Sound of silence

Siemens of Munich wants to make inaudible ultrasound carry audible speech (W0
00/1099). A tiny loudspeaker produces sound waves in a band well above the 20
kilohertz frequency limit of human hearing. Speech is converted into digital
code and spread over the ultrasound band using techniques developed for military
radio. Siemens says the system is ideal for connecting cordless phones: it has a
range of only a few metres, so there is no risk of interference between
neighbouring systems or wireless eavesdropping.