Conventional thermal microphones record the temperature increase generated by the absorption of acoustical energy. The microphone described in this paper measures the periodic change of the temperature that is generated by an amplitude‐modulated sound wave. This microphone is small, omnidirectional, free of drift, and extremely sensitive. Temperature variations in the order of 3×105C have been recorded by means of a narrow‐band amplifier tuned to the modulation frequency. The temperature distribution in a thermocouple enclosed in a sound‐absorbing material, and the optimum modulation frequency are calculated. A relaxation function is introduced to help interpret the results physically. Also, the electronic input circuit is discussed in detail.