Acoustic Resonance of a Building


by redier
Tags: acoustic, building, resonance
redier
redier is offline
#1
Jul10-11, 03:20 AM
P: 3
A friend of mine and I were having a discussion about acoustic resonance. He theorized that a building could be destroyed by acoustic resonance.

Where I disagreed. Thinking that a building doesn't really have an acoustic resonance because of its non-uniformity of being brick and mortar.

Although only after thinking about it did I surmise that maybe mechanical resonance of a slight tremor could do it, but still am unsure. But the thought of sound doing it seems absurd to me.

Being a software engineer I have been trained in basic physics, but this is not my expert area obviously.

So I just wanted to hear your thoughts on the matter. I probably will ask some of my structural engineer friends tomorrow.

Thanks!
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Studiot
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#2
Jul10-11, 03:39 AM
P: 5,462
Structures, including building can indeed be destroyed by resonance, induced by various agents.

The most famous in modern times is the Tacoma Narrows suspension bridge

http://www.google.co.uk/#hl=en&sugex...w=1024&bih=585

Which was captured on video.

The mechanism was the same as that causing the wind induced singing in telephone and electric wires.

It is traditional for a troop of marching soldiers to 'break step' when crossing a bridge.
The Romans originally discovered that the resonacne effect of many feet stamping in unison could induce bridge destroying vibrations.

go well
redier
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#3
Jul10-11, 04:29 AM
P: 3
Your not really answering the main question of this. Could SOUND do it. You are talking about general mechanical resonance, rather than the acoustic subset that I mentioned.

It would seem to me that the sound would have to be VERY intense and it could not be done through traditional methods.

Also on that note, it would also seem that different buildings would have different frequencies. So a slight tremor at the correct frequency would completely obliterate one building, but the surrounding buildings would be unharmed. I have not heard of that happening.

I would think that the quote "all materials have a resonant frequency" is taken out of context. Previously stated a non uniform object would have varying frequencies. The bricks would resonate at different frequencies that the mortar, and even the bricks would vary in frequencies. Do all these frequencies 'average out' to make an over arching resonant frequency?

What makes crystal glasses so easy to break using sound is that crystals are uniform so all the molecules will vibrate at the same resonant frequency. Strings also can be easily vibrated. But a brick? That doesn't seem to make much sense to me.

There is the Telsa earthquake machine, but mythbusters busted it then in another episode bumped it up to plausible. Which still makes some sense to me since buildings are known too sway so they would have some vibrating tendancies, but again....SOUND??? I could not imagine the magnitude needed to produce that at the perfect frequency.

That sway argument kind of dismisses some of my other notions...I am still formulating ideas in my head now

Studiot
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#4
Jul10-11, 04:49 AM
P: 5,462

Acoustic Resonance of a Building


Goo morning, redier.

Yes I did answer the question, although you may not have realised this.

Structures, including building can indeed be destroyed by resonance, induced by various agents.
Any mechanical agent vibrating at the appropriate frequency can (will) transfer energy by resonance to another mechanical system that it is mechanically coupled to.

This included sound.

There are many ifs and buts to this.
It is also important to distinguish between the architectural acoutic resonance theory of buildings, which is really about the air in a room, not the building itself.

You may have noticed, if you watched the video, that the resonance frequency was low. Less than one cycle per second.
Low frequency like this is called infa-sound any several countries have military research projects to use this to disrupt building structures.

I also said mechanically coupled. This is an acknowledgement that there is a mechanical resistance to the transfer of energy inherent in the coupling. Most coupling systems have a very low efficiency. So you not only have to generate the right frequency, you also have to transmit it and couple it to the structure, losing much of the energy in the process.

Finally there are the dissipative processes in the structure itself. This is called damping. As you have already pointed out the damping will depend upon the construction, brickwork having a much higher damping than homogeneous artificial panels.

The main agents that I know of, which do created mechanical vibrations at the appropriate frequencies, are those resulting from heavy traffic by road and rail.
redier
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#5
Jul10-11, 08:51 AM
P: 3
Okay,

I'm still having a hard time admitting I'm wrong to myself...Like most humans, I don't particularly like being wrong. But I do want to get this right in my head.

Here a few examples that I am thinking about.

A dumptruck drives past my 100 yr old house and shakes the whole building. Bits of brick and mortar come loose from the walls. (this happens quite frequently actually).

An F-15 or 18 flies over the house and everything shakes. (Not very frequent, but has happened to me in places that I have lived)

Those are not examples of resonance, are they? Just the fact that something really loud can shake other things.

Hope I wasn't coming off as a jerk. I just don't like to be wrong (imagine that!).

Thanks for your explanations!
AlephZero
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#6
Jul10-11, 09:50 AM
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Quote Quote by redier View Post
A dumptruck drives past my 100 yr old house and shakes the whole building. Bits of brick and mortar come loose from the walls. (this happens quite frequently actually).

