  The existing molecular relaxation models based on both parallel relaxation
theory and series relaxation theory cannot extract the contributions of gas
compositions to acoustic relaxation absorption in mixtures. In this paper, we
propose an analytical model to predict acoustic relaxation absorption and
clarify composition relaxation contributions based on the rate-determining
energy transfer processes in molecular relaxation in excitable gases. By
combining parallel and series relaxation theory, the proposed model suggests
that the vibration-translation process of the lowest vibrational mode in each
composition provides the primary deexcitation path of the relaxation energy,
and the rate-determining vibration-vibration processes between the lowest mode
and others dominate the coupling energy transfer between different modes. Thus,
each gas composition contributes directly one single relaxation process to the
molecular relaxation in mixture, which can be illustrated by the decomposed
acoustic relaxation absorption spectrum of the single relaxation process. The
proposed model is validated by simulation results in good agreement with
experimental data such as $\mathrm{N_2}$, $\mathrm{O_2}$, $\mathrm{CO_2}$,
$\mathrm{CH_4}$ and their mixtures.
