Active sound absorption

Sound attenuation is usually obtained by means of absorbing materials. This technique, which works quite well for sounds of medium and high frequencies, is very inefficient at low frequencies, because the thickness of absorbing material necessary to produce a constant attenuation increases with decreasing frequency. There is another technique by which destructive interference is used to silence sound fields: it is active sound absorption. Hitherto, this was based on Young’s principle of interference. This paper proposes a general theory, using Huygens’ principle, for three-dimensional sound-wave propagation. Some experiments are also reported and discussed.