Pirates Thwarted by Sonic Weapon

Kristian Hermansen was one of dozens to submit a story about would be pirates attempting to take control of a cruise ship of the coast of Africa, only to be twarted by some sort of sonic weapon known as an LRAD, or Long Range Acoustic Device.

Comments

Weekly piracy report

[__aagctu1952]

Oh yes. Pirates are as active as ever. Only nowadays they have machine guns and RPGs instead of blunderbusses and cannons...
Just read IMB's Weekly Piracy Report for an idea of how active and how dangerous modern pirates are.

Re:LRAD Countermeasure?

[Reverberant]

I wonder if something as simple as noise-cancellation headphones would provide significant protection against LRAD usage.

Noise cancelling headsets (and noise cancelling technologies in general) don't work all that well for high-frequency sounds since the impinging noise and the 180-degree shifted sounds need to be perfectly aligned at the ear to cancel each other out. That's difficult to do with high-frequency sounds because of the relatively small wavelengths involved.

Now passive hearing protectors (e.g. ear plugs) work very well at high frequencies. However, the best ear plugs reduce sound by 20-30 dB at high-frequencies. If the LRAD literature is true, and it can produce sound levels of 150 dB, than ear plugs can reduce it to 120 dB, which is still uncomfortably loud.

Lifting content from wikipedia?

[rsborg]

Since you're just cutting and pasting portions of your content from wikipedia, why don't you just paste the link?

Something you left out from the wiki content:
"At maximum volume, it can emit a warning tone that is 151 decibels (1000 W/m) at 1 metre, a level that is very capable of permanently damaging hearing."

I'm not sure I'd call that "an ideal use of this technology in private sector" as you put it... ethical concerns don't just go away if you're "private sector".

Re:LRAD

[Technician]

Can you elaborate?

Yes.

Sending a 1Khz tone a long distance would require a large reflector to produce a relatively tight beam. Sending a 30 KHZ tone requires a much smaller reflector to deliver the acoustic power the same distance. The small dish sends high power signals. For example if we sent 30Khz and 31Khz, they would both travel in a well defined beam as a ~30Khz signal. Unfortunately we can't hear 30 Khz ultrasonic signals (except for a few audiophiles ;-). ) Air at high volume is not linear. For example you can compress air to several atmospheres. The reverse is not true. You can not draw several atmospheres of vacuum. You are limited by the hard limit of pure vacuum. Air compressing and expanding is non-linear.

Now enter a couple very high intensity sound waves of two frequencies. Now you have non-linear mixing. In non linear mixing (just like in a radio reciever) you get out the 2 original frequencies + the sum and the difference. Therefore 30Khz + 31 Khz is 30Khz, 31Khz, 61Khz and 1Khz. Now you have delivered a very loud and painful 1Khz sound using the tight beam delivery of 30Khz with a small dish.