In electrodynamic microphones there are two major sources of thermal agitation noise, namely, the electromotive force due to the thermal agitation of the electrons in the conductor and the electromotive force produced by the motion of the diaphragm due to the thermal agitation of the molecules impinging upon the diaphragm. In commercial microphones, the thermal acoustic noise is far below the thermal electric noise. Therefore, in order to detect the thermal‐acoustic noise, a special high‐sensitivity ribbon velocity microphone was designed and built. The presence of the thermal acoustic noise was detected and measured in the microphone. The theoretical and experimental data correspond for both the thermal electric noise and the thermal acoustic noise.