We know that speed of sound is higher in a denser medium. So when a sound wave strikes a wall, why does it echo instead of passing through it with a speed faster than the speed of sound in air?

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    $\begingroup$ Tell that to our neighbours, the wall (concrete) transmits sound so good, you can hear a needle fall down on the other side, literally $\endgroup$ – PlasmaHH Feb 15 '16 at 8:43

Whenever a wave reaches a boundary between two mediums some of the energy is reflected and some is transmitted.
The important parameter is not just the speed of the wave on either side of the boundary but what is called the acoustic impedance (= density $\times$ speed). If there is a large difference between the acoustic impedances then you will get most of the wave reflected.
So if you have an air (speed of sound 330 m/s, density 1.2 kg/m$^3$) brick wall (4200 m/s, 1850 kg/m$^3$) boundary a lot of the sound will be reflected.

So when you walk down a tunnel the sound that you emit gets reflected off the walls and even off the open end of the tunnel and comes back to you as an echo.

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    $\begingroup$ I never understood impedance in EM or why there was reflection at an impedance boundary until I thought about acoustics and sound reflecting off of a wall, and then it all made sense :) $\endgroup$ – hobbs Feb 14 '16 at 21:28

Sound follows like everything else in nature. Minimize energy. So while sound can travel faster in a denser medium, the fact that sound is produced by mechanical waves mean that more energy is required to transmit the sound 100% through the wall with the same intensity.

And just like any other wave, when a sound wave hits a solid object like a wall, it will undergo reflection, dissipation and transmission. This transmission usually reduces the intensity of the travelling sound wave which is why when you are in an adjacent room, you can hear a fain part of the conversation from the next room. But at the same time, if the wall is made from a very dense material, then the sound waves don't have enough energy in creating mechanical waves in the material of the wall and thus you usually end up with only reflection (echo) and some dissipation.

  • $\begingroup$ Is it the fact that whatever portion of the original sound we receive, we get it a little bit earlier than we would have receive it through the same rarer medium? I mean, do an obstacle reduces intensity as well as carries the less intensified wave faster? $\endgroup$ – Priyankush Deka Feb 14 '16 at 11:49
  • $\begingroup$ The last line should be- I mean, does an obstacle reduce intensity as well as carry the less intensified wave faster? $\endgroup$ – Priyankush Deka Feb 14 '16 at 15:07
  • $\begingroup$ I think Farcher's answer describes this situation very well. And yes, this is exactly what it means. $\endgroup$ – user106422 Feb 14 '16 at 15:12
  • $\begingroup$ "Minimize energy" do you have a source? That sounds interesting. $\endgroup$ – PyRulez Feb 14 '16 at 15:29

When sound gets reflected by a wall, some portion of the sound is reflected back to air, some of it is converted into heat energy and lost in the atmosphere and some of it indeed gets transmitted through the wall. This is evident due to the fact that we are able to hear sounds coming from completely closed structures such as rooms. You might have noticed that loud sounds can make things vibrate.


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