Have you guys ever felt that it is quiet at night than in comparison to day?I'm known with the fact that reduction in people's activity makes night quiet...but is there something else which in a way amplifies the sound wave in night?
The difference between day and night can be pretty big, up to tens of decibels. But most of this is likely due to different activity levels - less traffic, industry, voices/animal sounds etc.
Outdoor sound propagation depends on a number of factors, but their impact in day and night will be variable.
In particular, there is a temperature dependency in how fast sound intensities are attenuated in air: as it gets colder attenuation goes up somewhat. So one might think that as the day cools off the range of sounds decrease. However, there are complications here like humidity changes (dampens some frequencies but not others).
Temperature also affects the speed of sound, making temperature gradients refract sounds in the direction of lower sound velocity (that is, lower temperature). In the night a temperature inversion is common, with the ground and the low air colder than the upper air, and this tends to make sound travel longer distances since it is focused along the ground rather than radiated upward.
A further issue is wind, which can amplify sound in some directions, add turbulent damping, and of course cause noise.
In short, there are various attenuation effects that could play a role. But I suspect the main cause is just less noise sources.