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04-06-2011 Post does not mapped to Knowledge Tree
Romy the Cat

Boston, MA
Posts 9,740
Joined on 05-28-2004

Post #: 1
Post ID: 15939
Reply to: 15939
Long Range Acoustic Device – how do they work?

There is a family of sonic weapons:



I wonder what kind principle they use to keep sound radiated at narrow angle at high distance. The explanation that I‘ve seen does not do to me a lot. If they claim the they use the same principle as phased array beamformers then I do not buy it.

"How the LRAD Unit Works

The LRAD's job is to make sound - lots of sound. It produces very loud sound that is audible over relatively long distances. But it's not limited to producing painful noise for use as a weapon. It can also amplify voices or recordings to a level that is loud and clear but not painful or debilitating.
Instead of using one big, moving device to make all this sound, the LRAD uses lots of little ones. A speaker usually uses one rapidly moving diaphragm to make sound. The LRAD uses has an array of piezoelectric transducers. A transducer is simply a device that changes one kind of energy into another kind of energy. In this case, it changes electrical impulses into sound.

Applying a charge to a piezoelectric material causes it to change shape.A piezoelectric material is a substance that is permanently electrically polarized -- it has a positively charged side and a negatively charged side. If you apply pressure to a piezoelectric material, it creates an electrical impulse. On the other hand, if you apply an electrical charge to it, its molecules move and it changes shape. Using electrical current from a battery, generator or other source, the LRAD applies electrical charge to lots of piezoelectric transducers. The transducers rapidly change their shape and create sound waves.

The LRAD has lots of transducers in a staggered arrangement.  All of these transducers are attached to a mounting surface. They're staggered to allow more of them to fit into a smaller space. This helps the LRAD create very loud sounds -- identical waves emerge from the transducers, and their amplitudes combine to create louder sounds.

So that's how the LRAD creates lots of volume. But the sound coming from the LRAD is also relatively directional. It doesn't disperse as much as sounds from typical speakers. Fifteen degrees outside the beam, the volume drops about 20 dB. People behind or next to the device still hear the sound, it isn't as loud. Even outside the beam, the sound can still be loud, so operators and nearby personnel often wear ear protection.

The outer transducers are not completely in phase with the inner transducers. The sound waves interact with one another, canceling out some of the outermost waves and making the sound less audible outside of the "beam."

The device's diameter is larger than most of the wavelengths it produces. This allows the device to create a wave front that's more flat than rounded, keeping the sound from dispersing.

Air interferes with sound waves as they pass through it. As the LRAD's sound waves interact with the air, they create additional frequencies within the wave. Such waves are referred to as parametrically generated, and many speakers try to prevent them. The LRAD uses them to create a greater range of pitches and to add volume.

The result is essentially a loudspeaker that can receive input from a microphone, a recording device or a Phraselator translation device. It can then amplify that input, allowing law enforcement, security and military personnel to give instructions and warnings or to clear buildings and disperse crowds. If those verbal instructions don't produce a result, the LRAD can produce a loud warning tone that approaches or passes the threshold of pain."

Romy the Cat

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
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