A model for the accurate measurement of low frequency sound spectrum is put forward and a simplified experimental device is built. Based on the principle of mechanical resonance, the natural frequency of the system is controlled, which realizes the function of both measuring frequency spectrum and monitoring noises at low frequency. The device is able to measure the acoustic source at a single frequency below 100 Hz as well as to analyze the spectra of acoustic sources at mixed frequencies, with a relatively high resolution of 1 Hz beyond 40 Hz.