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The difficulties encountered by acoustically coupled 103 compatible data sets are considered. In particular the limitations introduced by the harmonic distortion generated by the nonlinearity of the telephone's microphone are analyzed. It is shown that interference between the second harmonic of the transmitted signal and the received signal restricts the application of acoustic coupling to lines of relatively low attenuation. A method of cancelling the second harmonic by adding a third harmonic component to the transmitted signal is described, and shown to be independent of the microphone's nonlinearities. The effect of handset placement on the microphone's performance is also analyzed and experimental results are given. Incorporating the described method of reducing second harmonic distortion and choosing the optimal handset placement allow Applied Digital Data Systems acoustic coupler to transmit at full power and to communicate reliably in full duplex on lines of up to 30-dB attenuation.