Sound waves?

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I’m assuming this is the reason you “hear the ocean in a shell” or hear chimes when circling a wine glass. Is the only reason we can hear these is because of the phenomenon of putting them right next to our ears? Are there any weird facts based on age, etc?

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Yeah, it’s just the shape of the shell. The “ocean” sound you hear is really just the sound around you being altered by the acoustics of the shell and the unique shape gives it a wide variety of resonant frequencies.

Sound waves are a type of energy that travels through the air, or any other medium, as a vibration of pressure.

These both have to do with the shape.

In the case of the shell, it’s sound waves coming from elsewhere, reflecting around the inside of the shell, and into your ears.

In the case of the wine glass, the friction from your fingers is causing the wineglass to vibrate. It’s like when you flick a spring and it bounces back and forth, except the glass is very stiff and so the vibrations are high enough pitch to be heard as sound.

Okay, settle in because there’s a lot in this (depending on the level of detail you want)!

A “sound wave” is really just a type of wave that can be heard. So basically any time you hear anything, it’s a sound wave. (Exceptions are tinnitus, and electrical stimulation – which is how people with cochlear implants hear – but those are both very different things!)

So what is a sound wave. Well, [here](https://youtu.be/7EaQ5CNPJv0) is a really janky but informative video to explain it. (I know the guy that made it, and he’s well aware of how cheesy it is. But hey, it was aimed at kids for a science festival!) Basically, when stuff vibrates, it bumps into particles (bits of stuff) in the air. This makes pockets of stretched air, and pockets of squashed air. The stretching and squashing gets passed along by particles bumping into each other. It’s less like a wave in water, and more like what happens if you thrust a stretched slinky. The wave travels through the air until it gets to your ear, goes along the ear canal, and then pushes and pulls your eardrum. This moves some bones, which move another stretched membrane, and using ~~magic~~ science it gets turned into an electrical signal in the cochlea and fires up to the brain.

Now, we need to talk about resonance, which is pretty key to your two examples. [Here](https://youtu.be/dihQuwrf9yQ)’s a good video on that. Anyway, stuff likes to vibrate at particular frequencies. If you push a swing, it will go at a certain rate of back and forth. It doesn’t matter how hard you push it, it will take the same amount of time to swing. All your pushing does is change the height (intensity, or *amplitude*) of the swing. You can check this out by tying some kind of weight to a string, hanging it over a support (or your finger) whilst holding the other end, and letting it swing. If you change the length of the pendulum, it will change frequency. A shorter pendulum will swing quicker, and a longer one will swing slower. This is why the pendulums on grandfather clocks are all the same length! This is the resonant frequency of the pendulum. When you push a kid on a swing, you need to push them at the right point. This is you adding energy into the swing, which lets it increase the amplitude *but keeps the frequency the same*.

So we’ve had sound and we’ve had resonance. Now we can combine them. When you flick a wine glass, you chuck a bunch of energy into it. It uses this to vibrate at its resonant frequency. The circular rim of the glass deforms, with the front and back squashing together and the sides bulging out, then the sides squashing together and the front and back bulging out, hundreds of times a second. The bits of air beside the glass get pushed and pulled, and they pass this on to their neighbours, and we hear this as a sound wave. The analogue to pushing a kid on a swing would be using sound waves at the resonant frequency of the glass to shatter it. Yes you really can [break a wine glass](https://youtu.be/0o6-zmEHVGw) with [just your voice ](https://youtu.be/X6iJ0hPpGec). You just need to push and pull the glass with the right sound wave that it vibrates more and more until it rips itself apart.

But what you were talking about is slightly different. It’s more like how a violin works. The bow drags along the string. Friction means the string gets pulled with the bow, bending to the side. Eventually, the tension overcomes the friction and the string slips past the bow and pings back. Then friction pulls it again, then it slips again, etc. This happens hundreds of times a second, and these vibrations pass on to the air, etc, until we hear it. This is called stick-slip friction, and is how those woodpecker toys that slide down poles work. (I think there might be one [here](https://youtu.be/k0oNkO2YI-w) but I’m not sure.) When you drag your finger along the wine glass, the same thing happens. Your finger pulls the wine glass a little, then it slips, then it sticks, then it slips. This makes the glass vibrate at the resonant frequency, and you get the sound.

The seashell is also resonance. Here, the complicated shape means it actually has several different resonant frequencies. When we add together different frequencies, we hear this as [tone](https://www.reddit.com/r/explainlikeimfive/comments/o2kwbj/Eli5_how_exactly_speakers_recreate_let%27s_say_a_human_voice._I%27m_not_talking_about_analog_or_digital_conversion%2C_i_know_a_bit_about_those_subjects._But_the_physical_act_of_a_speaker_cone_moving_back_and_forth%2C_perfectly_recreating_a_human_voice_or_any_other_kind_of_recording_seems_like_magic_to_me/h274zz6/?utm_medium=android_app&utm_source=share&context=3). The different curves and turns in the shell help different frequencies to resonate, and we hear this as the distinctive wooshing sound.

As for if there are any other interesting things, yes, loads! Sound is really interesting. I did a PhD on sound localisation, which means I spend like 3/4 years researching [how we work out where sounds come from](https://www.reddit.com/r/explainlikeimfive/comments/ldzj17/ELI5%3A_In_8D_audio%2C_how_is_the_audio_able_to_sound_like_it%27s_behind_or_in_front_of_you_even_though_there_are_only_left_or_right_outputs%3F/gm8vmzd/?utm_medium=android_app&utm_source=share&context=3). And it was just one aspect of that. There’s so much cool stuff out there!

You specifically asked about age. Well, it’s no secret that we tend to get a bit deafer as we age. The way the ear is shaped, the bit that picks up the higher-pitched sounds is nearer the outside and the lower nearer the inside. So the higher part takes more of a battering, meaning we generally lose the ability to hear higher sounds first.