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Anechoic Chamber Walls: Should They Be Resistive or Reactive at Low Frequencies?
The theoretical design and the preliminary practical implementation issues of an anechoic chamber designed specifically for spherical-wave propagation are described. Conventional anechoic chamber design methods dictate that the acoustic impedance of the chamber's boundaries should be purely resistive (complete absorption) over the whole operational range of the chamber. For good loudspeaker measurements at low frequencies, this means large chambers and long absorptive wedges. Theory suggests that a relatively small spherically shaped chamber, with the source constrained to the center of the sphere, could be designed that operates down to any arbitrary frequency, if the chamber walls are mass reactive at lower frequencies where the wavelengths are much larger than the chamber dimensions, and absorptive at higher frequencies where the wavelengths are much shorter than the chamber dimensions. A first-order mechanical model of the wall impedance is a massless plate for the sound waves to impinge upon, connected to a free-standing mass through a damper. At low frequencies the whole assembly moves, thus presenting a mass reactance to the wave, while at high frequencies the mass would be essentially immobile, and thus energy would be absorbed by the damper. The crossover point between the two modes of operation occurs at the frequency where the wavelength is equal to the circumference of the sphere, or equivalently, the radius of the sphere is about one-sixth wavelength. Derivations show that the total movable mass of the chamber's walls should be exactly three times the mass of the air contained in the sphere. These ideas are explored.
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