Talk:Resonance

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Contents 1 Article scope 2 Increase in energy? 3 Tacoma Narrows Bridge not destroyed by resonance 4 Resonant frequency??? 5 Split into different articles 6 Rice resonance? 7 disambiguation 8 Graphical equation question 9 technical link

[edit] Article scope

can we add some more detailed stuff about string resonance, tube resonance, impulse response, frequency response, stuff like that? maybe a swing set example, since that is something people understand easily. i don't know the details that well. also can we explain how the energy moves around in a resonant system? For instance... well... I don't even know a for instance. I don't understand it and I wish I did. Clearly a resonant system is passive, and doesn't create more energy than you put into it, but it somehow builds up that energy, and I obviously need this concept explained more clearly. I will do some research and help write it, but other people add stuff you know too. - Omegatron 14:16, May 21, 2004 (UTC)

Actually, should we make an article for Acoustic resonance and move some of this stuff over there? Along with the resonance bits from Acoustics. - Omegatron 15:36, Aug 6, 2004 (UTC)

[edit] Increase in energy?

"is an increase in the oscillatory energy absorbed by a system"?

you mean an increase relative to other energy levels. resonance isnt an energy source... - Omegatron 18:11, Dec 9, 2004 (UTC)

One concept critical to understanding resonant absorption: in order to receive energy, the passive oscillator transmits. Resonant absorption might better be understood in the case of plane waves and diagrams involving diffraction patterns. If we have an incoming train of plane-waves, and if a small pointlike transmitter then sends out sphere-waves of the same frequency, and if the phase of the transmitted waves is adjusted in order to create a shadow in the region "downstream" from the transmitter... then we have resonant absorption occuring. By emitting a train of inverse waves, the "transmitter" has cancelled out some of the incoming waves; it has punched a hole in the plane waves, created a shadow, and a portion of wave-energy has gone missing. The missing energy ends up inside the point-like "transmitter." That's how resonant absorption occurs. If a passive oscillator such as an LC circuit is involved, then the oscillator is simultaneously "stealing energy" from the incoming wave while it also emits waves of its own. These concepts apply to resonant radio antennas, to resonant acoustic absorbers, and even to resonant atoms which "eat" light waves. The same explanation also applies to RLC circuits: a paired coil/capacitor acts as a passive oscillator which essentially sends out an inverse copy of the incoming signal. The two signals cancel. As a result, some energy has vanished from the original signal. The missing energy ends up inside the passive LC oscillator, and the oscillations grow larger until any further increase would "transmit" more energy than is being absorbed via the wave-cancellation process. (So, is this understandable? Too complicated?) --Wjbeaty 08:42, Apr 19, 2005 (UTC)

???

Can you draw or GIS some pictures? Passive systems clearly don't really transmit anything, so you're just using that idea as an example in some way, but I don't understand the example. - Omegatron 02:45, Apr 20, 2005 (UTC)

Ah, perhaps your first assumption is the problem. I meant what I said: passive systems definitely transmit, it's just that they cannot transmit anything more than they absorb. Also, as they are transmitting, they are simultaneously absorbing. For example, when a metal mirror reflects EM waves, the free electrons in the metal surface are oscillating coherently, and this is no different than the electric current in a radio transmitter antenna. In that sense, a mirror is an "energy source" since it radiates EM waves which it had absorbed. But obviously a mirror is not a *net* energy source. In many different systems the phenomenon of reflection is not a "bouncing" of waves, instead it's absorption combined with re-emission.

In the case of resonant absorption, the "absorber" behaves as a much better transmitter than it otherwise would, but this occurs only at one particular frequency. The "absorber" takes in wave-energy, then sends out an anti-wave which cancels out part of the incoming wave-energy, which allows the absorber to take in MORE wave energy, letting it transmit an even stronger anti-wave, etc. This is how all radio receiving antennas work. (Question: with antennas, why does a good transmitter make such a good receiver? Answer: It's because absorption is in fact based on the emission of an anti-wave. Whenever you design a good transmission antenna, you inadvertantly design a good receiving antenna, and you're amazed to find that the reception pattern is the same as the transmission pattern. ) --Wjbeaty 01:33, May 21, 2005 (UTC)

Hmm.. still confused. Reflection is not the same as transmission, and a transmitted wave cannot cancel out a received wave since they are going in opposite directions? - Omegatron 17:49, May 22, 2005 (UTC)

I reworded the definition slightly to remove the word "increase", which I admit was slightly ambiguous. --Heron 19:33, 22 May 2005 (UTC)

I must say, Wjbeaty has provided some food for thought - it will take some time to 'digest' what he has said above! W.r.t. Omegatron's question above, it is my understanding that a standing wave is due to the interaction of a transmitted and reflected wave where the nodes of the standing wave are due to the destructive interference of the two waves travelling in opposite directions.

