Can you picture the screech and whirr of a police car siren?
One vocalise (warm-up exercise) that I use with my voice students is called a portamento. (It is defined as: a smooth or uninterrupted glide from one tone to another.)
I use it in a pitch circle which means we don't just "slide" down or just up, we make the whole circle. It sounds very much like a siren. This relaxed sliding is a great way to warm up the vocal chords, but it also can be used in other ways. Vocal range can be widened and muscle control can be strengthened, especially at vulnerable register switchover spots!
So next time the kids are playing cops and robbers, join in w/ them and see how high or how low the sirens can go!!