Abstract: The mysterious “Maoshan Bugle” phenomenon, the clearly audible bugle-like sounds after a launched firecracker blast in the air about 39 m high in the square in front of the Southern Jiangsu Anti-Japanese War Victory Monument at Mount Maoshan in Jiangsu, China, has drawn attentions of architects and acousticians in the past two decades. In this report, we attempted to reveal the physical mechanism of bugle calls and proposed the concept of acoustic negative reflection to explain the phenomenon. First, a model of plane sound wave reflection from an infinite periodic rigid surface is built and rigorously solved using an exact method of variation of boundaries, especially in the resonance region where the sound wavelength is of the same order as the periodic spacing. Then, numerical calculation is performed for the model with its periodic unit being taken as regular steps. Attention is then paid on the singular phenomenon that most of the incident sound power is reflected to the side of incidence, rather than to the side of specular reflection. This is what we call the negative reflection of sound, which is similar to the effects of blazed grating in optics. We then show the mysterious bugle calls at Mount Maoshan are actually the negative reflections, which are most clearly heard on the central axis in front of the square. The six notes of the bugle call are a result of sound reflection from the six groups of steps in front of the monument, which serve as acoustic gratings. According to the characteristics of the grating reflection, only certain frequencies in the explosion spectrum can be received. We further proved that these frequencies are
integer multiples of the fundamental. Thus people hear the echo much like the rhythmic sound of a bugle, instead of the explosive sound. The spectrum characteristic of the reflected sound is theoretically analyzed by using the grating equation, with the calculated fundamental and harmonic frequencies well fitting with the audio samples recorded on site. Finally, we discussed the possibilities of finding similar phenomenon in other step-like structures.