Vibrations and hearing

[Bernard NebelKeymaster]

Ask: How does a vibrating rod or string affect the surrounding air? Have students analyze how each oscillation of a vibrating string necessarily pushes and “pulls” on adjacent molecules of air. They push and “pull” on the molecules next to them, and so on potentially for considerable distances.

This domino effect of molecules pushing and “pulling”one to the next, although they are not attached, can be demonstrated with a device called a Newton’s cradle as seen in the following video:

Again, have students analyze the process in terms of transfer of energy one ball or molecule in air to the next. When these vibrations reach the ear, they push and “pull” on the ear drum accordingly. In response to the mechanical movement of the ear drum, nerve impulses are sent to the brain which interprets them as sound.

Type into your browser: anatomy of the ear diagram

The human ear is capable of responding to vibrations in the range from abut 20 to 20,000 hertz, the higher the frequency the higher the pitch. (Dogs and other animals can hear even higher (and lower) frequencies. The following video produces sound according to Hertz. You are welcome to test your hearing. Can you/your kids hear the full range 20-20,000 Hertz? Most people can’t.

The following table gives how the total scale (20-20,000 Hertz) is divided into octaves and notes:

http://www.liutaiomottola.com/formulae/freqtab.htm

Whistles and pipes

Whistles and pipes have no parts that vibrate. Ask: how do they produce sound? Guide students to consider how air is elastic. If it weren’t, it would not transmit sound waves as it does. The secret of a whistle or pipe is that air blowing past or through an opening may set up and cause a vibration of the air itself–a rapid compression, decompression vibration of the air within the whistle/pipe. The pitch is determined by the size and shape of whistle/pipe.

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