Rumbler siren

A Rumbler siren is a type of emergency vehicle siren used primarily in the United States. Developed in 2007 by Federal Signal Corporation, and sounding at a low-frequency level, it is designed to be heard by motorists who may otherwise be unable to hear high-frequency sirens due to ambient noise, such as urban traffic.
Design.
The Rumbler is a pulsating, low-frequency tone that operates at 182 to 400 hertz, and sounds in tandem with a standard, piercing high-frequency tone. At the lower frequency, the sound penetrates hard surfaces like glass and doors more effectively than high-frequency tones, and helps generate attention in high-noise environments like "dense, urban traffic". In addition, the siren's low thumping sound often causes bodily vibrations in persons who are near to it. The Rumbler is about 10 decibels quieter than a standard emergency vehicle siren and is generally activated in eight-second bursts by depressing the horn in a Rumbler-equipped vehicle.
History.
The Rumbler was developed by Federal Signal Corporation as an "intersection clearing device". It was initially tested by the New York Police Department in 2007. According to Noise Off, a noise pollution advocacy group, more than 100 U.S. police departments had deployed the siren by 2012, including those of Washington, D.C.; Amarillo, Texas; Tulsa, Oklahoma; and Reading, Pennsylvania. As of 2016, the per-unit cost of the siren was between $400 to $700.
In 2015, emergency vehicles in New South Wales, Australia were outfitted with Rumbler sirens and, the following year, some ambulances in Singapore began to be equipped with the Rumbler. According to Singapore officials, the decision to adopt the Rumbler came after seeing its efficacy in U.S. police vehicles.
Wales Air Ambulance's response vehicles are now equipped with the rumbler siren. This is the second application of such equipment in the UK, having only previously been used by another response doctor charity in Hampshire.
"Rumbler" was a nominee for "People's Choice Award Word of the Year" for 2016 by the "Macquarie Dictionary".