Ultraslow medium with an acoustic membrane-like undamped dynamic vibration absorber for low-frequency isolation

Abstract

An extremely dense and thick sound panel is generally used to block low-frequency sound waves, according to the mass–density law. However, it is possible to block these sound waves with an ultralight thin panel using an ultraslow medium. In this study, theoretical and experimental methods are used to realize an ultrahigh-density acoustic metasurface consisting of thin membranes decorated with a coiled ring. When an incident sound wave induces membrane vibration, the coiled ring also vibrates. The natural frequencies of the two composite vibrating systems provide maximum membrane displacement, thus yielding zero density and extraordinary transmission. Near a frequency ω0=ks∕Ms determined by the elastic modulus ks and mass Ms of the coiled rings, the effective dynamic displacement of the membrane reduces to zero, thus implying maximum density of the membrane. At ω0, the coiled rings act as dynamic vibration absorbers because the induced force exerted by them is opposite to the identical pressure force. Thus, the acoustic metasurface provides a sound transmission loss of 48 dB at 65.7 Hz, a thickness of less than 20 mm, and a surface density of less than 1.27 kg/m2. By arranging eight unit cells in a parallel array, the proposed artificial acoustic waveguide provides a density 2,000 times higher than air density, thus realizing an ultraslow sound medium below 7.5 m/s. This medium can be used for ultralight sound proofing plates operating at the deep subwavelength scale.