We hear the echo only in the case where the reflected sound is perceived separately from the spoken. For this purpose, the period of time between exposure to the two sounds in the ear the eardrum was not less than 0.06 C.
To determine how long after the man uttered a short cry of reflected sound reaches his ear, if he stands at a distance of 2 m from the wall. The sound must go double the distance – to-wall and back, i.e. 4 m, spreading at a speed of 340 m/s. it takes time t=s : v, i.e.
t= 4 m : 340 m/s ≈ 0,01.
In this case, the interval between the two perceived sounds - spoken and reflected is significantly less than that required to hear the echo. In addition to the formation of the echo in the room prevents the furniture, curtains and other items, partially absorbing reflected sound. So in this room of people talking and other sounds are not distorted by echo, but the sound clearly and legibly.
Large half-empty room with smooth walls, floor and ceiling have the property to reflect sound waves. In this room due to the overrunning of previous sound waves on subsequent turns the overlay of sounds, and produces hum. To improve the sound properties of large halls and auditoriums often stone walls with acoustic materials.
On the ability of sound to bounce off of smooth surfaces based action of the horn – flared tubes are normally round or rectangular. When using the sound waves are not scattered in all directions and form a narrow beam, whereby the sound power increases and it extends to a greater distance.