Effect of different forms of absorption on the integrated spectrum of Cas A for a magnetic field value of 0.78 mG. At ~ 5 MHz, the synchrotron emission begins to self-absorb and has a slope of ν^5∕2. The blue line shows the spectral shape that the radio source with the synchrotron spectrum shown by the red line would have if it encountered ISM absorption along the line of sight, and the green is the shape it would have if 25% of its synchrotron shell were subject to internal free-free absorption. The yellow line is a combination of both effects.