Patent ID: 6850555

Claim:
A signalling system, comprising: a first signalling device comprising: i) an input terminal for receiving electrical message data having a power spectrum which lies substantially over a first bandwidth; ii) means for spreading the power spectrum of the message data substantially over a second bandwidth which is greater than said first bandwidth and which extends over a frequency band which is audible to humans; and iii) means for converting the data output by said spreading means into corresponding acoustic signals and for transmitting the acoustic signals into the atmosphere; and a second signalling device located, in use, at a distance from the first signalling device and comprising: i) means for receiving acoustic signals from the atmosphere and for converting the received acoustic signals into corresponding electrical signals; ii) means for despreading the power spectrum of the received signals; and iii) means for regenerating the message data transmitted by said first signalling device from the despread signals output by said despreading means; wherein, in use, said first signalling device is operable to transmit said acoustic signals at a power level which is chosen in dependence upon the distance between said first and second signalling devices, so that the power level of the transmitted acoustic signals at the second signalling device is below the ambient noise level and therefore inaudible to humans in the vicinity of the second signalling device.