In this paper, the Hermite-Gauss model TEM00 was treated as a signal light field, and a local light field composed of small displacement and balance homodyne detection system was studied. The experiment was carried out based on the system of sound frequencies (20 Hz to 20 kHz) transverse small displacement measurement. We discussed the shot noise of system, directivity noise and laser intensity noise effect on the displacement of the minimum detectable. According to the experimental results of noise measurement, when the SNR is 3 dB, the minimum measurable displacement at different frequencies in the sound frequency band is given, of which, the minimum measurable displacement at 1 kHz is 3×10－9?m, the minimum measurable displacement at 10 kHz is 1.2×10－10?m and the minimum measurable displacement at 15 kHz is 0.85×10－10?m. This paper lays a certain foundation for the experimental and theoretical research on the suppression of laser noise in acoustic frequency band and the reduction of directional noise in small displacement measurement system.