This, the second of two articles, gives an account of the experimental and theoretical work on which our present extensive knowledge of the sensitivity and analytical properties of the ear is based. Masking phenomena are surveyed and shown to indicate the broad asymmetrical pattern of response of the cochlea that has been observed physically. In all response activities, the ear behaves as though it involves a variable-frequency band-pass analyser with critical bandwidths. In many instancem of response to signals such as pure tones and speech it appears that the Uncertainty Principle for acoustics applies.