In a reed organ the free reed is mounted above the windchest and is typically surrounded by a reed cell cavity with small cross-sectional area. The length of this tubelike cavity varies from just slightly longer than the reed in some instruments to several centimeters longer in others. A series of measurements of the frequency and spectrum of the radiated sound have been made on reeds from American reed organs mounted on a laboratory windchest, using a simulated reed cell of varying dimensions. The presence of the reed cell cavity generally results in a lower frequency of vibration at the same blowing pressure and increased amplitudes of the higher harmonics relative to the fundamental, although some anomalous effects are observed for longer reed cells. Some hysteresis effects are observed as the blowing pressure is continuously increased and decreased.