Materials:

• Speaker
• Vacuum jar
• Frequency generator or amplifier
• Vacuum pump (use large vacuum pump on cart for best results)

Setup:

1. Position and wire the speaker in the jar on vacuum plate (add some vacuum grease if necessary for tight seal).
2. Connect the leads from the vacuum plate into the Amp or frequency generator.
3. Connect vacuum plate outlet to pump with rubber tubing.
4. Turn on the speaker to create sound waves and listen to them diminish as the air is evacuated.

Demo:

Turn on the frequency generator and adjust the amplitude until you can hear the tone clearly. At a frequency of about 750Hz it actually resonates with the jar and can be heard very clearly. Once you are satisfied that the class can hear the speaker under normal circumstances, turn on the vacuum pump to evacuate the air. Using the large grey vacuum pump, a good vacuum should be created almost instantly (a few seconds is all that is necessary).

Once a vacuum has been created in the jar, the speaker will (ideally) not be heard entirely. Because this is an imperfect system and it is impossible to get a perfect vacuum with the materials we have, you will continue to hear some noise but it will be substantially decreased from before.

Once you are satisfied that the class cannot hear the speaker in a vacuum you can slowly release the vacuum inside the jar with the vacuum release valve that is on the tubing from the vacuum pump to the jar. Often because the vacuum pump makes a lot of noise while it is working, this is when the students will notice the clearest difference between the speaker ringing inside the vacuum compared to the speaker ringing in normal air pressure.

Explanation:

To understand why things do not make noise in a vacuum, we first have to understand the physics behind how sound travels. Sound, like light and ripples in a pond, travels in waves. Imagine dropping a stone into a pond and watching the ripples move outward. Now imagine dropping the stone if there was no pond there. Obviously there would be no ripples created because there is no water. Sound (unlike light) requires a medium to travel through. In the case of this demo, air is the medium.

Sound waves react somewhat similarly to water waves, but instead of their medium being water, the medium of sound waves is the molecules in the air around us. Sound travels in compression waves, by vibrating the molecules in whatever medium it is being propagated through. Sound however, unlike water waves, is a pressure wave. This means that instead of going up and down with crests and troughs, sound waves transmit energy through repeating low and high pressure areas.

Imagine a tuning fork that has been hit on a table and begins producing sound. The vibrations of the metal fork (even though you may not be able to see the vibrations with your eyes) push and pull on the air molecules in their vicinity. When the tine moves ‘forward’ it pushes the molecules in the air causing a temporary compression in the molecules of air in the area. Similarly, when the tine moves ‘backward’ it causes an area of low pressure so the molecules pull back. These high and low pressure areas are known as areas of compression and rarefaction, respectively.

Because the tuning fork will vibrate at a constant rate and amplitude consistent with its resonant frequency, the distances between the compressions and rarefactions created will be equal in size. The distance between the compressions and rarefactions is the wavelength, and the number of complete wavelengths per second determines the frequency of the sound. The amplitude that the tuning fork vibrates with dictates how much energy is transferred in the waves. The greater the amount of energy that is transferred, the louder the sound will be.

This demo works with a speaker but the physics is basically the same as for a tuning fork. When the speaker vibrates, it vibrates the air molecules around it evenly in all directions, much like the ripples in a pond spread out in a circle around where the stone first hits the water. Sound waves, however, spread out evenly through all space, not just in one plane. They  create pressure waves that spread out in a sphere around where the vibrations initially occur.

Because sound is just a pressure wave, it can travel whenever the thing emitting sound is surrounded by particles, whether those be molecules of gas in the air or water molecules in an ocean. Sound waves can even travel through solids!

Because sound relies on the individual molecules vibrating each other and bumping into one another to travel, the more densely packed the molecules are in the medium, the better and faster the sound will travel. For example, because water is much denser than air, many marine mammals (whales, dolphins, etc) can communicate across far larger distances than they would if they were communicating through the air. Similarly, sound will travel faster through cold air than through warm air because cold air is slightly more dense than warm air.

The vacuum pump that is attached to the jar removes all of the gas from inside of the chamber. Because it removes all (or most) of the molecules of nitrogen and oxygen in the jar there is nothing for the vibrations of the speaker to compress, and therefore the pressure waves have nothing to move through and cannot travel. Even if the vacuum is not perfect, the sound of the speaker will be greatly lessened because there are fewer molecules of air for the speaker to compress, and the molecules are so far apart that they do not interact strongly among one another.

Our eardrums act as simple membranes that can feel pressure waves traveling through the air. They then send signals to our brains that are interpreted as the sounds we hear. If there are no pressure waves for our eardrums, we will not hear a sound.

Written by Sophia Sholtz