![]() This type of metamaterial is composed of Helmholtz resonators of which the group and phase velocities are opposite in direction near the resonant frequency. manufactured a one-dimensional acoustic metamaterial with negative effective dynamic modulus at ultrasound frequencies. Various acoustic metamaterials with negative effective mass density were engineered by periodically arranging the acoustic “meta-atoms” 16. The resonant frequency of each unit is primarily determined by its geometry and size. In particular, each resonance unit - which is equivalently an artificial “meta-atom” - that constitutes the acoustic metamaterial is orders of magnitude smaller in size relative to the effective operating wavelength. proposed the idea of localized resonance, which later became the foundation of acoustic metamaterials. Since then the research of left-handed materials boomed, resulting in electromagnetic metamaterials 11– 15 being one of the mostly studied area. first manufactured a left-handed material with periodically arranged metallic wires and split rings, and experimentally demonstrated the phenomenon of negative refraction 10. presented that periodically arranged metallic wires and split resonance rings can realize negative permittivity and negative magnetic permeability, respectively 8, 9. In 1968, Veselago theoretically predicted that the inverse Doppler effect 7 could be observed in materials with negative refractions ( n < 0). Nowadays, this phenomenon is utilized in many fields, including space technology, traffic control, disease diagnosis, to name a few 3– 6. In 1843, Doppler first generalized the change in frequency of a wave for an observer moving relative to its source 1, 2. The Doppler effect is a fundamental phenomenon in wave propagation. ![]() This newly discovered phenomenon would also offer a clue into designing next-generation smart broadband double-negative acoustic metamaterials with varying refractive index. This study suggests that recorder and clarinet may be the earliest man-made acoustic metamaterials known so far, offering a new explanation why pipe instruments have enjoyed wide popularity in Europe and Asia over the past hundreds and thousands years. The calculated effective refractive indices of these two pipe instruments are negative and varying across a set of pitches, exhibiting a desired characteristic of broadband acoustic metamaterials. Our study shows that the inverse Doppler effects can be detected at all seven pitches of an ascending musical scale when there is a relative motion between a microphone (observer) and abovementioned two pipe instruments (source). Built on the foundation of previous flute and flute-like acoustic metamaterial models, we herein report the experimental results of the inverse Doppler effects discovered in two common pipe instruments - recorder and clarinet. The pipe instrument is one of the most popular musical instruments of all time. Music is older than language, and for most of human history music holds our culture together.
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