First Echo

The human brain is very good at fitting together sound which arrives at the ear directly and sound which arrives slightly later; the delay can be "accidental" caused by a reflection from a room surface or "intentional" as part of the design of a sound system.

You can test this for yourself

get two radios and tune them to the same radio station - use a local station which broadcasts on a single frequency.
set them to similar volumes and then take one outside the room, say into the hall, but less than 8m away.
go back into the room and sitting say 1m from the "inside" radio, you should have no sense of the other radios existence.
however, turn the inside radio off, and the outside radio "springs into life".
naturally both radios were playing all the time, but you did not hear the outside radio; your brain joined the two sounds together and you only "heard" the sound which arrived first.

Sound engineers exploit the above phenomenon, by ensuring that sound from supplementary speakers arrives at the listener's ear slightly after sound from the main system on stage; that way, the volume at the ear is louder and in echoy (reverberant) conditions such as churches the sound is clearer; however, the listener still perceives that the sound is coming from "the front" and not from the supplementary speakers. At large outdoor concerts the banks of supplementary amplification are known as "delay towers" because of the artificial delay introduced into the sound system to ensure this effect.

All well and good, until strong reflections arrive at the ear more than 50 milliseconds after the direct signal; our brains can no longer join the two sounds together, and there a loss of clarity and confusion as to where the sound is coming from.

Acoustics and Noise Control - 2nd Edition is a standard acoustics textbook, published in UK by Addison Wesley Longman Limited and is available from this site here. Chapter 3 relates to room acoustics. At section 3.2 it is stated

"In large halls care must be taken to make certain that no strong reflections of sound are received by the audience after about 50 ms, otherwise confusion is likely between the direct and reflected sound for speech... This corresponds to a delay of about 17 m. A member of the audience sitting at 8.5 m from a good reflecting rear wall of a hall will find it difficult to understand speech."

If it is necessary to have large spaces, the methods of overcoming the problems are to shape the room such that strong echos are broken up, or profile "problem" walls with say shelving or other structures to break up the surface profile and thereby reduce echos from that surface; or to cover the "problem" wall with very acoustically absorbent materials, e.g. heavy curtains, again to reduce the echo.

The directory can be queried for consultants able to assess room acoustics and/of for suppliers of sound systems and absorbent materials.

Some of these problems can be heard on the BKLA website.

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