Technology

Cover your ears and discover how sound can be used to stop riots and brawls

To disperse crowds and prevent
rioting, various forms of non-lethal
weapons (NLW) are used. Among
these, interestingly, is sound. Known as an
acoustic or sonic weapon, infrasound (superlow
frequency) and ultrasound (super-high
frequency) greatly affect human ears. They can
disorientate a target and have psychological
effects as well as physical effects of nausea and
damage to blood vessels.
Ultrasound can be increased to 120 decibels
(the same volume as a jumbo jet taking off),
which is considered the human pain threshold.
In contrast, infrasound feels like a damaging
vibration or pressure wave, as it’s at a frequency
too low for humans to hear. When exposed to
high levels of sound, the influx of energy has a
painful effect on the body.
As well as huge decibels of volume, the
variations in frequency can also cause damage.
For instance, an infrasound of seven or eight
Hertz can rupture blood vessels. Certain types
of sound within the frequencies we are able to
hear also have psychological effects, acting a
sort of musak that can calm people and helps
prevent drunken brawls.
In New York City, piezoelectric loudspeakers
are used by the NYPD. They focus sound waves
in a particular direction to control large crowds.
Known as Long Range Acoustic Devices (LRAD),
some types can reach a maximum of 162dB.
Another device is the sonic bullet. Made by
inventor Woody Norris, it sends a beam of up to
145dB into its target. Prototypes are already in
production to be used in the US Army to help
enforce safety areas in warzones.