Phonon spectroscopy measurements have been made on a variety of chromium-doped gallium arsenide samples over the range 3 cm-1 (90 GHz) to 24 cm-1 (720 GHz) at 1 K. In semi-insulating samples and in weakly p-type samples a cluster of three partially resolved resonant absorptions at 4.2 cm-1 4.9 cm-1 and 5.6 cm-1 are found, together with a weaker but broader absorption at 10 cm-1. These absorptions are not affected by sub-band gap illumination. In n-type samples these features are not found under dark conditions, but may be induced by illumination