Joe Bader tried setting the two tones of his invention four notes apart on the musical scale, but the result sounded like music, not a siren. Same thing when he played around with a five-note interval. But when he set the two tones apart by two octaves and gave the siren a test run outside the Florida Highway Patrol headquarters in Tallahassee, the effect was so attention-grabbing that people came streaming out of the building to see what the strange sound, with its unfamiliar vibrations, could possibly be.

Which was precisely what Mr. Bader, a vice president at the security firm Federal Signal Corporation, was going for: a siren that would make people sit up and take notice  even people accustomed to hearing sirens all the time. Even people wearing ear buds or talking on the phone. Even people insulated from street noise by a layer of glass and steel. Even New Yorkers.

Rumblers, as Mr. Bader called his invention, achieve their striking effect with a low-frequency tone, in the range of 180 to 360 hertz (between the 33rd and the 46th key on a standard piano keyboard), which penetrates hard surfaces like car doors and windows better than a high tone does. When it is paired with the wail of a standard siren, the effect is hard to ignore  like the combination of a bagpipe’s high chanter and low drone, or perhaps like a train whistle and the caboose that moves that whistle through space.

Following the lead of some other municipalities, the New York Police Department gave the devices two limited test runs beginning in 2007. It liked what it heard, with the result that a Rumbler will be coming soon to a police car near you  perhaps one speeding right at you in a high-speed chase through traffic- and pedestrian-clogged streets. And eventually to about 5,000 of the department’s more than 8,000 vehicles.