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Cell phone ringers at frequencies that only teenagers can hear.  These are for secretive use in class, but I am sure my students have never done this…

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I am extremely skeptical of this. Most cellular phone loudspeakers can't get anywhere near 18kHz, and the low-bitrate mp3s that they use to encode the ringtones these days have terrible frequency response in that band, too.

It's possible that some of the higher-end music-oriented phones, especially recent Motorolas and Nokias, can do this. But I'd be surprised if it were at all widespread.

This is really cool, if it works! I know what I want on my birthday now!!

What good does it do to be able to hear the phone ring? Presumably the student does not answer the phone and hold a conversation in 18khz tones.

Gee, I can always hear an 18Khz tone. I hear it now even though my computer isn't playing anything.

Six months ago, some of us were already hearing about this in a slightly different context.

http://www.theage.com.au/news/world/rowdies-buzz-off-as-the-mosquito-bites/2005/11/29/1133026467657.html

I never heard about this kind of telephones and frequencies.Although this kind of things seem to exist.These days I was surfing on the web and found Internet Yellow Pages, I never thought that there is such thing online.In my opinion the yellow pages was a yellow book full of different informations.

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