An x‐ray acoustic detector was found to be sensitive to extremely soft x‐ray flashes (5 eV–10 keV,T≤1 &mgr;s). The total dose of the flash is measured by the x‐ray acoustic effect. The absorbed energy induces an acoustical pulse that is registered with a microphone. The microphone signal only depends on the absorbed energy. It is independent of the wavelength of the x‐rays. The dynamic range is limited only by the linearity of thermal properties of the x‐ray absorber and by the noise of the preamplifier. The measuring of continuous x radiation is possible using a chopper and lock‐in techniques.