The absorption curves of an acoustic resonator were measured for air, nitrogen, and helium gases. An empirical equation for the curves was proposed. Simple discussion showed that this equation was valid for cases of small dissipation of energy in a resonator. The dissipation was composed of two terms: one due to the incomplete reflection of sound waves at both ends of the resonator, and the other proportional to the length of the resonator indicating the effects of the side wall. The latter was evaluated by employing thermoacoustic theory. The theoretical results showed quantitative agreement with results of experiments. This experiment therefore supports thermoacoustic theory quantitatively.