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We report that Hybrid membrane resonators (HMRs) made of a decorated membraneresonator backed by a shallow cavity can function as Helmholtz resonators (HRs)when mounted on the sidewall of a clear waveguide for air ventilation. When twosingle-frequency HMRs are used in the same scheme as two frequency-detuned HRs,asymmetric total absorption/reflection is demonstrated at 286.7 Hz withabsorption coefficient over 97 % in a waveguide 9 cm x 9 cm in cross section.When two multiple-frequency HMRs are used, absorption in the range of near 60 %to above 80 % is observed at 403 Hz, 450 Hz, 688 Hz, 863 Hz and 945 Hz.Theoretical predictions agree well with the experimental data. The HMRs mayreplace HRs in duct noise reduction applications in that at a single operationfrequency they have stronger strength to cover a much larger cross section areathan that of HRs with similar cavity volume, and they can be designed toprovide multiple frequency absorption band.

Hybrid membrane resonators for multiple frequency asymmetric absorption and reflection in large waveguide