Ἠχώ

"The echo occurs because of a reverberation of the air struck by the voice, from the bodies being opposite and smooth, especially the hollow ones. For the speaking person thrusts away almost all of the air surrounding him, and especially the air in front of him; this is the reason why we hear more clearly when the speaking person is looking at us, than when his face is turned aside. Now, this air in front, when thrust away, whenever it is brought to some object opposite provided with a hollow -- like caves -- since it is continuous and can remain unscattered due to the fact that it is contained by the hollow, running to the previously mentioned opposite body rebounds back, just like the ball which is thrown against a wall.¹ Thus, [the air] moves back to the person who struck it, obviously because the air is set in motion in both directions according to its faculty of conducting sound; for if the air rebounds back in the opposite direction just in the same condition as it has been brought by the effect of the repercussion of the solid body, it is not only because the hollow space does not admit it to be dispersed, but because when it hits this hollow object, it hits [the object] not as an empty [thing], but as full of air, and this air² is continuous and kept united by the surrounding space. Thus, the air which has been struck by the speaking person and carries the sound,³ dividing this [air contained in the hollow] and prevented by that which is continuous and united from being scattered, falls upon the solid body, and since it cannot move ahead, it is reflected in the opposite direction with its own energy -- I mean, the energy of the sound -- and thus, it happens that the sounds come back. But Alexander offers a different explanation."⁴