Sound travels at the speed of light when transmitted through electrical wiring. With these fact at hand, we can now study the echo. Therefore a wall that is 70 feet away from where the sound starts can produce an echo. At this distance, the echo may return soon enough that it doesn't interfere with the program. As you get further away, the echo becomes more apparent.
Most churches have foyer's at the back of the church.
The foyer will have either doors or a direct path into the worship area of the church. It is common to have speakers on a volume control to allow people to know when to enter the worship area and they often work as overflow speakers when the sanctuary is packed. When standing near the foyer speakers, the foyer speakers will get the sound first to the foyer. The sound from the main sanctuary speaker system has to travel though the air which is a slower medium for sound transmission. As a result, people sitting near the back of the church who can hear both the foyer speakers and the live sound at the same time will hear an echo.