Using Power to Fight the Power: Countering the LRAD

by Tiffany Montgomery

What do you do when the people that are supposed to protect and serve the public are the ones endangering the public's safety?  How do you protect yourself?

The device that is being pointed out in the photo is what is known as a Long Range Acoustic Device (LRAD).  Described by the LRAD Corporation as "a high-intensity directional acoustic hailer designed for long-range communication and issuing powerful warning tones", various models of the LRAD have been used for many purposes such as defense from Somalian pirates and by the U.S. Army for crowd control in Iraq.  More recently, however, the LRAD has been used against citizens in Ferguson, MI, engaged in peaceful protest against the death of Micheal Brown, an unarmed 18-year-old boy, by a Ferguson police officer on multiple nights of protest in August . The use of the "sonic weapon" in Ferguson is depicted below:

        Although people like Major Joe Schrantz, writer for the Army Lawyer, claims that the LRAD "is not a weapon if it is used for its intended purpose" , Amnesty International still describes the weapon as posing "serious health risks which range from temporary pain, loss of balance and eardrum rupture, to permanent hearing damage".  In the same article a St. Louis police officer also warns a member of Amnesty International by saying "This noise will make you sick" after providing this same member with earplugs.  It is clear that the Ferguson police department is aware of the dangers of this device, but the usage of another LRAD at similar protests against the death of Eric Garner on Thursday December 4th (also killed by a police officer despite being unarmed), prove that the police don't plan on changing their tactics any time soon.  If military grade weapons like the ones used here are the first signs of a budding police state, then we outta learn to protect ourselves right?

Using the Wave Model to Calculate the Power Output of the LRAD 500X

Amnesty Intentional describes the LRAD as being "mounted about 8 to 10 feet off the ground on top of an armored truck" and being "pointed at group of stationary protest on the street approximately 15 feet away" from 10:00 pm to 10:15 pm.  Using this description in combination with the specifications of the LRAD 500X, the model that was most likely in Ferguson, you can begin to visualize the situation like so:

Calculate the area of the sound wave at 1 meter:

The LRAD 500X is able to put out 148 decibels (dB) at 1 meter as its maximum output. Using the definitions of decibels (B and and sound intensity, we can calculate the total power output of the LRAD:

Power is defined as energy over time. In a closed system, energy is conserved, and while some of the energy will be lost due to our system of a protest not being completely closed, the energy that is lost is negligible at the extremely close distance the LRAD was being operated at. Therefore, using the same methodology as before, we can use the power that we calculated to solve for the amount of decibels that the protesters are exposed to:

We can then convert this intensity to decibels:

By plugging this equation into the Desmos graphing calculator you can create a graph of the power the wave that you need to produce:

Despite the LRAD “not being a weapon” it is still used to expose protesters to sounds that cross the threshold of pain for the human ear (130 dB) and are well over the 85 dB hearing damage limit set by the CDC. Obviously no one wants hearing loss to be the price for exercising our fourth amendment rights, so what is a poor protester to do?

It is possible to cancel out the sound wave from the LRAD, by creating your own wave that matches the intensity of the LRAD, with only a fraction of the power that the device actually uses.  By using the definition of intensity to solve for the amount of power that you would need to produce this wave you can arrive at an equation for power that varies based on the distance from the protester (x) and the beam width (b) of the wave:

And there you have it!  You now know how to use power to fight the power!  

¡Viva la Revolución!

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