A signalling system is provided which can be used to update the prices of goods which are electronically displayed on supermarket shelves. The signaling system may include a first signalling device having an input terminal for receiving electrical message data received from a central computer system, a spread spectrum encoder for encoding the received message data and an electroacoustic converter for converting the encoded data into corresponding acoustic signals and for transmitting the acoustic signals from the transmission medium and for converting the received acoustic signals into corresponding electrical signals, a spread spectrum decoder for decoding the received signals and a message regenerator for regenerating the message data transmitted by the first signalling device from the signals output by the decoder.