The hum that helps to fight crime

So if one were to set up a continuous recording of mains electricity, and then provide a UI that lets you grab a slice of the hum (a time range), and sell that, you could make some rather evil money. Or, if you were the police, have your "real" recordings verified very easily.

"This buzz is an annoyance for sound engineers trying to make the highest quality recordings."<p>It hit me a while back when I was writing the software for a test stand for a hearing aid as a summer intern. Mysterious 60/120/180 Hz frequencies appeared on our analysis, soon to be discovered as the motors for the building's ventilation unit. It was barely noticeable to anyone, but it was quite obvious to our test equipment, even in an insulated box.

OT, but I've seen this come up before and I wonder if HN could explain/justify this grammatical curiosity:<p>&#62; <i>"A gang were accused of selling weapons..."</i><p>"A gang" implies a singular entity (gang), but "were" is a pluralized use of was, as if "gang" was plural (as in, 'several gang<i>s</i>'). (I lack the vocabulary to properly articulate myself, since grammar is not my strong suit. I am probably not describing terms completely accurately). I've noticed this more and more in regard to singular forms of entities (typically compromised of many singular parts, such as corporations). For example: "Apple were..." or "Google have..." or "Microsoft are..." I notice that this seems to be more of a British English phenomenon.<p>My question is this: Why are people using what I will call pluralized modifiers on what I would consider singular nouns? What I would consider the "more correct" forms of the above examples are: "The designers at Apple were..." or "Google's board of directors have..." or "Employees of Microsoft are..."<p>Is this just a cultural clash between American and British grammatical conventions, or is there an elusive (to me) practical reason why one version is "better" than the other?<p>I apologize for thread jacking. Hopefully the more relevant comments will rise above this one.

Last time this was posted on HN, this question went unanswered: wouldn't variations in the AC line frequency be dominated by variations in recording speed? Even digital recordings have to be perfectly clock synced or they drift out of sync, and not necessarily monotonically. I've made digital recordings just minutes long that sounded fine to the ear alone, yet even after lining up the beginning and end, the middle was noticably mismatched.

It sounds cool, but I wonder how reliable this method is. What's the false positive and false negative rate? Errors here would have real consequences -- for example, according to the article, it was used as a crucial piece of evidence in putting several men behind bars for decades.

Great. It's a modern day equivalent of the lie detector for recordings.

So why is an article about the Metropolitan Police forensic lab in south London (UK) illustrated with a stock photo of what appears to be a US power socket? (It might be a Euro socket, but it's certainly not a UK one.) Do they not have electricity sockets anywhere at the BBC that they could have taken a picture of?

The mains frequency is an interesting topic. While it is true that it floats still its daily average is purposefully kept at quartz-like stability so that clocks can use it for synchronization. It also needs precise phase synchronization across the whole network (otherwise generators would blow up.)

Curious, is this "mains frequency" the same ringing I often hear when in alleged silence? And, no, I'm not talking about my hearing because I've had conversations with others who witnessed it :P - more like the ring old tube TV's used to make.

Application idea: locating criminals, hostages using video / audio recordings

I'm a little disturbed that they are using this as evidence, yet one of the "forensic scientists" is quoted as saying<p><pre><code> Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper </code></pre> That is incorrect. Normally it is <i>exactly</i> 50Hz. That's why the time displayed on battery-powered clocks drift over time, but clocks plugged into the wall stay correct.

Any thoughts on how "unhackable" this system is?

&#62; If millions of people suddenly switch on their kettle after watching their favourite soap, the demand for electricity may outstrip the supply, and the generators will pump out more electricity, and the frequency will go up.<p><a href="http://en.wikipedia.org/wiki/TV_pickup" rel="nofollow">http://en.wikipedia.org/wiki/TV_pickup</a>

Obviously, if a building is off the grid then there wouldn't be the same background hum, but what if the building has its own energy generation system (e.g., solar) and feeds back to the grid?

I wonder if the same technique could work for the microwave range and the cosmic microwave background. That is, is there a discernible "hum" in microwave signals from the CMB?