In article , Phil Allison
wrote:

"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.

Yes. The significant point being that if you look in court papers, and
journal papers for the AES and Forensic Science you can see that the
variations with time show patterns that can be reliably used to tell the
date and time recordings were made since many of the details change
unpredictably from day to day.

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.

Indeed. And in Europe sometimes cover a series of linked countries.

That is one of the useful features as it means Forensic bodies can monitor
and collect data on the patterns for later comparison with evidential
recordings. As has happened in a number of cases already reported in the
journals.

AES members will have seen papers on this recently. The September 2009
issue of JAES discusses the points I make above. Contains a couple of
papers and a feature article on Audio Forensics. Must admit I was surprised
to read just how useful the technique had turned out to be!

Slainte,

Jim

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