Many media outlets credited the LRAD with warning off the pirates, leaving the Seabourn Spirit unscathed. Some of the coverage was pretty dramatic. News stories described the LRAD as a sonic weapon that fired a beam of sound at the pirates and drove them away.
 Photo courtesy American Technology Corp |
What makes sound a weapon? In this article, we'll review the basics of sound and discuss exactly how the LRAD produces its "beam of sound." We'll also explore LRAD's hailing and warning abilities and other uses for sound.
A Review of Sound and Hearing
No matter what creates it, sound is always made of waves. These waves move through matter, such as air, water or the ground. They interact with the matter (and in some cases with each other) as they go. The animation below will show you the basics of sound waves and how they travel. If you already know how sound waves work, just skip to the next section.
Sound waves can pass through one another without much distortion or change. But in the right conditions, sound waves can change each other dramatically. For example, identical sound waves that are out of phase (their compressions and rarefactions are reversed) can cancel one another out. On the other hand, identical waves that are in phase combine their compressions and rarefactions, doubling their amplitude.
 Wave addition. (This example uses transverse rather than longitudinal waves because their differences are easier to see.) |
As sound waves travel, they spread out in all directions in a curved wave front. The farther they travel from the source, the more they spread and the quieter the sound becomes. But high-frequency waves don't spread as much as low-frequency waves. Also, waves with long wavelengths generally travel farther than ones with short wavelengths.
There are plenty of other things to learn about sound, but this is what you need to know to understand the LRAD. We'll look at how the LRAD creates sound and takes advantage of these physical properties next.
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