Its bandwidth of 428 kilohertz means a single segment can carry a bare-bones video picture, compact disc-quality audio, and text for news flashes, weather details and emergency announcements.

ECONOMIST: America is finally poised to get mobile television

So if you blast inaudible ultrasonic frequencies of 100 kilohertz and 101 kilohertz, you get an inaudible 201 kilohertz overtone plus an undertone of 1 kilohertz that is well inside the range of hearing.

FORBES: Magazine Article

Now, Dr Cummer and his colleagues have shown off an acoustic cloaking technique that works in air, for audible frequencies between one and four kilohertz - corresponding to two octaves on the higher half of a piano.

BBC: Acoustic 'cloaking device' shields objects from sound

The machine that Dr O'Brien and Dr Frazier have designed to get round these constraints consists of a transmitter that sends out a pulse of sound at a frequency of 6 kilohertz, and an array of 52 receivers that collect the reflections from anything buried beneath the surface.

ECONOMIST: Archaeology