We present transmission and phase measurements on a birefringent $\mathrm{Y}\mathrm{Al}{\mathrm{O}}_3$ crystal that is illuminated with linearly polarized terahertz radiation in a broad frequency range $(10{\mathrm{cm}}^{-1}\text{\hspace{0.5em} to \hspace{0.5em}}16{\mathrm{cm}}^{-1})$ . The spectra exhibit pronounced and complex Fabry–Perot resonances that depend on the polarization orientation of the incoming light. We present a theoretical description to analyze these data and to obtain the optical constants of the crystal in the terahertz range.