In one embodiment, the present invention provides a portable speaker including a speaker driver, a first cylindrical ring coupled to the speaker driver, a base plate configured in parallel with the speaker driver, a second cylindrical ring affixed to the base plate, and one or more interposed unaffixed cylindrical rings, wherein in a first expanded state the sidewalls of adjacent rings form frictional seals and a substantially airtight rigid chamber having a height substantially equal to the sum of the sidewall heights of the cylindrical rings, and in an unexpanded state the sidewalls of the cylindrical rings are substantially parallel to one another. Embodiments of the present invention may be used in an audio system including a class-D amplifier.