A delay line, in which the delay is accomplished acoustically, as in a long tube with a speaker at one end and a microphone at the other.
"Chris - Just got the CD, it is really good. I have to listen closely to hear even a bit of digittized sound. The Roger Wagner Chorale and it's music is long gone. I was not able to get anything on them, and I looked pretty good. You have truly resurrected this album. I have a couple more I will send as soon as I can get them packed up. Thanks ..."