focus

Turning Sound Into a Laser



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As you've probably been told light and sound are both waves

Light waves can be spread out like a light bulb or focused like a laser

But can you do the same for sound?

Surprisingly the answer is yes, and I'm going to show you how I built this device and how you could build your own

So normally when sound travels it disperses and spreads out like you'd expect

But this is actually dependent on the frequency

The higher the frequency the more focused the beam is I don't know why the math works out this way

But I just accept it

But the big issue is that the frequencies needed to get sound focused are actually in the ultra sound range

So it's beyond the range of human hearing

But there's this really weird physics glitch, okay

It's not a glitch but it's a weird property where when the frequency and amplitude are high enough

The air itself becomes a nonlinear medium is able to demodulate a modulated waveform

Yeah, it doesn't really make sense to me either but I'll link the Wikipedia article

But anyway that aside it means we can encode audible sounds by modulating them with an ultrasound carrier wave

This is actually exactly the same as how FM radio works only the ultrasound frequencies are in the kilohertz instead of megahertz

And since individual ultrasonic speakers are pretty small and not very loud. I actually have a bunch of them connected in parallel

Designing a circuit for this was pretty straightforward. The audio comes in here with a simple high-pass filter to get rid of DC offset

That signal travels to a standard five five five timer to modulate a 40 kilohertz square wave

Finally, it's fed into an amplifier to drive the speaker array

It's powered by a 12 volt power supply for the speakers with a 5 volt regulator for the logic parts

So how well does it work?

really well

It's so hard to show in a video though

You really have to be there to experience it, but I can at least describe what it's like first thing to notice

Is that the sound travels pretty straight through the air but as soon as it hits a surface it demodulates it becomes audible

So if I point out a wall, it really sounds like the sound is coming from the wall. Not from the speaker

And as I rotate it, it sounds like the sound is crawling along the wall to the ceiling if I pointed at someone's head

They can tell it's coming from the speaker but to another observer. It sounds like it's coming from that person

pretty cool

But that means if you're trying to make only one person hear

It has to be pretty quiet to eliminate reflect itself another interesting thing compared to normal speakers

Is that very small objects can completely block the sound just putting a piece of paper in front blocks it completely

Anyway, hope you learned something new

It was a really fun project to put together and I'd encourage anyone who wants to make something similar to give it a shot

minus some tuning it wasn't as difficult as I thought it would be. Thanks for watching