I Made a Lens, But for Sound

A Patreon supporter of mine, Zack Murphy, noticed something strange while handling helium balloons. If you put one between it and a distant

sound

, the

sound

became quieter, and more importantly, it's not something that happens with normal air-filled balloons. So what is going on? My theory is that it is a form of

lens

es. Then you can use a

lens

to focus the light. Light travels as a wave, and in principle you should be able to focus any type of wave, as long as you can build the equivalent of an optical lens into our system. case an acoustic lens or a sound lens.

And check out these helium-filled Mylar balloons. They looked like lenses. Specifically convex lenses instead of concave lenses. But here's the strange thing: in optics, a convex lens focuses light, so if you look at a light through a lens, it should appear brighter as long as you're a little closer to the focal point. Except that's not what happens. with helium. With helium the sound does not become louder, but quieter. Which is equivalent to a light dimming when you put a lens in front of it. So something else is happening. The reason a convex lens focuses light is because glass is optically denser than air or has a high refractive index;

To use more formal terminology, when light passes from a low refractive index medium to a high refractive index medium, such as from air to glass, the light bends toward this perpendicular line sometimes called the normal in optics. There is more than one way to explain why light bends this way. But the intuitive one I like involves looking at the wave front. It is important to know that light travels more slowly in a medium with a high refractive index. You see, as light passes from air to glass, it slows down and the peaks and valleys of the wave should clump together a bit.

In other words, the wavelength is shortened if the light arrives at an angle. So this slowing down of light actually changes the angle of the wave front and if we assume that light always travels perpendicular to the wave front then the direction of travel must also change. This is true when going from a high refractive index to a low refractive index medium; In other words, light is deflected from the perpendicular to the normal. Hopefully, you can see that the geometry of a convex lens will cause the incoming parallel light rays to bend inward to a focal point.

If you wanted to blur the light, then you would use a concave lens and hopefully you can see from the geometry here. That's what you get. So why does a convex sonic lens seem to behave like a concave optical lens? Why does it spread the sound and reduce the volume at any point in space? Well, it's because helium is less acoustically dense than air, which is the opposite of what we have with glass and air, where glass is more optically dense than air. Helium is less acoustically dense than air, so we need to change the geometry to get the same thing. effect, so hopefully you can see that here, when the sound rays enter the convex geometry of the globe, the rays bend away from the normal and then bend towards the normal on the way out.

Let's test this hypothesis by trying to build a focusing sound lens that we could do by building a concave helium lens or We could build a convex lens that is filled with a gas denser than air. The easiest thing to get is carbon dioxide, so that's what I'm going to use. And the easiest way to supply carbon dioxide. It's with a fire extinguisher. A quick detour about the geometry of the lens, which is important for the experiment. This surface here is a section of a sphere, so you can imagine a big sphere like this and we only have a part of it here and that bottom surface.

It is also a section of a sphere. And the reason a lens geometry tends to be spherical is because it's easy to make. Imagine you have a spherical indentation and you have a piece of rough glass and you are polishing it inside that indentation. It doesn't matter how you orient the lens because it is a spherical slit. That wouldn't be true for other geometries, but it turns out that spherical geometry is not the perfect geometry for focusing light on a point. what you really want is for one surface to be elliptical and the other hyperbolic. But for a thin lens like this it's a pretty good approximation.

So spherical geometry is fine. But the lens I have is a full sphere and you're going to end up with something called spherical aberration where you don't get perfect focus, so I also wanted to see if I could create a more traditional lens shape, so I

made

it with a mylar balloon , but I reinforced the edge with copper. tube That's so it doesn't get rounded around the edge if you're wondering how I got the copper tube on the globe. I do it like this. To test the lens I need a sound source. I used my phone for that and I have software that generates white noise.

I biased the white noise towards higher frequencies. That's because for the lens to work, the wavelength of sound must be substantially smaller than the geometry of the lens, which also applies to optical lenses. And anything below a thousand Hertz probably won't work. One thousand Hertz is a wavelength of about 35 centimeters. Ironically, it sounds like a balloon deflating. I did some rough calculations to calculate the focal length of the lens just to make sure I could do the experiment in my garden. As if I didn't have to move tens of meters away from the lens to find focus and all that. it was fine But to determine the exact focal point I had to do a little trial and error With the microphone mounted on the lens You can hear the moment I hit the sweet spot of the sound with my camera And here it is with more traditional balloon shaped lens.

In fact, I think the spherical globe works better perhaps because it is larger and therefore picks up more sound. I'm going to show you the yellow balloon experiment again this time from the perspective of the handheld camera and at the same time I'm going to show you a frequency analyzer. I'm going to freeze the frequency analyzer at the low amplitude point and at the high amplitude point when we get to the focal point. And there you have it, you can clearly see the difference in amplitude. It really seems to have a lensing effect. To top it all off, here's the mylar balloon filled with helium and you can actually hear the amplitude decrease as the sound goes out of focus.

So, there you have it. Thanks Zack for the idea and thanks to all my patrons on Patreon. for your support and ideas. Here's a question for you. Do you use the Chrome browser? Statistically speaking, you probably do and I used to, but I don't anymore. You might think that switching browsers is a huge headache. But I want to explain why it doesn't have to be and is actually really interesting. So Chrome is not an open source project But it is based on an open source core called chromium and other people can create browsers based on this open source chromium for example the sponsor of this video is brave and the best of a chromium-based browser is that Chrome extensions work on Chrome-based browsers.

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I hope you enjoyed this video if you did, don't forget to subscribe and I'll see you next time.