DENJIRO: September 1st is Disaster Prevention Day in Japan. Everyone has heard this sound before, right? This is the siren that emergency vehicles like fire trucks make when there is an emergency. Today, the sound is digitally generated, but in the past, a handcrank siren like this one was widely used.
What is the mechanism behind this unique siren noise? I made this model to demonstrate how a siren works. This disk with holes around its perimeter is attached to a motor. If I turn the motor on, making the disk spin, and blow air toward it through a straw... the air is repeatedly being blocked and unblocked from passing through the holes in the disk. As a result, the air vibrates and produces this noise.
The pitch of the siren is dependent on the number of holes in the disk and the speed at which the disk is spinning. In the beginning, it spins slowly, so the siren is at a low pitch. As it speeds up, the pitch increases. This is how a siren works.
Look at what I've made here. It is a disk of concentric circles with many holes in it. If I blow air at it from the center, outward...
ONSCREEN TEXT: Do, re, mi, fa, sol, la, ti, do.
DENJIRO: The inner circles have fewer holes, and the number of holes increases as you move outward. So, as you change where you blow the air at, the pitch changes.
ONSCREEN TEXT: "Hotaru no Hikari."
DENJIRO: I hope energy will bring you all happiness. Our magic word is "Happy Energy!"