Microphone's output (recorded audio) is not stable -Acoustics Forum-Acoustic Fields Forum

We are developing an audio processing system. As a part of its process, we play a pure sinusoidal signal with constant amplitude in a speaker, and record the audio emanated from the speaker with a unidirectional microphone.

When the distance between the microphone and the speaker is (almost) zero, we get consistent audio recorded in the microphone. However, when the distance is increased, we see that the audio in the microphone fluctuates in terms of its amplitude, and the higher the distance, the greater the fluctuation.

Is it happening due to any environmental reason like reflections? How can we make the microphone input stable?