L.R.A.D

[patentman]

I read in the post today that a cruise ship off somalia used a device called the LRAD, or Long Range Acoustic Device to repel pirates who assaulted the ship. Apparently this device can project a "beam" of noise at earsplitting volume over relatively short distances. Apparently it was developed as a countermeasure to prevent small terrorists from bombing U.S. Ships in port after the USS Cole was bombed in Yemen a few years back.

Does anybody have any idea how this thing actually works? The only thing I can find on the net is that the beam width can be controlled with magnets. I realize wikipedia has an article on this device and that there are a few pages on the web describing it. However, none of these sites actually tell you actually how the beam is produced.

 

[PottedMeat]

A while back I had a acoustics lecture that explained the 'audio spotlight' effect - i'm guessing the LRAD is just a more powerful version of this. Essentially you have a low frequency audio signal ( in the range of human hearing ) modulating a ultrasonic local oscillator. The emitted signal consists of the audio frequency at sidebands centered about the ultrasonic frequency. Someone found that because of the nonlinear portions of the wave equation the signal will demodulate in the air - > so you have a highly directional audible sound. I guess you could do this with normal speakers with a phased array just at the desired frequency ( no modulation ) but this ultrasonic method may give more distance with smaller drivers? - someone correct me, its been a while.

Audio Spotlight Basics - McGill University Powerpoint Presentation

It's even got plots of array directivity.

 

[patentman]

OK, so basically what you are saying is that LRAD is the same as radio or other rf communications, except that instead of using an ultrasonic carrier wave modulated by an ultrasonic information signal which is then demodulated by a detector and converted to a intrasonic frequency by speakers or other similar means, LRAD uses an ultrasonic frequency carrier high (GHz) frequency modulated by an intrasonic information signal.

As a result, the ultrasonic carrier provides means to directionally transmit audible sound over a relatively long distance (as compared to audible frequencies which are not piggybacked on a carrier, such as those emitted by normal speakers.)

If thats the case that makes complete sense and is fairly easy to understand.

one question, how exactly does an information signal stay piggybacked on a carrier wave? Anyone want to explain the physics to me?

Btw: If its not already clear, I'm not an engineer. My background is in analytical chemistry and Materials Science. So forgive me if some of these questions are a little basic.

 

[Newfie]

Heres some more info from Wikipedia L.R.A.D

 

[dkozloski]

Many years ago shaft turbine engines and a propeller were adapted to a Republic RF-84F in place of the normal turbo jet engine. When the device was started and operated for the first time, everyone in the area immediately became violently ill and had severe balance problems. The whole operation was banned from activity anywhere near the main base at Edwards AFB and was shuttled five miles out into the desert. The nature and intensity of the noise coming from the super sonic propeller was such that it was cavitating the fluid in the inner ears of everyone but the pilot. The performance of the aircraft was as desired but was not remarkable enough to overcome the downside of the operational problems. I have seen the aircraft in the Air Force musem at Wright-Patterson AFB in Dayton, Ohio.