The Null Device

Dual-use technologies

The street finds its own uses for ultrasonic teenager repellants; now some enterprising hoodie-wearing troublemakers have apparently sampled them into mobile phone ringtones inaudible to teachers and authority figures, allowing them to text each other and organise happy-slapping parties and such in class with the teachers remaining none the wiser. Or so the Metro (a throwaway tabloid given out on public transport in the UK) says:
Schoolchildren have recorded the sound, which they named Teen Buzz, and spread it from phone to phone via text messages and Bluetooth technology.
A secondary school teacher in Cardiff said: 'All the kids were laughing about something, but I didn't know what. They know phones must be turned off during school. They could all hear somebody's phone ringing but I couldn't hear a thing.
I'm somewhat skeptical about this. Wouldn't the MP3 format's psychoacoustic compression algorithms wreak havoc with subtleties such as ultrasonic tones?

Anyway, I wonder how long until the Teen Buzz sound is heard in grime records, making the first form of teenage music that's actually (partly) inaudible to elders.

There are 1 comments on "Dual-use technologies":

Posted by: Jim Fri May 26 11:47:01 2006

Just did some tests - the max frequency for human hearing drops from 20kHz to about 14kHz by middle age, so if you had a sound of (say) 19kHz that might work. So, generating that tone as a sine wave in Audacity using a 96kHz sampling rate, it's completely inaudible.

Generating more tones, I can hear up to 16kHz just about. 17kHz is completely inaudible for me (I'm 37 years old).

That rate's a bit high, though. Dropping down to a more sensible 48kHz rate and we start to approach the Nyquist limit - and I can hear artifacts.

So it is possible, but only if

(a) you can generate sounds at 96kHz sampling rates (b) your hardware can reproduce those sounds (c) your encoding doesn't screw it up.

Hmm.. I haven't got time to look up the details for any of that, sadly.