Abstract: In this paper, to establish a mathematical model of a spatial filter with smooth pass band and steep transition zone with vanishingly small separations and small-scale based on dual microphone system, the relationship between the phase difference of the two signals and attenuation coefficient is analyzed and derived using acoustic propagation characteristics of sound waves in space. Considering the advantages and disadvantages of step-type attenuation coefficient formula, another expression of attenuation coefficient with exponential type is provided, which reduce the voice interruption by using step-type formula, with smooth pass band and steep transition zone at the same time. A method to control the width of the main lobe of the spatial filter using the exponential type expression is proposed, which will adapt to various environments with different purpose.