The high-frequency sound from the transmitter produces sharp reflections of small objects for the same reason that an electron microscope (which operates at high frequencies) can see smaller things than a light microscope (which operates at low frequencies).

ECONOMIST: Archaeology

At the time, prosecutors allege, Mrs Heidrun was sitting in her study in front of a wireless transmitter that was receiving encoded messages on a shortwave frequency and was hooked up by cable to a computer, reports the German news website Spiegel Online.

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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

The transmitter has a range of two meters to three meters, and operates in the unlicensed 2.4GHz frequency band.

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