The experimental system of our optical beating Brillouin spectroscopy was improved, which can observe the phonon propagation with the resolution of 1 kHz. The increased sensitivity and frequency resolution were demonstrated in the observation of phonon spectrum in solids of fused silica and also in gases, nitrogen, and freon CHClF2: for their weak light‐scattering ability, these substances are very tough specimens for Brillouin scattering studies, and the classical spectrometer of Fabry‐Pérot etalon has been so far used for the frequency analysis. The phonon peaks were observed with the present light beating system in nitrogen and freon near 300 K over the wave‐number range from 2π=6.20×103 to 3.09×105 m−1, the pressure range from 14.8 to 177 kPa. The observed spectra were well fitted with the Lorentzian curve representing the phonon lifetime, and the phase velocity. The phase velocity of phonon in fused silica agrees well with that obtained by ultrasonic measurement, while the phonon dispersions obtained for gases show the relaxational behavior due to the coupling to the rotational and vibrational motion of molecules.