Described embodiments relate to a method operable in a biometric authentication system. The method comprises initiating generation of an acoustic stimulus for application to a user's ear; and determining a quality measure of a response signal to the acoustic stimulus. Responsive to determining that the quality measure is inadequate for performing a biometric process, the method comprises one or more of: (i) modifying one or more properties of the acoustic stimulus to improve a signal to noise ratio, SNR, of the response signal and (ii) cancelling the effect of noise from outside the ear on the response signal of the user's ear to the acoustic stimulus to improve the SNR of the response signal.