We report a theoretical and experimental investigation into the fundamental physics of why loose granular media are effective deadeners of structure-borne sound. Here we demonstrate that a measurement of the effective mass,M (ω), of the granular medium is a sensitive and direct way to answer the question: What is the specific mechanism whereby acoustic energy is transformed into heat? Specifically, we apply this understanding to the case of the flexural resonances of a rectangular bar with a grain-filled cavity within it. The pore space in the granular medium is air of varying humidity. The dominant features ofM (ω) are a sharp resonance and a broad background, which we analyze within the context of simple models. We find that: a) On a fundamental level, dampening of acoustic modes is dominated by adsorbed films of water at grain-grain contacts, not by global viscous dampening or by attenuation within the grains. b) These systems may be understood, qualitatively, in terms of a height-dependent and diameter-dependent effective sound speed [∼ 100 − 300 (m · s −1 )] and an effective viscosity [∼ 5 × 10 4 Poise]. c) There is an acoustic Janssen effect in the sense that, at any frequency, and depending on the method of sample preparation, approximately one-half of the effective mass is borne by the side walls of the cavity and one-half by the bottom. d) There is a monotonically increasing effect of humidity on the dampening of the fundamental resonance within the granular medium which translates to a non-monotonic, but predictable, variation of dampening within the grain-loaded bar.