A method of detecting a suitability of a signal for live speech detection, the method comprising: receiving the signal containing speech from a transducer; measuring a signal characteristic of an audible component of the received signal; estimating an expected signal characteristic of an ultrasonic component of the received signal based on the measured signal characteristic of the audible component; determining, based on the estimated expected signal characteristic, whether the ultrasonic component is suitable for detecting whether the speech is live speech.