It is true that the farther away from the sound source the smaller the measured sound level.
Sound wave is a kind of energy, it is propagated in the actual medium, due to diffusion, absorption, scattering and other effects, so that the energy of the sound wave with the increase in the distance from the source of the gradual attenuation of the amount of attenuation of the sound energy with the propagation distance and the frequency of the sound wave. According to the law of conservation of energy: long distance consumes more energy, the sound is small.
Sound level is a physical quantity consistent with people’s subjective perception of the strength of sound, the unit is decibel. When the intensity and frequency of the sound source must be certain, the intermediate medium under the same conditions, the farther the source is from the detection point, the smaller the sound energy transmitted to the detection point, the smaller the measured sound level.