Hi All,
While you may not hear a discrete delay (echo), assuming equal levels of both sources) until about 50mS (about 55 feet) or more, the common sources being combined but out of time will cause significant comb filtering (cancellations in the frequency response). Even a time arrival difference of 1mS causes combing to begin at 500 Hz and have intervals every 1k on up.
A good way to observe the impact of misalignment is to take two recordings of the same show, align them perfectly and then offset one of them by .5mS, then 1mS, and more to see how it works. This can be achieved with a good software package that allows working with more than two audio channels. The offset can be performed by dragging one file or through setting a delay on an insert or effects send.
It won't be until about 40mS or more that you can actually detect a discrete echo.
While the initial tendency is to hear the combined board/audience ("matrix") recording as fuller and richer, due to the level increase of two common sources, you will gradually start noticing the changing frequency response. I often find that it is one way to take two good recordings and make one mediocre one, unless great care is given to proper time alignment, including the changes that occur as the show goes on (heat and humidity), and changing the time offset as needed to maintain alignment.
As far as those who are interested in finding a delay unit, they are somewhat hard to come by. Those used for sound system driver and cabinet alignment often include numerous functions that will not be of use to most home users, unless you also want a great crossover and EQ. Then, expect to pay over $1000.00
Less expensive units like those from Rane and Behringer, available for $200-400.00, are known for their functionality and price point but not necessarily for optimal sound quality.
Still once you are in that price range, assuming you have a computer, you are well on your way to a recording package that would perform the exact functions and have more beneficial qualities to offer as well.
Happy Recording Everybody,
Marc