Why Do Some Sound Waves Reflect While Others Are Absorbed?

[adjacent]

I think they(longitudinal waves)hits the air,but they are not reflected back because air is very light.In the same way,when they hit a wall,as the wall is not going to vibrate vigorously,Some of the waves are absorbed(i.e. they make the wall vibrate)and the others are reflected back.
If I am right,I don't understand why the wall reflects others back.
Can you help please!

 

[mfb]

Sound waves in air, hitting a wall? Air is light, so the wall does not start to vibrate significantly. This is good for reflection - at a perfect wall, pressure differences come in from one side, and start waves going backwards (as there is no way to "escape" through the wall).

Sound waves in the wall, hitting the border to air? Air is light, so the wall can vibrate without moving too much air. This is good for reflection - at a perfect boundary (to vacuum), velocity differences come in from one side, and start waves going backwards (as there is no way to "escape" through the boundary).

 

[adjacent]

What's that start waves?

 

[sophiecentaur]

When a wave (any kind) encounters a change of material, there are 'conditions' at the boundary that must be continuous e.g. the displacement must be the same on both sides of the boundary. If the wall cannot move much (it's dense and stiff) then the air right next to the wall cannot either (you can't have a vacuum there so the air will 'stick' to the surface). The only way this can happen is if another wave is produced in the air which is traveling in the reverse direction in the air (reflection). The smaller the difference in the characteristics as you cross the boundary, the more energy can get through and the less the reflection will be. An air to air interface will allow all the energy to pass with no reflection; all the sound energy will propagate through uniform air (but you can get slight reflections at a hot/cold or damp /dry air interface even).
There's a similar thing when going from a dense, stiff medium like a wall into air. The pressure in the air cannot get very high with the small displacement of the pressure waves in the wall material so a reflected wave is formed at the surface, sending energy back into the wall.
The term used for this effect is Impedance Mis-match or Impedance Matching. The nearer you can get the two impedances to 'match', the more wave energy gets through. A loudspeaker cone is made as light and wide as possible for this reason. Also, a microphone diaphragm is made as light and with as big an area as practical. If you take a tuning fork, you can hardly hear it at all because it doesn't 'couple' its energy into the air until you use a sounding board that is less dense and can shift a lot of air over a large area.