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The creation and propagation of sound waves are often demonstrated through the use of a tuning fork. A tuning fork is a metal object consisting of two tines capable of vibrating if struck by a rubber hammer or mallet. As the tines of the tuning forks vibrate back and forth, they begin to disturb surrounding air molecules. These disturbances are passed on to adjacent air molecules by the mechanism of particle interaction. The motion of the disturbance, originating at the tines of the tuning fork and traveling through the medium (in this case, air) is what is referred to as a sound wave. The generation and propagation of a sound wave is demonstrated in the animation below.

In some demonstrations, the tuning fork is mounted on a sound board. In such instances, the vibrating tuning fork, being connected to the sound board, sets the sound board into vibrational motion. In turn, the sound board, being connected to the air inside of it, sets the air inside of the sound board into vibrational motion. As the tines of the tunig fork, the structure of the sound board, and the inside of the sound board begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles which can be made to vibrate, the louder or more amplified the sound.

In the tuning fork demonstrations, we know that the tuning fork is vibrating because we hear the sound which is produced by their vibration. Nonetheless, we do not actually visibly detect any vibrations of the tines. This is because the tines are vibrating at a very high frequency. If the tuning fork which is being used corresponds to middle C on the piano keyboard, then the tines are vibrating at a frequency of 256 Hz - 256 vibrations per second. We are unable to detect vibrations of such high frequency. But perhaps you recall the demonstration in which a high frequency strobe light was used to slow down the vibrations. If he strobe light puts out a flash of light at a frequency of 512 Hz (two times the frequency of the tuning fork), then the tuning fork can be observed to be moving in a back and forth motion. With the room darkened, the strobe allows us to view the position of the tines two times during their vibrational cycle. Thus we see the tines when they are displaced far to the left and again when they are displaced far to the right.

A distinction is made between two categories of waves: mechanical waves and electromagnetic waves. Electromagnetic waves are waves which have an electric and magnetic nature and are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy. Mechanical waves are waves which require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through regions of space which are devoid of particles. That is, mechanical waves cannot travel through a vacuum. This feature of mechanical waves was demonstrated in class using a segment from a laser disc. A ringing bell was plaed in a jar and air was evacuated from the jar. Once air was removed from the jar, the sound of the ringing bell could no longer be heard.