Why does sound reflect and create an echo in certain conditions?

[Misr]

1-why does sound intensity increase at night?
2-what makes diffraction of sound more observable than diffraction of light?
3-reflection of sound takes place when it falls on a reflecting surface whatever the distance between the vibrating source and the surface
but in case of echo there are certain conditions to be heard,,,so am I understanding this in the right way??

 

[Integral]

1-why does sound intensity increase at night?

This depends or air density, humidity, temperature and wind direction. Denser air transmits sound better. Generally night air is usually denser then day air, so sound usually carries better at night.

2-what makes diffraction of sound more observable than diffraction of light?

Wavelength.

3-reflection of sound takes place when it falls on a reflecting surface whatever the distance between the vibrating source and the surface
but in case of echo there are certain conditions to be heard,,,so am I understanding this in the right way??

Please rephrase question 3.

 

[Misr]

Denser air transmits sound better. Generally night air is usually denser then day air, so sound usually carries better at night.

what makes denser air transmit sound better?

Wavelength

I know.but how?if the wavelength of a sound wave is bigger so what makes this observable?

Please rephrase question 3.

I mean, echo is a result of sound reflection...but sound can reflect also without making an echo (depends upon the distance between the vibrating source and the reflecting surface)
hope it's clear now
Thanks very much for your help