The purpose of this study was to check if attenuation analysis is sensitive enough to make a distinction between photon energy spectra of standard accelerators and accelerators operating without a field flattening filter. Attenuation measurements were performed in two operating modes of a medical linear accelerator: with a flattening filter and without it, at nominal energy of 6 MV. Numerical and Laplace transform pair methods were used for spectra reconstruction. Although the difference between two attenuation curves is very small, as expected both reconstruction techniques produce slightly softer spectra at flattening filter free mode. The most important conclusion of this pilot study is that attenuation analysis can be a sensitive tool which is able to detect spectral differences between photon beams of an accelerator operating in two regimes. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. 174019]