Question

when is reflection of sound harmful?

Answer 1

reflection of sound is called an echo.
An echo is not harmful/ first if you produce sound in high frequency you can't hear. second when you try reflecting it. the freq reduces. So the reflection of sound becoming harmful is of low possibility.

instead the sound - Volume+Intensity of sound when I reached more than 100db it's harmful

Answer 2

reflection of sound as harmful when multiple sounds get mixed together due to reflection