Abstract: This paper describes an acoustical array structure with four elements that have the directivity of cosine function over certain frequency band.The acoustical axis directions of these elements are stagged off 90°each other,so their main beam can be covered with in 360°in the horizontal plane.It is easy to detect the bearing of incident acoustic signal with amplitude-bearingme-thod.A reflective rubber is adopted as a back baffle of the array.It has been verified,in practice,that the constant beam-width for a ratio of high to low frequency of 8:1 may be obtained.Moreover,this paper also gives a mathmatic model for the array structure.It is consistant with theoretical calculated and practical measured values.