A Supporter of mine on patreon - Zack Murphy - noticed something strange while handling helium balloons.
If he put one between himself and a distant sound the sound got quieter and importantly it's not
Something that happens with normal air-filled balloons.
So what's going on?
My theory is that it's a form of lensing
So you can use a lens to focus light
light travels as a wave and in principle
You should be able to focus any
kind of wave so long as you can build the equivalent of an optical lens in our case
an acoustic lens or sound lens.
And look these helium-filled Mylar balloons. They kind of looked like lenses
Specifically convex lenses as opposed to concave lenses
But here's the weird thing:
In optics a convex lens focuses light
so if you're looking at a light through a lens
It should appear brighter so long as you're somewhat near the focal point
Except that's not what's happening with helium.
With helium the sound isn't getting louder,
It's getting quieter. Which is equivalent to a light getting dimmer when you put a lens in front of it
So there's something more going on.
The reason a convex lens focuses light is because glass is more Optically dense than air
or it 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 like from air to glass
the light bends towards this Perpendicular line
sometimes called the normal in optics
There's more than one way to explain why light bends in this way
But the intuitive one that I like involves looking at the wavefront
It's important to know that light travels more slowly in a medium with a high refractive index
So look as the light goes from air to glass it slows down
And so the peaks and the troughs of the wave need to bunch up a little bit.
In other words the wavelength shortens
if the light comes in at an angle
Then this slowing down of light actually
changes the angle of the wavefront and if we assume that light always travels
Perpendicular to the wavefront then the direction of travel must change as well
The opposite is true
when you go from a high refractive index to a low refractive index medium in other words, the light bends away from
Perpendicular away from normal.
So hopefully you can see that the geometry of a convex lens will cause
Parallel rays of light coming in to bend inwards to a focal point
If you wanted to defocus the light then you would use a concave lens and hopefully you can see
From the geometry here that that's what you get.
So why does a convex sound lens appear to be behaving like a concave optical lens
Why is it spreading out the sound and
Reducing 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 switch the geometry around to get the same effect
so hopefully you can see that here as the rays of sound enter the
Convex geometry of the balloon the rays bend away from normal and then they bend
Towards normal on the way out
Let's test this hypothesis by trying to build a focusing sound lens
we can do that by building a
concave helium lens or
We could build a convex lens that's filled with a gas denser than air
The easiest to get hold of is carbon dioxide - so that's what I'm going to use
And the easiest way to supply carbon cioxide is with a fire extinguisher
A quick detour about lens geometry
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've just got a part of it here
and that bottom surface is also a section of a sphere
And the reason the geometry of a lense tends to be spherical is because it's easy to make.
Imagine you've got a Spherical indentation and you've got a rough bit of glass and you're grinding it inside that indentation
It doesn't matter how you orient the lens because it's a spherical indentation. That wouldn't be true for other geometries
but it turns out that
Spherical geometry isn't the perfect geometry to focus light to a point.
what you actually want is one surface to be
elliptical and the other to be hyperbolic
But for a thin lense like this
It's a pretty good approximation
So spherical geometry is fine
But the lens that I've got 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
lense shape, so I did that with a mylar balloon, but where I
reinforced the rim with copper pipe
That's so that it wouldn't become rounded around the edge if you're wondering how I got the copper pipe in the balloon
I do it like this
To test the lens
I need a source of sound I used my phone for that and I've got some software that generates white noise
I skewed the white noise towards higher frequencies
That's because for the lens to work the wavelength of the sound needs to be substantially smaller
Than the geometry of the lens that's true for optical lenses as well by the way
And so anything below about a thousand Hertz
Probably isn't gonna work a thousand Hertz is a wavelength of about 35 centimeters
Ironically, it sounds just like a balloon deflating.
I did some rough calculations to figure out the focal length of the lens just to make sure that I could do the experiment in my garden
Like I wasn't gonna have to go tens of meters away from the lens to find the focus and that bit was fine
But to pinpoint the exact focal point I needed to do some trial and error
With the microphone mounted on the lens
You can hear the moment that I hit the sweet spot the focal point of sound with my camera
And here it is with the more traditional lens-shaped balloon
I actually think the spherical balloon is working better perhaps because it's larger so it's gathering 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 point of amplitude and at the high point of amplitude when we hit
the focal point
And there you go you can clearly see the difference in amplitude it really does seem to have a lensing effect
For completeness here's the mylar balloon filled with helium and you really can hear the amplitude go down as the sound is defocused
So, there you go
Thanks Zack for the idea and thank you to all my patrons on patreon for your support and your ideas
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