louder).

If the sound source moves, the waves still move out in circles, but from the position where the source was when the wave was emmitted.

The effect is that the waves are bunched up in front of the sound source and spread out behind it. To a stationary listener, the sound seems higher in pitch before the sources passes and lower after it passes.

If the speed of the sound source is equal to the speed of the sound waves (that is, the speed is sonic) then the sound waves in front of the source go no distance before the source catches up with them. They bunch up on one another at the source.

If the speed of the sound source is higher than the speed of the sound waves (that is, the speed is supersonic) then the source immediately overtakes the sound waves in front of it.

A listener hears nothing until some time after the sound source has passed. Very suddenly, they will hear the sound, but very strongly compressed at first and then rapidly decreasing to a low pitch.

This is only part of the story behind the sonic boom: the intensity of that phenomenon is due largely to a shock wave produced by the supersonic motion of a body through air (in addition to the sound being otherwise emmited by the body). But the sonic boom moves as a wave much as do the normal sounds being produced by the source, and so is likewise inaudible until some time after the source has passed the listener.

A common misconception about the sonic boom holds that it happens at the time and place an aircraft “breaks the sound barrier”. In fact, the sonic boom follows behind the aircraft as long as it is travelling at supersonic speed, and different listeners on the ground hear the boom as the aircraft passes them regardless of where the aircraft was when it began to travel at supersonic speed.