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There is this device I saw when I was a child at a big music store, that makes me curious.

It was meant to listen some CDs to try them out before the purchase. You stand under a bell-shape kind of "speaker" to hear the music, but as soon as you step out you aren't able to hear anything anymore.

I've been searching the net, but probably I haven't used the appropriate keywords because I didn't find anything. What's this device called, and how is does it work?

I'm interested in particular about the sound and how it's possible to transmit it only in a very limited area, and being able to hear it well only inside that little "circle"

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    $\begingroup$ useful search terms : beamforming, phased array. (apply to radar antenna ... and speakers too!) $\endgroup$ – Brian Drummond Nov 4 '16 at 11:02
  • $\begingroup$ @BrianDrummond: thanks. I'm trying them now. Not looking so good atm, most of the links I'm getting are not what I'm seeking $\endgroup$ – Zorgatone Nov 4 '16 at 11:06
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    $\begingroup$ See also LRAD en.wikipedia.org/wiki/Long_Range_Acoustic_Device for a less congenial application of this technology, which is in the news at the moment. $\endgroup$ – Brian Drummond Nov 4 '16 at 11:12
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    $\begingroup$ It's a Cone of Silence. Nice idea, but they are known for reliability problems. $\endgroup$ – Olin Lathrop Nov 4 '16 at 12:02
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    $\begingroup$ I usually hear these called 'parabolic speakers' even though that may not be geometrically correct. It seems to work as a google search term. $\endgroup$ – Ethan48 Nov 4 '16 at 14:12
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Something like this? http://www.browninnovations.com/sound-dome/single-localizer-sound-dome

As others have said, it is a speaker with a parabolic reflector to direct the sound down at the listener. You can make your own fairly easily: http://www.instructables.com/id/umbrelAudio-Sound-Dome-Umbrella/

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With a description as vague as that, it was probably a parabolic reflector with a speaker at its focus. Pointed straight down, the "beam" of acoustic energy would be reflected from the floor right back into the "bell", with remarkably little leakage out the sides.

A reflector with a diameter of 4-5 feet would be directional for frequencies down in the hundreds of Hz, which would be fine for auditioning music. They probably rolled off the bass response deliberately in order to prevent problems from the lack of directionality at those frequencies.

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