Question

Sound travels fastest in solids...yeah?
Then why a person sitting in room not able to listen the sound of other person sitting in another room as walls are also solids. So sound should travel and it should be listen to other person but this doesn't happen. Why so?

Answer 1

╔═══❁═❀═✪═❀═❁════╗
.......Here You Go Ur Answer............
╚═══❁═❀═✪═❀═❁════╝

The difficulty of hearing your friend is an example of the importance of “impedance matching”. Because air is much less dense than wallboard, the ratio

Sound pressure (fluctuation of air pressure) / 
Displacement (fluctuation of air location),

called the “impedance”, is correspondingly lower in air than in wallboard. Impedance is roughly proportional to density. When sound in air impinges on wallboard, the sound pressure in the wallboard is about the same as in the air, and the displacement is lower in proportion as the impedance (proportional to density) is higher.

Since the product

Sound pressure × Displacement

is the sound’s intensity (power per unit area), the reduced displacement means that the intensity in the wallboard is lower than in the air, approximately in proportion as the impedance is higher. The rest of the sound power in the air goes mostly into reflection of the sound from the wall, and some sound power is lost, i.e. turned into heat, at the surface.

As the sound propagates from the wall into the air of the adjacent room, there is another loss of about the same amount. The complex structure inside the wall leads to other losses at inner surfaces. Again, the losses are mostly in the form of reflection back toward the source.

A funnel held against your side of the wall, with the narrow end against your ear-hole, provides some impedance matching by gathering the displacement over a wider area of the wall and concentrating it into a narrower area near your ear. Thus, your friend is more clearly audible. Sound power is conserved, with theoretically perfect impedance matching, because the funnel imposes higher reactive pressure against the wall and thus reduces the reflection back into the wall.

In related matters,

Your sound-sensing nerve ends are immersed in liquid in your inner ear, where the impedance is much higher than in air. Your external ears contain a funneling geometry, and your middle ears contain a membrane (the ear drum), and levers (the “stirrup”, “hammer”, and “anvil”), to match impedance between air and your inner ears.