Detecting liveness of a speaker comprises: generating an ultrasound signal; receiving an audio signal comprising a reflection of the ultrasound signal; using the received audio signal comprising the reflection of the ultrasound signal to detect the liveness of a speaker; monitoring ambient ultrasound noise; and adjusting the operation of a system receiving the audio signal, based on a level of the reflected ultrasound and the monitored ambient ultrasound noise. The method can be used in a voice biometrics system, in which case detecting the liveness of a speaker comprises determining whether a received speech signal may be a product of a replay attack. The operation of the voice biometrics system may be adjusted based on a level of the reflected ultrasound and the monitored ambient ultrasound noise.