Investigation On Strange Echoes From Steps-like Structures

When a firecracker is launched in front of the Maoshan martyrs' monument,people below the steps can clearly hear echoes like a bugle call.Yang et al.had used the negative reflection theory proposed by our laboratory to explain the phenomenon.We believe in the wide existence of this unusual echo phenomenon,not just only in the Maoshan martyrs' monument,but also in other large step-like structures of similar periodic or quasi-periodic structures.On the light of the negative reflection theory,echo experiments are carried out in this work on other similar structures,all confirming the validity of the negative reflection theory.At the same time,we also discover some soundscapes with other peculiar echoes.Finally,we proposed based on the negative theory a scheme for designing the steps-like structure that can produce specific music echo based on the reflection characteristics of the acoustic waves in the steps.We for the first time observe strange echoes much like bugle calls in the front steps of the mausoleum of LiangXiaoWang in Yongcheng,Henan province,by using explosions of fireworks as the sound source.The reflected sound from the ladder structure is analyzed theoretically.By considering the step structure as an acoustic grating,the Kirchhoff tangent plane method is applied to approximate the reflected sound field and study the reflection of the spherical wave of the source radiation at the quasi-periodic step surface.The results show that the steps have a comb filtering effect on the incident sound waves.And there is a significant negative reflection phenomenon that the incident and reflected sound field of the middle and high frequency are on the same side of the normal direction.The quasi-periodic steps change the spectrum of an explosion,so the echo sounds like a bugle call.According to our negative reflection theory developed earlier,the fundamental frequency of echo reflected by the steps is determined by the incident angle,reflected angle,and the size of the steps.The experiment in the mausoleum of LiangXiaowang shows that the fundamental frequency of the bugle is consistent with the theory,thus further verifying the validity of the theory.Also for the first time we heard burgle-like-echoes in our experiments conducted in the famous Sun Yat-sen Mausoleum in Nanjing,by using explosions of balloons as the sound source.Further experiments are carried out on similar other structures and architectures,e.g.,the the stony steps located at He-shan in Jimo,Shangdong and some ancient towers like the Yin Yin Towers in Yongji,Shanxi,the Baolun temple in Sanmenxia and the Fawang temple in Dengfeng,Henan province,etc.While clapping hands at the position of 20m in the front of the steps at He-shan and the Guanque tower,we can clearly hear crane-like echoes.On the other hands,at the ancient towers with several layers of lamination of overhanging eaves,we heard frog-like calls in response to hitting stones.All these experimental results show the strong dependence of the timbres of the echoes on the spectrum of the incident sound and the size of steps and the structures,with the fundamental frequency much consistent with what the negative reflection theory predicts.From the directivity diagram of the sound waves reflected from the steps,a large portion of the sound energy is returned almost along the opposite direction to the incidence,thus further confirming the validity of our negative reflection theory.The negative reflection theory also provides theoretical guidance for us to design soundscapes of special sound effects in constructions and structures,like those wonderful echoes as reflected from periodic or quasi-periodic steps of appropriate sizes.Based on this theory a scheme of design is proposed in this thesis to build a step structure that can produce a specified music rhythm.When the sound source and the observation point are far away from the steps,the incident and the reflection angle at the different steps do not change much.Therefor the fundamental frequency of the echo generated by the steps also does not change much.The fundamental frequency of desired echo is adjusted mainly by adjusting the size of the steps,and is corrected according to the incidence and reflection angle of the acoustic waves at the steps.Then,a finite element analysis is used to verify the design,.With the modulation of Gaussian pulse,the echo of the steps is consistent with the theoretical design.With the negative reflection theory,we can find more structures that reflect unusual echoes,and we can use negative reflection theory to build echo buildings that produce more interesting echoes.