Reference: Timothy Mellow, Olga Umnova, Konstantinos Drossos, Keith Holland, Andrew Flewitt and Leo Kärkkäinen, “On The Adsorption - Desorption Relaxation Time Of Carbon In Very Narrow Ducts”, in proceedings of the Acoustics 08 conference, Jun. 29–Jul. 04, Paris, France, 2008
Abstract: Loudspeakers generally have boxes to prevent rear wave cancellation at low frequencies. However, the stiffness of the air in a small box reduces the diaphragm’s excursion at low frequencies. Hence the box size is generally a compromise between low frequency performance and practicality. Activated carbon has been found to increase the apparent size of a given box through adsorption of the air molecules when the pressure increases and likewise desorption when it decreases. However, the exact viscous effects in the granular structure are difficult to model. Thus it is impossible determine the high frequency limit due to the natural adsorption/desorption relaxation time in the absence of viscous losses. In this study, a tube model is presented which takes into account viscous and thermal losses with boundary slip together with adsorption. Impedance measurements are performed on an array of 12 million holes, each 2 micrometers in diameter, etched in a 0.25 mm thick silicon wafer so that the viscous and thermal losses can be verified against the model without adsorption. Impedance measurements are then performed on an array of holes coated with graphite in order to create an activated carbon- like structure, thus enabling the adsorption/desorption relaxation time to be evaluated.
AI-Generated Summary: This paper explores a way to make very small loudspeakers produce better low-frequency sound without requiring a larger enclosure. Activated carbon can make a small loudspeaker box behave as if it were acoustically larger by absorbing and releasing air as the pressure inside changes, but this effect becomes much weaker at higher frequencies. The paper develops a model to better understand what limits this behaviour, separating the adsorption process itself from other losses that occur as air moves through very small structures. To do this, it considers a simpler and more controlled structure made of extremely narrow graphite-coated channels rather than conventional granular activated carbon. The simulations show how the speed and capacity of the adsorption process affect both the amount of apparent volume increase and the frequency range over which it remains useful. The paper also proposes an experimental method for measuring these properties directly, although the measurements were not yet available when the work was published.
