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This article theoretically and experimentally characterizes direct contact, packed bed thermal gradient attenuators in which a fluid control stream, subject to broadband thermal disturbances, flows through a thermally attenuating packed bed medium. The theoretical model decomposes the device into three subsystems: an upper mixing volume, the packed bed, and a lower mixing volume. Analysis of the packed bed subsystem leads to a coupled system of equations that describe local heat transfer within the control stream and within individual spherical elements comprising the packed bed; the system of equations obtained has the same general form as that derived in an earlier investigation of tube-in-shell, noncontact thermal gradient attenuators [J. Heat Transfer 123, 796 (2001)]. Associated subsystem transfer functions are derived, and in analogy with observations reported in the earlier investigation, are shown to provide simple, unambiguous criteria for designing and optimizing packed bed attenuators. It is found that, for 52 different sets of experimental conditions, theoretical performance of the device matches experimental performance to within 2.5 dB root mean square, and that the device provides several orders of magnitude attenuation for flow rates on the order of 1 l/min and disturbance frequencies higher than approximately 10 mHz. The results of the present study and the earlier investigation together provide a useful basis for analyzing and designing large and small, contacting and noncontacting, thermal gradient attenuators. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]