An acoustic coupling sensor 200 for a portable electronic device is described which includes an acoustic transmitter 22, a reference signal generator 20 coupled to the acoustic transmitter, and an acoustic sensor 24 wherein in operation, the reference signal generator is operable to transmit the reference signal to the acoustic transmitter, to detect an acoustic level of the transmitted reference signal via the acoustic sensor, and wherein the detected reference signal level corresponds to a value of acoustic coupling between the acoustic transmitter and the acoustic sensor. The acoustic coupling sensor may be used for example in a mobile phone to detect acoustic leakage in order to adapt the acoustic level in the mobile phone during use.