Rising Pitch Siren Electronics Circuit Explanation
Here's the circuit: -



See the notes I made on the circuit for explanation why you get a gradual rising pitch over several seconds.

To my knowledge loudspeakers only have polarity markings so they can be paralleled and pump sound in phase with each other correctly. For this circuit it won't matter but please do note that the circuit that drives the loudspeaker is not ideal in that there will always be a dc component of current flowing through the speaker.