Acoustic pressure response of diluted hydrogen-air diffusion flames is investigated numerically by adopting a fully unsteady analysis of flame structures in a counterflow. Without the restriction of the previous quasi-steady approach, acoustic response to pressure oscillation is investigated in a wide range of frequencies and for two representative pressure regimes, namely, low- and medium-pressure regimes. Acoustic amplification/attenuation and their intensities are judged by the normalized amplification index derived from Rayleigh's criterion. The numerical results show that acoustic pressure response has a complex pattern depending on acoustic frequency in each regime. In the low-pressure regime, the amplification index remains low and constant at low frequencies, which indicates a relatively weak acoustic amplification. As acoustic frequency increases, finite-rate chemistry is enhanced through a nonlinear accumulation of heat release rate, leading to a high amplification index. Finally, the flame responses decrease at high frequency because of the response lag of the transport zone. For a medium-pressure operation and low-frequency excitation, the amplification index is low and constant. The value is smaller than that found in the same frequency range for the low-pressure operation. It then decreases at moderate frequencies since the transport-zone response becomes dominant over the reaction-zone response. As frequency increases further, the amplification index increases appreciably because of an intense accumulation effect, and its peak value exceeds that found for low-pressure operation. The results predicted in the previous quasi-steady approach agree with acoustic responses to low-frequency pressure oscillations predicted in the present unsteady analysis. However, the accumulation effect occurring at high frequency, which could not be predicted in the quasi-steady approach, is here identified as a key amplification mechanism.