Directed Sound

yawningyellowyak writes "Technology Review has an interesting article on directed sound. Ultrasonic 'sound' is sent out from a 'speaker' and the distortion encountered on hitting the air produces hearable sound, but only in certain spots. You could be standing right next to someone and they would hear nothing. One step closer to the cone of silence!"

Comments

Cone of Silence? More like cone of annoyance.

[Maestro4k]

Seriously, marketers will be in heaven, able to target ads at passerbys. Now you can look forward to being inundated with directed sound ads while walking the street. It'll be far more annoying because it'll be harder to ignore than ambient noise (ads running on outside speakers, people hawking their businesses on the sidewalk).

And we think spam is bad...

Hope it will work for "boom cars"

[old man of the c]

But I doubt the car owners would want it. Why have a loud sound system in a car if the whole world can't hear (and know about) it.

Already in use

[Big Nothing]

This kind of technology (although not as refined as mentioned in the article) has been in use for quite some time. For example, in Oslo Lufthavn (Oslo/Norway international airport) there are "quiet spots" where a speaker is used in combination with a parabole to create "sound spots" in the airport lounge area. The sound (sea waves, bird song, etc.) is basically only audible to the person standing directly under the speaker/parabole.

Frequency change=nonlinearity=high levels

[dpbsmith]

Linear transformations cannot create new frequencies, only alter the relative intensity of frequencies that already exist. If ultrasonic sound is being heard, some nonlinearity somewhere is converting it to audible sound.

The thing that I have to wonder about is that this kind of nonlinearity implies fairly high (ultrasonic) sound intensities, and suggests that stuff inside your head is being driven beyond its elastic limit. The big thing that seems to me to be missing from the article is any statement of the ultrasonic sound power level, in decibels, that is being delivered to your head (and the ratio between the actual ultrasonic sound level and the apparent audible sound level).

How does this compare, for example, to the sound levels used for ultrasonic imaging in medicine?

I'm not suggesting that the process is necessarily dangerous, but it isn't obvious that it's intrinsically safe, either. It's one thing to be subjected to high-power ultrasound a few dozen times during your lifetime for the purpose of preserving your health. It's quite another to be subjected to it day in and day out, for your convenience in listening to music, or for some advertiser's convenience in interrupting your train of thought.

neither one

[ashot]

will dominate this market, because there is a new technique in acoustics that will eclipse the ultrasound method. Using something called time reversal, you can pinpoint the output of sound to a single location in 3 dimensional space, focusing around objects, people whatever; no beam, no drop off.

This focusing can be done with more than just sound waves however, and the first applications are in medicine, however, it does apply to sound as well.

The basic idea is that if you create a sound from some source location, and record all of the noise at another location, then play this noise signal backwards from the recorded location the sound will reappear in the one spot from which it was originally played. An analogy is that if you take a pool ball, put it right in front of a pocket, and then bounce it outwards really hard, letting it bounce against the walls multiple times, but noting the exact location of the last bounce of the ball, then if you reshot the ball at the exact spot where you last saw it bounce, it would go back in the whole.