[BlueDragon]

woops -- I see I posted a badly garbled sentence.

how far away from substantial enough in size sound reflective that could be recording small echoes?

my apologies. But maybe you can easily enough decipher what I intended to mean...

The "sound reflective" material (a wall, or anything hard and flat enough, and of enough size, and angled just so) would not itself be recording any sound, but instead, (possibly) only reflecting it towards a microphone.

[Diogenesis]

Any reflecting surface would have the reflecting
pulses match in time. The reflection would be
third order, very small, but the time sequences
would match, making this an important video presentation.