An apparatus comprises a receiver (203) which receives a microphone signal from a microphone (201) where the microphone signal comprises a test signal component corresponding to an audio test signal. A divider (215) divides the microphone signal into a plurality of test interval signal components, each of which corresponds to the microphone signal in a time interval. A set processor (217) generates sets of test interval signal components and a similarity processor (219) generates a similarity value for each set. An interference estimator (221) determines an interference measure for individual test interval signal components in response to the similarity values. The interference measure may be used to select signal segments that can be used to adapt an audio processing algorithm which is applied to the microphone signal, such as e.g. speech enhancement or echo cancellation. The approach may allow for a reliable interference estimate to be generated while maintaining low complexity.