Applet is designed to show user how a particular sound can be transformed into a completely new sound by playing that sound repeatedly over a relatively short amount of time. The concept involved is called frequency modulation. In this experiment, the sound of a stick hitting a fence post repeatedly (when being swept across a fence) creates a whole new sound wave, whose frequency is a function of the distance between posts and the speed of the stick.
To try this
simulation:
To try this simulation:
• Left-click the start/stop button to start or stop the sweeping stick.
• Left-click the stick speed button to switch control of the sweeping stick's speed to the group of three buttons labeled "1x", "2x", and "3x". In this mode, the number of posts in the fence and the distance between posts remains constant.
• Left-click the post spacing button to switch control of the spacing between posts in the fence to the previously-mentioned group of three buttons (labeled "1:1", "1:2", and "1:3"). In this mode, the stick speed remains constant.
What's Going On?
All sound is a result of vibrations. As you walk along the fence, dragging the stick on the fence slats, you are creating vibrations. Each time the stick strikes the fence, you get a nice "Tap!" As you continue to walk, you get a series of taps in succession. The time between the taps is controlled by the distance between fence slats and the speed at which you are walking. This time is called the period of the vibration.
When the slats on the fence got closer together, the sound was a higher pitch. Since the slats are close, the taps happen more frequently. The sound you hear depends on how frequently the taps occur. The pitch is called the frequency of the sound. You get the same effect of changing frequency if you move faster, too.
There's More!
You might have noticed this effect while riding in a car across a bridge with a grating roadway.
As the car goes across the bridge, the tires run over the ribs of the grating and make a noise each time a rib is struck. This creates a humming sound, the frequency of which is controlled by the speed of the car and the spacing of the ribs. Can you think of other examples of sound being produced in this way?
