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?
Answer
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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