Air raid siren!

On Thursday, we had an air raid siren practice mid-way through the afternoon. 'A' wound up the siren in the middle of year 6, and everyone had to get under the tables. It was very interesting to see an air raid siren up close.

Here is some research by 'A' on how it works...

An electro-mechanical siren is a fairly simple device. It consists of an electric motor which turns a fan called the "rotor" or "impeller", spinning inside a slotted drum called the "stator". The first job of the rotor is as a centrifugal fan. It pulls air into the siren axially through the intake, and blows it out radially through the holes in the stator. The second job of the rotor is to chop the incoming air stream into impulsive bursts. The rotor is segmented by vanes that periodically cover and uncover the holes in the stator. Each time the rotor and stator holes align, a burst of air is forced through. The frequency of these bursts is the pitch of the siren. The siren in the diagram to the right is my dual-tone Model 2. To create two distinct tones, this siren has a two-level rotor and stator assembly. The top half of the rotor has 5 vanes, and the bottom half has only four. Side note: The above siren animation is way slowed down. In actual use, it spins at over 7000 rpm. No wonder Federal puts those big DANGER OF AMPUTATION! signs on these things! What Gives the Siren its Unique Tone? The siren gets its unique tone, or timbre, from the from the shape of the sound wave it produces. The chopping action of the rotor produces a triangular waveform that is rich in both odd and even harmonics. The first few harmonics are especially strong. The waveform below is from a single-tone 580 Hz fire siren.