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.
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. The Rumbler is no louder than a standard siren. In fact, it’s quieter — 10 decibels lower, which translates to only half the volume. But because low-frequency sound waves penetrate cars better than those at a higher pitch, drivers experience the Rumbler as much louder than a standard siren.
That’s good news for pedestrians who might prefer not to be deafened, though not necessarily for the officers in Rumbler-equipped cars. To spare the officers’ ears, the device cuts off after eight seconds. But the officers who demonstrated it for me said they had used it in repeated intervals for longer durations. And though Federal Signal describes the Rumbler as an “intersection-clearing device,” the officers also recounted using it while zipping up long stretches of highway.
“It’s like the Red Sea parting,” Capt. Christopher Ikone said. Low-frequency sound can have physical effects, like making you feel queasy. Enough, in fact, to be of interest to some weapons manufacturers, but their experiments take place at much lower frequencies and much higher amplification than the Rumbler employs.
In fact, despite the siren’s name, the rumbling effect is subtle — far less than what you experience when an Escalade rolls up beside you at a stop light, tinted windows lowered, custom speakers blaring and thunder bass thumping. Hearing a Rumbler while standing on the street, I felt a slight tingle under my ribs; in Officer Gallagher’s car, I felt a gentle reverberation on the seat. – Full Article Source
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