  This paper reports an experimental observation of broadband reversed Doppler
effects using an acoustic metamaterial with seven flute-like
double-meta-molecule clusters. Simulations and experiments verify that this
locally resonant acoustic metamaterial with simultaneous negative elastic
modulus and mass density can realize negative refraction in a broad frequency
range. The constructed metamaterial exhibits broadband reversed Doppler
effects. The frequency shift increases continuously as the frequency increases.
The assembling of double-meta-molecule clusters introduces a new direction in
designing double-negative acoustic metamaterials in an arbitrarily broad
frequency range and other various applications.
