The propagation and interaction of acoustic modes in ducts is of importance for the measurement and control of sound generated by ducted fans and compressors. A method is presented by which the modal spectra (pressure and power spectral densities) for the first nine cross modes can be determined from pressure cross‐spectrum measurements in the duct. The flow noise, a potential source of errors, is suppressed by placing the two microphones sufficiently far apart in the duct. The modal spectra and some of the modal interaction terms (modal correlations) for a number of fans are presented. The method should have many applications in verifying modal propagation, absorption of linings, and scattering of modes as well as in identifying sound sources.