Based on the fundamental concept of modulation, we have analysed the related facts and concluded that the modulation of acousto-optic sounding wave modulator is a broad frequency modulation. It is shown that the criteria of broad band modulation of this type are v<0.9 (v=2πL△μ/λ is the independent variable of diffracted light beam I(θ,t) ).A certain part of travelling wave is generally contained in the diffracted light beam of ordinary standing wave modulators. But in the range of Raman-Nath diffraction the diffraction angles abide by the formula sin θmi=±mλ/Λ, so that diffracted beams by both standing wave modulation and travelling wave appear in the same space range. In this paper a method of sprectal analysis for measuring the modulation index in the frequency-domain is proposed. The formulas of measurable values for zero-order and first-order diffracted light beams are given and verified experimentally.