The LRAD or the long range acoustic device that has been developed by American Technology Corporation is capable of emitting a maximum of 151Db sound within 30 degrees of where the device is pointing. This device can be used as a combatant deterrent weapon or crowd-control device by emitting sounds that are painful to the ears.
The LRAD weighs about 210 kilograms and is capable of emitting sound within a 15 to 30 degree beam. The range of the LRAD is 300 to 500 meters and, at maximum volume, it can emit sound 50 times greater than the human threshold for pain so it can cause permanent damage.
Sound Propagation Principles used by LRAD
Sound waves spread out in all directions as they travel through a medium and lose strength proportional to the distance from the source. High-frequency sound waves, however, spread out less than low-frequency sound waves. Moreover, sound waves with longer wavelengths travel a lot farther than those with short wavelengths. Sound waves can also either cancel each other out or merge together into a sound wave with higher amplitude depending on whether they are out of phase or not.
LRAD applies all the abovementioned properties of sound to make the device work.
The Technology behind the LRAD
Instead of the usual diaphragm that normal speakers use to make sound, the long range acoustic device uses a set of piezoelectric transducers which are capable of converting electrical energy into sound. These piezoelectric transducers are permanently polarized, so any distortion of the shape of such transducers creates an electrical impulse and vice-versa. By using a power source to supply this electrical impulse, piezoelectric transducers can rapidly change their shape, therefore, creating sound waves in the process. The transducers are also arranged so that they are in phase with each other so that the resulting sounds they emit can combine to make the projected sounds louder.
The sound that the LRAD produces can be directed so there's less-than-normal dispersion. This results in a 20-dB drop in the volume of sound 15 degrees outside the beam. This directional sound propagation stems from the fact that the LRAD employs outer and inner transducers in creating sound waves that are not completely in phase with each other. This enables other sound waves to cancel out those that are in the outermost portion of the beam. The resulting wave front of the sound is also flatter than usual, preventing the sound from being dispersed as it propagates. Moreover, as the LRAD-produced sound waves interact with the air, they create additional frequencies within the wave and thus amplify the sound and pitch.
Advantages and Disadvantages of LRAD
The primary advantage in military and law enforcement circles is that the LRAD is a less than lethal solution that has the potential to prevent antagonists from continuing their unwanted activities, without endangering the friendly personnel.
The major disadvantages of the LRAD system are:
- The loud sounds the LRAD emits may cause permanent hearing damage to those within its range.
- The LRAD sound wave can be cancelled altogether through the use of common earplugs. The better the earplugs are at blocking sound waves, the less effective the LRAD is.
- Sounds emitted from the LRAD can be reflected back to the source by using a flat solid object.