Compared with the waveguide invariant, the interference structures in low-frequency acoustic field are more comprehensively described by four system functions: coherent function, power response function, spread function and dual-frequency function, from space-time filter theory. A stable interference structure in the low-frequency acoustic field is testified based on theoretical analysis, simulation and sea trial data processing. When the target moves in a uniform rectilinear motion from far to near then from near to far, the interference structure of coherent function (LOFARgram) appears to be a family of quasi hyperbolas; the interference structure obtained by power response function represents the group delay difference and the dispersive characteristic of the normal mode; and the phase slowness difference can be exhibited by dual-frequency function. Each system function owns special characteristics, which can give prominence to certain features of the interference structure.