# What makes a origami paper popper produce loud sound?

A paper popper like the one shown in this YouTube video makes a loud sound when moved quick through the air.The air pressure inside the 2 conical structures in the popper would be higher when this popper is moved fast. But how exactly is this difference in pressure converted to sound? Will having different paper types produce different frequencies?

Whilst physLads answer is correct, it is perhaps too succinct to be helpful with those unversed in gas dynamics, so I will attempt to explain what a 'blast wave' is in this case.

"My understanding currently is that the pockets created in the popper actually become high pressure pockets ( like a balloon) when moved very quickly through the air. This high pressure releases a small flap of paper suddenly when the differential pressure ( inside the pocket pressure is higher than what is just outside) is enough to push out the paper flap against frictional resistance."

This much is correct. However, as the flap of paper moves away, the high pressure gas pocket that had built up must rush in after it, pealing away layer by layer in a 'rarefaction' that means that, as the paper is moving it is followed by a moderately low pressure, but crucially very fast moving air. The paper is then brought to a very sudden halt (in the frame of reference of the persons hand), but still has a mass of fast moving air piling in towards it. This creates a 'blast wave', in this case a region of stationary high pressure air between the paper the region of fast moving low pressure air. The blast wave is the discontinuity in velocity and pressure between these two regions of air. As more air piles in across the blast wave the region of high pressure air adjacent to the paper increases in size, and the blast wave moves outwards through the air. This 'blast wave' or 'pressure discontinuity' is the loud pop heard.

Edit: you state in comments that you believe the sound to be produced by the region above the paper popper as the paper accelerates into it. It is true that this will also create a blast wave. However the acceleration of the paper is much slower than its deceleration, thus this wave will be smaller. In addition, the rarefaction caused when the paper halts will reduce the size of it yet further. Recordings of a paper popper being used may contain an ear splitting pop followed or preceeded by a much quieter one, the quieter one will be produced by this method.

Pretty much exactly off wikipedia, let me know if you have any further questions.

What happens is that air rushes in a blast wave to fill the vacuum created in the pocket that opens. The sound is described as the crack of a whip, which is an example of breaking the sound barrier, although the causes are different.

So its basically the rushing air into the newly created pocket that makes the noise.

• I just read the wikipedia page and I'm pretty unconvinced by this explanation... My intuition would be that it's the paper unfolding abruptly that creates a noise. I guess trying with cloth won't make a sound, while very rigid cardboard will be harder to pop, but make a bigger sound. – Klodd Nov 18 '16 at 13:17
• cloth would also allow air to permeate through. how about trying with a sheet of plastic? – physLad Nov 18 '16 at 23:40
• I read the wikipedia page again. I do not seem to agree or probably not getting the language right. My understanding currently is that the pockets created in the popper actually become high pressure pockets ( like a balloon) when moved very quickly through the air. This high pressure releases a small flap of paper suddenly when the differential pressure ( inside the pocket pressure is higher than what is just outside) is enough to push out the paper flap against frictional resistance. This released flap causes the air just above the flap to suddenly compress causing the sound to be produced. – Ravichandran K Nov 19 '16 at 1:00
• Ah really useful you compared it to a balloon actually. Read this on why balloons pop when they're burst. – physLad Nov 19 '16 at 1:05
• When you stick a pin in the side it creates a tiny hole. The rubber around the edge of the hole isn’t being pulled uniformly in all directions any more because there isn’t any force exerted from the centre of the hole. So the net force pulls the rubber away from the hole, which makes it bigger and the force imbalance increases. In a fraction of a second, the entire skin of the balloon has contracted all the way back to a point on the opposite side from the pin. The high-pressure air that was inside the balloon is now free to expand and this creates a pressure wave that our ears hear as a bang. – physLad Nov 19 '16 at 1:06