"Jim Lesurf"

Electric Network Frequency. In effect, allowing the sound recording to
have a small amount of 'mains hum'. (Which tends to happen anyway, even
when you *don't* want it. ;- )

The frequency of mains hum varies with time,


** It wanders a small amount, like 0.1%, around the nominal frequency.


and differs from time to time and with the location (i.e. which mains
network covers your area).

** Mains networks cover huge geographical areas, sometimes whole countries -
within which the frequency is locked in phase.


Hence
it can and has been used as evicence that recordings were recorded when
and where claimed and have no breaks or edits - or not! Bit like an audio
forensic version of dating a tree by its growth rings. Surprisngly, this
technique has been used a number of times in court, etc, and apparently
works well. Even for recordings made on battery systems where the mics
picked up the hum as interference.


** Sure - you can say if it was in a 50Hz or a 60 Hz country.

That really narrows things down.



...... Phil