A first electronic device is operated in the presence of a second electronic device. Both are configured to transmit acoustic signals, the first being configured to transmit signals with a first characteristic. The first device determines the presence of the second device and thereafter transmits acoustic signals having a second, signal characteristic, different from the first characteristic, and giving a reduced interference between signals transmitted from the first and second devices respectively than the first signal characteristic. Acoustic signals comprising reflections of the transmitted signals from an object are received at the first device, and are used to characterise the motion of the object and thereby control a function of the first device.