Quarter Lambda Absorbers Rejuvenated as Multi-Frequency In-Plane Arrays 971879
A re-examination of the functioning of quarter wavelength tubular resonator absorbers did highlight the destructive interference near the tube opening (area S2) between the wave reflected from the resonator termination and the one reflected from the surface adjacent to the opening (area S1). Analytical considerations showed that, for perpendicular incidence, 100% absorption is achieved when the ratio of surface areas equals the specific termination impedance of the resonator absorber .
This same mechanism is also effective when the resonator lies in the reflecting plane. This latter point has important practical consequences because it allows one to integrate practical and effective absorbers in relatively thin treatments. Experimental work has shown that S1/S2 ratio's of 20 are readily achieved, and that regarding bandwidth, one or two sets of tubes per 1/3 octave are usually adequate. Multi-frequency arrays are created by interlacing sets of tubes tuned to different frequencies. Absorber arrays should always have the openings pointed downward in mounted position in order to prevent the accumulation of liquids and dirt in the tubes.