As with any other wave, sound waves, upon reaching a fixed obstacle, such as a wall, are reflected.
Sound reflection happens with phase inversion, but maintains the same propagation speed, frequency, and wavelength as the incident sound.
A well-known effect caused by sound reflection is the effect of Echo It consists of the reflection of the sound that hits a far wall.
When a person makes a sound towards an obstacle, that sound is heard at the time of emission, called direct sound, and the moment the reflected sound through the obstacle returns to him.
We know that the velocity is given by the distance traveled by the sound in a given time, this distance is given twice the distance to the reflecting obstacle, since the sound goes back and forth. Like this:
And the speed is the propagation of sound in the air.
Upon receiving a sound, it "stays" in nodes for about 0.1s, this interval being known as acoustic persistence.
By speed ratio:
If this time interval is less than acoustic persistence (t <0.1s), the sound heard after being reflected will seem only as a prolongation of the direct sound. This effect is called reverberation. For intervals longer than acoustic persistence (t> 0.1s) it is instinctive to realize that this reflection will be heard as an echo.
The other phenomena happen in the same way as for the other waves studied. Having a well-known use of sound interference where it is possible to apply an anti-noise frequency in order to soften the ambient sound.