Low frequency sound propagation in activated carbon

Activated carbon can adsorb and desorb gas molecules onto and off its surface. Research has examinedwhether this sorption affects low frequency sound waves, with pressures typical of audiblesound, interacting with granular activated carbon. Impedance tube measurements were undertakenexamining the resonant frequencies of Helmholtz resonators with different backing materials. Itwas found that the addition of activated carbon increased the compliance of the backing volume.The effect was observed up to the highest frequency measured (500 Hz), but was most significant atlower frequencies (at higher frequencies another phenomenon can explain the behavior). An apparatuswas constructed to measure the effective porosity of the activated carbon as well as the numberof moles adsorbed at sound pressures between 104 and 118 dB and low frequencies between 20and 55 Hz. Whilst the results were consistent with adsorption affecting sound propagation, otherphenomena cannot be ruled out. Measurements of sorption isotherms showed that additional energylosses can be caused by water vapor condensing onto and then evaporating from the surface of thematerial. However, the excess absorption measured for low frequency sound waves is primarilycaused by decreases in surface reactance rather than changes in surface resistance