was 1/2 of a set of jensen headphones, the singer would have to hold it in his hand and scream into the cup. The recording sounded like Led Zeppelin doing a Three dog night song.
I guess we're dealing with reciprocal devices here in that any speaker can serve as a crude mic and any mic can be a crude speaker. of course all bets are off for any form of impedence match, but the coil works both ways.