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"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 air[2] 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]

*)hxw/: o(/ti gi/netai h( h)xw\ kata\ a)ntana/klasin th\n a)po\ tw=n a)ntitu/pwn kai\ lei/wn swma/twn, ma/lista de\ koi/lwn, tou= plhge/ntos u(po\ th=s fwnh=s a)e/ros. o( ga\r dialego/menos w)qei= me\n sxedo\n pa/nta to\n pe/ric a)e/ra, ma/lista de\ to\n e)/mprosqen: dio\ ma=llon a)kou/omen pro\s h(ma=s o(rw=ntos tou= dialegome/nou h)\ a)pestramme/nou. ou(=tos dh\ o( e)/mprosqen a)h\r w)qou/menos, o(/tan e)nexqh=| e)pi/ ti a)nti/tupon e)/xon koilo/thta, oi(=a/ e)sti ta\ a)/ntra, sunexh\s w)\n kai\ a)diaske/dastos mei/nas dia\ to\ perie/xesqai u(po\ tou= koilw/matos, prospesw\n au)tw=| a)ntitu/pw| o)/nti sw/mati w)qei=tai ei)s to\ e)/mpalin, w(/sper h( r(iptome/nh kata\ tou= tei/xou sfai=ra, kai\ ou(/tw xwrei= e)pi\ to\n plh/canta, e)nergou=ntos dhlono/ti tou= a)e/ros di' a)mfo/tera kata\ th\n dihxh= au)tou= du/namin. ou) mo/non ga\r dia\ to\ mh\ sugxwrei=sqai u(po\ tou= koi/lou to/pou diaskedasqh=nai, oi(=os h)ne/xqh u(po\ th=s a)ntitupi/as tou= sklhrou= sw/matos ei)s tou)nanti/on a)pwqei=tai, a)ll' o(/ti kai\ tw=| koi/lw| tou/tw|, w(=| prospi/ptei, ou) kenw=| o)/nti prospi/ptei, a)ll' a)e/ros plh/rei, w(/sper a)h\r sunexh/s e)sti kai\ h(nwme/nos dia\ to\ perie/xon. tou=ton ou)=n u(po\ tou= fwnh/santos plhgei\s o( a)h\r kai\ yo/fwn dielw\n th=| fora=| kai\ kwluqei\s u(p' au)tou=, sunexou=s o)/ntos kai\ h(nwme/nou, diaskedasqh=nai prosptai/ei tw=| sklhrw=| sw/mati, kai\ mh\ duna/menos pro/sw xwrh=sai a)ntanakla=tai ei)s tou)pi/sw meta\ th=s oi)kei/as e)nergei/as, le/gw dh\ tou= yo/fou, kai\ ou(/tw sumbai/nei palindromei=n to\n yo/fon. *)ale/candros de\ e(te/rws fhsi/n.

Philoponus, On Aristotle's De anima 360.20 - 361.5 Hayduck (on De anima 419b25). For divergences, see the notes below.
[1] Aristotle, De anima 419b27; cf. Alexander of Aphrodisias, On Aristotle's De anima 48.18.
[2] The Suda reads an unlikely w(/sper "like", instead of the relative o(/sper of Philoponus, translated here.
[3] Here too the comparison with Philoponus' original text is helpful with a difficulty in the Suda's text, since it reveals that yo/fwn should be yofw=n "carrying the sound". Cf. Alexander of Aphrodisias, On Aristotle's De anima 48.7 e)/ti fula/sswn yo/fon, "(the air) still keeping the sound".
[4] (This last sentence is the lexicographer's summary of what Philoponus goes on to say.) Alexander of Aphrodisias, On Aristotle's De anima 48.7ff. explains that "we may say that the air moving to the hollow body and to the air contained in it is not the first air, the one which received the strike [...] rather, the first air, being continuous and not scattered because of the velocity of the strike, scatters the air in front of it with a similar strike, and this thrusts the air in front of it and so on [...] The last part of the air, prevented from transmitting the strike any more by the vessel, is thrust back [...] like a ball by a rigid object". The phenomenon of echo, continues Alexander, is related to the "emptiness" of air, since an empty space is of crucial importance in the production of hearing.

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