Embodiments of the disclosure relate to methods, apparatus and systems for biometric processes. The invention relates to initiating generation of an acoustic stimulus for application to a user's ear and extracting features for use in a biometric process from a measured response signal. The measured response signal may be used to derive one or more quality metrics and the quality metrics may be used to validate features extracted from the measured response. The quality metrics may be used to provide feedback to the user seeking to carry out the biometric process.