An F-15 or 18 flies over the house and everything shakes. (Not very frequent, but has happened to me in places that I have lived)

Those are not examples of resonance, are they? Just the fact that something really loud can shake other things.
Those are not really examples of reasonance, because the energy in the noise is spread over a wide range of frequencies.

To get a significant resonance effect, you need to supply energy at a specific frequency for a long enough time so the amplitude of the motion can build up.

In some situations (for example wind energy, as in the Tacoma Narrows bridge) there can be a feedback loop between the energy being supplied and the motion of the structure, so the energy is "automatically" supplied at the right frequency.
Studiot
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#7
Jul10-11, 10:00 AM
P: 5,462
No problem.You are prepared to discuss your worries and listen as well as expound.

A single truck or F15 is unlikely to be regaded as resonance as there is only one strike or pulse.

But consider this also.

Years ago, my physics teacher came into class an suspended a heavy metal weight from a bar propped acoss two lab stools.

It simple hung there. He did not say anything he took his handkerchief, knotted one end and proceeded to whack the weight rhythmically with the knotted end of his handkerchief at the apex of each swing.

Pretty soon he had the weight swinging merrily backwards and forwards.

He then told us (correctly) that this was a resonance phenomenon.

The secret of resonance is that all the little packets of energy add up or reinforce each other because of the timing. If the timing is wrong ie if he struck sometimes against the swing and sometimes with the swing then resonance will not occur.

go well
simonmcnair
simonmcnair is offline
#8
Oct4-11, 04:17 AM
P: 2
I'm a complete numpty when it comes to physics, but I thought it came down to the following:

Each of the materials in the house will have a different frequency that they would resonate to (http://en.wikipedia.org/wiki/Sympathetic_resonance). This would change depending on when and if they were damped or amplified (an RSJ being damped by mortar or bolts for instance).

In addition to this you would have groups of items that would resonate at multiples of different frequencies (say 9hz for substance A and 12 hz for substance B they would both resonate at 36hz).

My final destination on this circuitous route would be that if you could find the frequency that most of the building resonated at, and maintain it for long enough it may, theoretically, fall apart. That would only really apply to bricks and mortar where you could have a large amount of damage from the loss of many small pieces of the building (sand/cement). In a building that was glass and RSJ's I'd say you'd be limited to broken glass unless the frequencies went seismic and moved the ground upon which the building sat.

Insights and critiques welcome :-P I failed physics ;-)
Simon
russ_watters
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#9
Oct5-11, 12:04 PM
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Quote Quote by Studiot View Post
Any mechanical agent vibrating at the appropriate frequency can (will) transfer energy by resonance to another mechanical system that it is mechanically coupled to.

This included sound.
Perhaps more to the point, what defines "sound" is if you can hear it, which is a matter of frequency and intensity. Otherwise there is no difference between "sound" and any other mechanical vibration. So if the goal here is to win an argument, you can claim that the frequency required to destroy a building is too low to be considered "sound".
Studiot
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#10
Oct5-11, 12:23 PM
P: 5,462
So if the goal here is to win an argument,
Pardon me, please amplify this comment in an aging thread.
simonmcnair
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#11
Oct5-11, 12:42 PM
P: 2
I wasn't arguing, I thought I was adding to a conversation, maybe even (shock horror) debating. As an aside, you're saying that a low frequency is not sound because we cannot hear it ? I would disagree with this, there are many sounds above and below the range of human hearing, that does not stop them from being sound imho.
Studiot
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#12
Oct5-11, 01:09 PM
P: 5,462
Hello Simon, and welcome to Physics Forums.

I think Russ' comment was aimed at me not you, although I agree with your interpretation of sound.



What do you call acoustic vibrations that a dog or bat or whale can hear that I can't if not sound?

Otherwise there is no difference between "sound" and any other mechanical vibration.
But I can't agree with this statement however.

The fluttering of a say a cantilever may result in sound in the air but is a mechanical vibration that is not sound. It is a vibration of a mechanical body as a whole.

Sound is one form of mechanical vibration of elements of a body, transmitting energy from one point to another within that body. If one of these points is mechanically coupled to another body some of this sound energy may be transferred to the other body.

go well
johnbbahm
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#13
Oct5-11, 01:26 PM
P: 135
I think if you swept what we could call the earth frequencies, (.1 to 110 Hertz) you could find something that would disagree with almost any structure. resonate on the offending frequency long enough, and things will start to come apart. I remember watching a seismic land crew at work, when they swept through the frequency of my inner ear, it was hard to keep my balance.
Travis_King
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#14
Oct5-11, 02:19 PM
P: 764
Possible? Yes. Will anyone ever knock a building down with sound alone? Almost certainly not. There are more efficient ways to do this.


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