"Anyway, if you're still here, thanks for the ad and page view."
always a pleasure.
Vibrations emitted by Britain’s most famous bell have been mapped for the first time by engineers from the University of Leicester. As everyone must know by now, Big Ben is housed at the top of Elizabeth Tower, London, and chimes faithfully every hour over the capital. Its deep bong can be heard whenever a giant 200kg hammer …
already done. Sort of...
Discovery Channel show "Time Warp" which films things in ultra slow motion to see what happens did this with superstar drummer Mike Mangini. (currently of Dream Theater). Even HE was impressed at the magnitude of deflection.
Can they point their kit at a rock band and give us the sonic picture in 3D?
Actually I guess that's what my brain does already.
Likewise. And not just Hawkwind[1].I'd suggest that Yes' Siberian Khatru might be a good place to start. Along with Barbers Adagio - especially the voice-only Trinity College version[2].
[1] To be honest, I never understood the attraction of Hawkwind. They were a competent band sure, but nothing revolutionary or different.
[2] Also called Agneus Dei - has an incredible bit of harmony where a dissonant peak resolves into a beautiful harmony..
Also the relative movement (vibrations) between two buildings would deteriorate the results unacceptably.
Btw. subways cause a lot of vibrations even in buildings that are quite far from the tracks. This I found out long ago when attempting to make holograms, it failed all night and suddenly worked in the few hours when the trains paused. All of this despite using a massive stone table that was vibrationally damped.
"All of this despite using a massive stone table that was vibrationally damped."
It's all about the base.
Except when it isn't, when the inertia of the base combined with the characteristics of the vibration damping is insufficient to hide the applied excitation from the extraneous external effects, and the signal you wanted to observe is therefore dwarfed by the extraneous effects. OK in your case it wasn't so much a signal you wanted to observe as a movement you wanted not to happen, but the same principle applies.
Hey, it's Friday. Have a great weekend.
"Pah! They've been bouncing lasers off reflectors on the moon since the 1970's. And IIRC measured moonquakes with them too."
But to what level of precision? I believe the precision needed with the Big Ben experiment was on the scale of sub-millimeters. Could they measure the deflection of the lunar surface during a moonquake to the micrometer?
nearby skyscraper and point it through the window?
Light-scattering from the pollution introducing errors I would imagine. And less bragging rights - "sat in a nice comfy coffee shop and did the odd measurement remotely" sounds a lot less rufty-tufty than "labouriously carried hunderds of KG of equipment up hundreds of steps on our backs before doing lots of intricate measurements in a freezing cold and draughty bell tower".
But there isn't a Reg standard for the origin of time, or even a Reg standard for measuring time intervals. (http://www.theregister.co.uk/Design/page/reg-standards-converter.html). If there were, it would be possible to express the frequencies of the bell in sensible units rather than boring old Hertz.
Perhaps time should be measured in b@gg@rs, where 1 b@gg@r equals the time interval between pressing the 'send' button on an email and, on realising that you've sent to the wrong address, shouting "b@gg@r!"
"and use a spectrum analyser to find the resonant frequencies?"
At a guess.
That would also register any resonances in the bell tower too. There would also be mixing in the air to give other frequency products.
The laser map shows extra information about how different areas of the bell produce which frequencies and at what volume. Presumably any flaws in the casting would show up as localised distortions of the picture. That might actually be a practical use for the technique.
The laser map shows extra information about how different areas of the bell produce which frequencies and at what volume.
Yes, indeed. The idea was to map out the different (simultaneous) vibration modes of the bell. Essentially a high-tech version of the famous Chladni experiment:
https://www.youtube.com/watch?v=wvJAgrUBF4w
(pouring sand on a bell doesn't work so well).
"Any reason why it wouldn't have been easier to just point a microphone at the bell and use a spectrum analyser to find the resonant frequencies?"
Because you can't hear the shape of a drum. I mean this literally, the shape of a drum cannot be determined from the sound it makes. (It can in reasonable general circumstances, but anyway.) Knowing the resonance of the bell ring gives little information about how it is being produced.
> Because you can't hear the shape of a drum.
There. I spot a mathematical commenter!
For those puzzled: You Can't Always Hear the Shape of a Drum.
Yes.
That gives you the frequencies it emits.
This gives you the exact points on the bell it emits from.
This tech's useful for things like tanks and other pressure vessels. Hit the structure with a hammer and scan for weak spots on the shape.
Even used to test the integrity of the vacuum in food tins.
