Focused Sound

[rathfon]

A while ago, probably atleast a year and a half, I remember seeing a news report on "focused beams of sound" in which sound was not being spread, but sound was actually focused into a "beam" so that your ears must be directly in front or "in-line" with the "beam of sound" to actually hear the sound. Now I was wondering, why don't we see anything using this? I mean, it doesn't seem useful in some things, but simple things could make it useful (probably most useful for pranks). I was wondering if anyone else knew anything about this.

Edit: I found this: http://www.atcsd.com/tl_hss.html

 

[KoolAidKid]

The military uses it on ships to provide directed warnings to possible intruders. The intensity can be increased to harmful levels.

 

[PottedMeat]

my acoustics professor demoed an acoustic spotlight ~5 years ago. - i think it was by the guy that developed it at mit. i think he said that one of his collegues was up on ~3rd floor pointing the transducer array at him walking down the street.

i dont believe its in the market yet because of a patent dispute between

this guy: http://web.media.mit.edu/~pompei/spotlight/ and some other guy ( i cant remember his name )

 

[rathfon]

Well, patents on the "general idea" is ridiculous. But different designs, unless there should only happen to be one, should be patented. I was hoping to see this in simple things, like objects the common consumer could buy. Being in high school, my thoughts were obviously the ones where as I wanted to mess with people. But that's beside the point.

I was also wondering on the basic design. I read up on this a little and the most obvious thing is the ultrasound part. They are just having,.. sounds let off at ultrasound levels that are changed in a way so they distort to become audible?

 

[onix]

I am suspicious of the ultrasound becoming audible, although I've heard about this before. Ultrasonic frequencies are out of the audible range, and it would take some seriously non-linear, possibly chaotic system to create sub-harmonics which humans can hear.

Regarding your first query. Yes, this technique is not so new. LASER is a good example of a optical analog of sound. It's based on distructive and constructive interference, and related phase cancellation techniques. One practical benefit might be to deliver different types of audio to a group of people in the same room. Imagine a United Nation's conference where each audience member was delivered a message in his native language.