This demo deals with sound, of course. Some of the material may be a little advanced for younger kids, but the information can be simplified, so don’t shy away from this just because the kids are young!
We want to start by talking about what exactly sound is. Before you start, ask the students what they think sound is. Their answers tend to end up being synonims for sound (noise, etc), so follow up with a question such as how does sound travel from my mouth to your ear? The goal is to get them to mention waves. If they don’t, just bring it up. You should then ask: What are sound waves made of? After their guesses, explain: Sound waves are air waves. The reason we hear something is because the air around us is vibrating and producing sound waves. Now you want to show them these waves, which is what those slinkies are for. Ask someone to hold the slinky stretched out for you, and firmly but quickly push one end of it towards the other end. You will see a wave travel all the way to the other end. Explain that this is exactly how air moves, only in the case of air we hear it as sound instead of seeing it. You can also use the slinky to illustrate volume (simply shake it both more vigorously and very lightly), as well as echo (shake it hard enough that the pulse reaches the other end and bounces back). Explain that echo is air doing the same: bouncing back.
Next you want to investigate what happens to sound when there is no air. You’ve been mentioning how it’s made of air a lot, so they should get it right when you ask what do you think happens to sound when there is no air? Just collect some answers, and then test this out by using the bell in the jar and the vacuum pump. Turn the bell on, cover it up and run the pump for a while, so that we have as good a vacuum as possible inside the jar. Then turn it off and ask for quiet, and whether anyone can hear the bell. If you have a volunteer, point out to them how the hammer is still hitting the bell, so we know it’s still working. Then remove the hose so as to let the air back in. As the air goes in, we can hear the sound of the bell getting louder and louder. Make sure to explain afterwards: When there is no air, there is nothing to vibrate, so there is no sound. The bell was still moving, but it was not making sound. When we let the air back in, the sound came back. You can also include that this is why there is no sound in outer space (and how all those sci-fi movies are so wrong!).
The next concept we want to get across is pitch, and how it’s related to the speed of vibrations. The first thing we can do is ask a volunteer to put their finger on the little speaker (gently!) and then turn on the frequency generator. While the kid has their finger on the speaker, vary the frequency produced, so they can feel how it tickles in different ways, faster and faster, until it’s so fast they can’t feel it vibrate anymore. Explain: what we call pitch is actually the speed of the air vibrations. The faster it moves, the higher pitched it sounds. You can follow this up with the air pipe. The idea is the same, only that now we’ll use our arms to move the air, rather than a frequency generator. You should do it yourself first, to demonstrate (just spin the pipe in the air, at varying speeds), and then ask for a volunteer to try it out, too. Repeat the explanation: faster movement means higher pitch.
Finally, we can play with the tuning forks. If this is done at an assembly show, absolute silence is needed for this part, since the sound of the tuning forks is very subtle. If you have a microphone, use it. If it’s a science night, bring the tuning forks right up to the kids ears, so they can hear them better. There are two forks, tuned to frequencies only 3Hz apart, so they sound almost identical to us. You can play each one separately and ask if the two sounds are the same or different. Then play both of them at the same time, and we’ll be able to hear the beat frequency. To clue kids in as to what they should be looking for, tell them to listen to the wa wa wa the sound makes. Make sure to point out how the individual tuning forks don’t do that, and it’s only wen both are played at the same time that we hear that. You can explain: these two are not truly identical, only very similar. This is one way of knowing that. Instrument tuners use this fact to tune instruments, because when they stop hearing the wa wa wa they know all the sounds are exactly identical.
One last thing we can do with the tuning forks, again to point out how they are different, is to try and use the resonant cavity with both of them. Only one of them works, because the cavity is built to work with a very specific frequency, so even though the other one is close, it’s not what it should be, and it does nothing. But when we bring the correct tuning fork near the resonant cavity, the sound gets amplified a lot. You should not only point out how it works with only one of them, but also how there is no need to touch the two objects. The vibrations travel through the air in the form of sound, so they reach the cavity all the same. (If you can’t make this work, it’s probably because you’re holding the resonant cavity too much and not allowing it to vibrate. Lightly hold it with two fingers from the middle.)