Resonance

[madorangepanda]

Lets not forget that you need to generate a powerful enough signal at the correct frequency.

 

You're looking at a system that can generate under 20Hz here for a concrete and steel building, even lower for brick. That will take some sound system.

[Matthew Robinson]

You're looking at a system that can generate under 20Hz here for a concrete and steel building, even lower for brick. That will take some sound system.

 

Or harmonics, which would be higher pitched.

[Jivemaster]

The modern sports stadia are calculated to accommodate such things as 45000 people doing mexican wave round the balcony -now that really is big amplitude low frequency but whether it ever reaches resonance....

[Matthew Robinson]

I'd suspect not - mexican waves happen once, but surely resonance requires a periodic driver at the driving frequency or harmonic thereof?

[madorangepanda]

@mr7526

You increase the frequency by a factor of n and you get 1/n times energy at the resonant frequency with Harmonics.

 

To work it out/explain it fully really is quite complicated. The Maths and Physics is beyond A level I'm afraid. I do recall a good article in the New Scientist that explained how buildings are damped from earthquakes that explained things fairly well though.

 

 

 

Would be interesting if to see if a Mexican wave could, after all the Millennium bridge resonated with people walking .

[Matthew Robinson]

Just realised that if you talk about people jumping, you need to talk about the speed of the music. Duh!

 

So, if (for sake of arguament) you had a building with a resonant frequency of 1Hz and you played music with 60BPM, you could get resonance through people jumping? Obviously, if you have a resonant frequency of 10Hz (as has been suggested) then you'd be wanting a track with 600BPM, which is quite absurd!

[Andrew Edwards]

The Millennium Bridge was down to simultaneous repeated lateral impetus from all the people on it walking in step. They new it moved, they just didn't anticipate EVERYONE to be walking in sync.

 

Mythbusters did the Millennium Bridge (sort of) in the very early days. They also attached a oscillating weight to a huge suspension bridge. Obviously it did not fall down but they did dial in the right frequency to create vibrations that could be felt on the tarmac over a hundred metres away. I can't remember the weight but it wasn't much bigger than a lump hammer.

They did the brown note too...it turned out to be a load of...

 

I think that there would have to be more going on than just a sound system to bring a building down. Afterall, if you could drop a building with sound then that's probably how they would do it. Certainly cheaper and safer than a lorry load of explosives.

 

Drew.

 

PS We've got some 'good' LF room modes in our venue. I regularly hook up my synth and use it to shake down confetti.