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In this research, we propose and design an acoustic metamuffler (AMM) by coupling a micro-perforated plate and a composite waveguide formed by a main waveguide and a Helmholtz resonator. The proposed mechanism and the deliberately designed structure are conducive to generating multimode resonances which help to improve the coupling absorption effect and lead to a broadband (4 octaves) sound insulation. We develop an effective circuit model to analytically predict the insulation bandwidth and put forward numerical and experimental measurements that demonstrate the effectiveness of the proposed concept. The designed AMM produces sound insulation with an average of 20 dB of sound transmission loss at a low frequency range extending from 100 to 1600 Hz while having an ultrathin thickness of 6.2 cm (1/55λ for the lowest working frequency). Our findings could have pragmatic applications for acoustic insulators or absorbers.

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