"Any reason why it wouldn't have been easier to just point a microphone at the bell and use a spectrum analyser to find the resonant frequencies?"
It was a follow-up to a visual demonstration of vibrational modes of a flat plate. The plate itself was pretty dreadful sounding with a lot of non-harmonic modes so the obvious follow up was to look at the modes of something made to be euphonious. The spectrum analyser in itself wouldn't show the mechanical basis of the modes. Having said that, they whisked through the results PDQ.
It's been far too long since I read such a contemptuous article from el reg. Almost back to the good old days of getting put on the naughty list for attempting to inform lord jobs how to say words. Available in the archieves for those of a disbelieving nature.
Now on that note.... get of my lawn! *grumble, grumble*
Mines the one with the werthers ordinals in the pocket.
Aaaaaand... cue the over-sensationalized 60 minute History Channel "investigation" featuring that exact phrase "...or something far more sinister?", creepy lighting and music and snippets of historical recreations in which badly-costumed actors run across a field to, in this case, carry hods of bricks up spindly wooden scaffolding to build a tower.
Love the turn of phrase in the article, had me giggling.
However I instantly thought of one application area. Music synthesisers (do they still call them that) do (at least used to 20 years ago - I'm a bit out of touch these days) model instruments mathematically, in effect to recreate the same sound you get from piano strings, woodwind instruments etc.
Not that they're likely to have much call for path 3485656438598587: "Big ben chime", but in general the more they know about the physics, the better the result.
And then people in their 40s who used to play instruments might have a mid-life crises and go and buy some hugely expensive Nord thingy, and stimulate the economy. Hmm....
"However I instantly thought of one application area. Music synthesisers (do they still call them that) do (at least used to 20 years ago - I'm a bit out of touch these days) model instruments mathematically, in effect to recreate the same sound you get from piano strings, woodwind instruments etc."
True synthesizers can still do that, and there's plenty of computational power out there to more properly mimic the physics of instruments. Having said that, it's also for most purposes overkill. Sample-based playback works well enough for most and for those who demand the retro "synth" feel can still use well-equipped synthesizers that are based more on modulating sound waves and mixing them together rather than trying to physically simulate an actual instrument.
There were some odd aspects to the programme.
At one point it suggested that Lesley Garrett's vocal range extended from 80Hz to 1kHz. That's some range. 1kHz is probably about right for a soprano but 80 is at the bottom of the bass range.
Also at some points they were illustrating natural sounds, in the sea for instance. Why did they cover them up with music?
1kHz is probably about right for a soprano but 80 is at the bottom of the bass range.
They might not be audible but I'd be surprised if there were not *some* lower frequency component to even very high-pitched voices. I'm sure someone who knoiws about acowsticks[1] will be along shortly to explain to us ignorant types..
[1] Sorry - been listening to Flanders & Swann "Reproduction Song" lately[2]..
[2] "A flutter in your bottom" still raises a smile[3].
[3] And not of the "Have some Maderia m'dear" sort.
"The racket is instantly recognisable, earning it a Twitter account that inexplicably has nearly half a million followers. All it does it tweet "BONG" on the hour. Half. A million. Followers."
My guess is that all of them were stoned when clicking "follow".
Anyway, have a nice weekend, everybody!
I visited Whitechapel Bell Foundry a few years ago, where they explained how part of the bell making process is to tune each bell to play a chord.
The unique sound of Big Ben is because the officials at the Palace of Westminster ignored the experts' instructions (some things never change) and used a hammer that was too heavy and with too long a drop. Which cracked the bell.
(This is after doing the same with the original bell and breaking it. What is it called when you do the same thing again and expect different results?)
The bell was then turned through 45 degrees so that the part the crack is on will vibrate the least. So it chimes, but the tone is not what the bell foundry intended it to be. The sound though is instantly recognisable as Big Ben.
Nobody comment on that a normal measuring microphone, or for that matter any bad microphone would ve picked up those frequencies and toss them in a FFT, or Cubase or something.. This is a nice engineering effort to pinpoint the mechanical behaviour causing the acoustic effects........... In Eindhoven, The Netherlands, some student picked up a degree, something like a PhD, on modeling this kind of clocks YEARS ago and would have found the mechanical behaviour just by simulating the thing. This was just a PR stunt, good one imo.............
Sure, the mathematical analysis is in terms of "eigenmodes". I think they may have been studied first by d'Alembert in the mid-18th century. Simpler eigenmode problems can be solved analytically, more complex ones (like a bell) can be solved numerically (although that's computationally intensive).