Police Use Low-Frequency Sound to Clear the Road

New technology warns distracted drivers of approaching emergency vehicles.

by / October 6, 2008 0
It's a ... Car? Photo by Terence Brown

San Francisco police officer Rich Lee vividly remembers the first time he tested the Rumbler in real traffic conditions.

It was a day when the manufacturer, Federal Signal, was in town to show off the Rumbler (or if you prefer, the Intersection Clearing System) to officials in various city departments and there was a photo session scheduled near the Golden Gate Bridge. It also happened to be budget week for the city and a day when the San Francisco police chief was at City Hall procuring dollars.

On his way to the Golden Gate Bridge for the photo session, Lee's BlackBerry alerted him. It was the chief at City Hall requesting Lee's attendance.

"I said, 'Can it wait 20 minutes?'"

The answer was no.

Lee knew what he had to do.

He tapped his horn, activating the Rumbler. On went the siren, red and blue lights flashing, and then the unique feature of the Rumbler -- the high-output speaker and subwoofers bounced low-frequency sound waves off buildings and vehicles.

"I'm watching people two and three blocks ahead of me, frozen with their hands covering their ears," Lee said. "Cross traffic was at a standstill as I went back to City Hall. They were stopping to see what was going on. If it had been a regular siren, people would have pulled in front of me like, 'What do you want?' but everyone got out of the way."

That day Lee knew the Rumbler would work as needed during a crisis.

Now San Francisco has two 2008 Ford F-450s equipped with the Rumbler, which consists of a slave amplifier, a timer that's activated when the driver taps the horn, two speakers that operate like subwoofers and a mounting bracket. The list price for the whole shebang is $685, according to Carlee Sanchez, senior district sales manager for Federal Signal.

"It's an additional tool to combat distracted drivers, whether they have the stereo on, air conditioning, cell phones or general ambient noise," Sanchez said. "It just provides a contrast to the typical high-frequency siren. It mimics the tone but it's done at a much lower hertz level. That alone gets attention, but then you will actually feel it. It's not an overwhelming violent shaking, but a subtle feeling like a freight train kind of a rumbling."

According to manufacturer specifications, the Rumbler interacts with 100/200-watt siren amplifiers and provides a secondary, low-frequency tone. The low-frequency tone can penetrate and shake solid materials, allowing anyone nearby to actually feel the sound waves and perhaps even witness their rearview mirror shake.

The Rumblers are being tested or used in a few cities across the country, including New York, Washington, D.C., and Elk Grove, Calif. Californians are just now getting a taste because of more stringent vehicle codes concerning light output and sound that had to be addressed. California agencies were free to use the Rumbler as of December 2007.

San Francisco purchased the trucks and equipped them with the Rumbler in case the need arose to haul equipment through city streets during a disaster. For example, the medical examiner has a mobile morgue and trailers that would need to be towed by a heavy-duty vehicle. "We have trailers with weapons of mass destruction chemical suits and medical equipment ready to haul, but we had no way to tow them," Lee said.

"We are a target," Lee explained. "The Golden Gate Bridge is a target, and we have other buildings that are considered high-risk and are targets."

San Francisco's trucks are equipped with the Rumbler and a light bar with two modes, one for police and one for other city personnel, including the medical examiner. Police have a special key that activates the red

and blue lights and the siren. Other city officials use the amber-colored lights. Other than that, the trucks look like a regular city vehicle; they're white with city decals.

"The concept of these trucks is to be available citywide for any employee who needs to get in the truck and tow trailers full of equipment," Lee said. Since it was a homeland security grant that yielded the funds, the police department was tasked with purchasing and maintaining the vehicles so it asked that they be vehicles police could use, Lee said, thus the red and blue light bars.

The two trucks were purchased in March with the grant, and Lee said the city plans to buy two more, one for crime scene investigation and another for the SWAT team.

There's really nothing for the officer to learn, except that when he taps the horn, it produces 10 seconds of rumbling, and if he wants 10 more seconds, he taps the horn again. "Traditionally when the officer taps the horn, the siren would change tone," Sanchez explained. "It would change from a wail to yell, so a faster pattern, and then the light bar pattern would speed up. Now when the horn is tapped, the Rumbler will kick on and the light bar will speed up and go into intersection clearing mode.

"It's a low frequency, so -- in addition to providing a contrasting sound where you have a high-low -- it's actually a subwoofer, so there's actually tactility as well as sound," Sanchez said. "You actually feel this as you would feel the bass coming from a vehicle."

The Rumbler's intensity depends on the surroundings, Sanchez said. "It was designed for urban areas. The ideal performance is in a very heavily congested area where there's something to reverberate off of, such as heavy traffic, a road with K rails [concrete barriers], in between buildings, semitrucks, when there is something for that sound to actually bounce off of, it becomes more effective. The more concentrated the area, the more you pick up on the feeling of it and not just the sound," Sanchez said.

"It doesn't blast your ears when you're inside because the speakers are on the front," Lee said. "But it's a different feeling."


Jim McKay, Justice and Public Safety Editor Justice and Public Safety Editor