Low frequency sound absorbers

Several kinds of effective absorbers of very low frequency (20 Hz-100 Hz) sound waves are examined. Comparisons show that a surface absorber of electromagnetic induction type is a feasible choice. The theoretical limitation of the absorption coefficient of the single surface absorber is 50%. A resonator system of sound absorption can be constructed by the surface absorber and a rigid wall on the back of and parallel to the surface. Absorption coefficients larger than 55% for the entire range of very low frequency may be obtained by the system. The absorption coefficients at some frequencies may be larger than 90%. The motion of the surface can be controled by an electronic circuitry in order to obtain optimum values of the equivalent parameters of the surface. It is therefore rather easy, based on the electromagnetic induction surface absorber, to develop a sturdy and perfect absorber system of very low frequency sound waves.