arranged in such a way that the fraction of sound they do reflect is directed towards another baffle instead of back into the room. This makes the chamber almost devoid of echos which is useful for measuring the sound pressure level of a source and for various other experiments and measurements. Anechoic chambers are expensive for several reasons and are therefore not common. They must be isolated from outside influences (e.g., planes, trains, automobiles, snowmobiles, elevators, pumps, ...; indeed any source of sound which may interfere with measurements inside the chamber) and they must be physically large. The first, environmental isolation, requires in most cases specially constructed, nearly always massive, and likewise thick, walls, floors, and ceilings. Such chambers are often built as spring supported isolated rooms within a larger building. The National Research Council in Canada has a modern anechoic chamber, and has posted a video on the Web, noting these as well as other constructional details. Doors must be specially made, sealing for them must be acoustically complete (no leaks around the edges), ventilation (if any) carefully managed, and lighting chosen to be silent. The second requirement follows in part from the first and from the necessity of preventing reverberation inside the room from, say, a sound source being tested. Preventing echoes is almost always done with absorptive foam wedges on walls, floors and ceilings, and if they are to be effective at low frequencies, these must be physically large; the lower the frequencies to be absorbed, the larger they must be. An anechoic chamber must therefore be large to accommodate those absorbers and isolation schemes, but still allow for space for experimental apparatus and units under test. Electrical and mechanical analogy The energy dissipated within a medium as sound travels through it is analogous to the energy dissipated in electrical resistors or that dissipated in mechanical dampers for mechanical motion transmission systems. All three are equivalent to the resistive part of a system of resistive and reactive elements. The resistive elements dissipate energy (irreversibly into heat) and the reactive elements store and release energy (reversibly, neglecting small losses). The reactive parts of an acoustic medium are determined by its bulk modulus and its density, analogous to respectively an electrical capacitor and an electrical inductor, and analogous to, respectively, a mechanical spring attached to a mass. Note that since dissipation solely relies on the resistive element it is