Interestingly, it is always possible to obtain the solution for a system with reflection at the boundaries using the 'Method of Images' where each reflected wave is replaced with a transmitted wave from a source outside the boundaries of the system. Perhaps this is the key to Wjbeaty's post above.

One last comment - As mentioned above, a good absorber is necessarily a good transmitter. Consider the ideal Black body. Alfred Centauri 14:43, 21 August 2005 (UTC)

[edit] Tacoma Narrows Bridge not destroyed by resonance

http://www.kuro5hin.org/?op=special;page=random#bridge

Here someone explains that the Tacoma Narrows Bridge is not a good example of resonance. Should the reference (or perhaps the whole paragraph) be removed?

The destruction of the Tacoma Narrows Bridge was indeed not caused by resonance, but the article is not claiming that it is. However, the bridge did suffer from resonance at other times, which is where the nickname "Galloping Gerdie" came from. At least, that is my understanding from reading Tacoma Narrows Bridge; perhaps a mechanical engineer can correct me. You may be right that we should not mention the Tacoma Narrows Bridge because it is a rather confusing example. However, the London Millennium bridge seems to be a proper example. -- Jitse Niesen 17:06, 7 Feb 2005 (UTC)

The Tacoma Narrows bridge was destroyed by resonance, but not in the way we might imagine. If we place an object in a flow of air, at certain wind speeds the downstream air turbulence takes the form of periodic counter-rotating vortices called a "Von Karman vortex street." (Imagine a flapping flag, then imagine a series of tornadoes shed by the flag and which continue far downstream.) At just the right wind speed the periodically varying wind direction and pressures caused by the wake-turbulence would have the same frequency as the bridge. Imagine a flapping flag which is connected to a pendulum: at certain wind speeds the flap-frequency would match the pendulum frequency, and the pendulum would go wild! If the pendulum frequency was very low, then only a very slow air motion would hit the right frequency. So, the bridge was resonantly pumped into motion by puffs of air, but these puffs of air were coming from downstream, and they were part of the natural (but invisible) turbulent wake that exists behind most objects exposed to wind. The same effect is often seen in power lines on days with almost no wind. The lines start swinging mysteriously because the very slow wind is creating some slow turbulence, and there is an "AC signal" in the turbulent air which matches the natural frequency of the swinging wires. The wires are pumped into large motion by their own air turbulence. --Wjbeaty 01:43, May 21, 2005 (UTC)

You are wrong Wjbeaty. I corrected the wikipedia article. I added the most important reference about that mistake that the Tacoma failed due to resonance. It failed due to fluttering. Robert Scanlan, father of bridge aerodynamics wrote an article on the subject... see main text:

http://www.ketchum.org/billah/Billah-Scanlan.pdf

Diego Torquemada 07:51, 5 April 2006 (UTC)

After browsing the linked document, it appears to me that, in EE language, the poles moved into the RHP. That is, with the wind as an energy source, the bridge became an oscillator. I assume that's what self-excited means in this context - there is an output at some frequency without a corresponding input at that frequency.

BTW, Diegotorquemada, beginning a sentence with "You are wrong..." is a great way to make friends around here. Wish I had thought of it. Alfred Centauri 03:44, 6 April 2006 (UTC)

sorry Alfred and Wjbeaty, next time I will measure better my words. What you say about the poles is right... when the poles of the bridge go to the right hand side, and have imaginary parts, fluttering is produced. There is another failure for bridges, that is when the pole in the RHS is purely real. In this case a failure called "divergence" is produced. It can be understood as a really flying bridge, because the deck just lifts on the presence of strong wind, making it literally fly, and since the tension in the hangers is lost, the bridge collapses by overturning. Diego Torquemada 07:42, 8 April 2006 (UTC)

[edit] Resonant frequency???

It appears that there is no such thing as a resonant frequency:

http://users.ece.gatech.edu/~mleach/misc/resonance.html

I recommend that the various 'resonant frequency' terms in the article be replaced with 'resonance frequency'. Alfred Centauri 02:41, 21 August 2005 (UTC)

I don't.

"The frequency at which resonance occurs is called the resonant frequency." -- A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press.

"At the resonant frequency, ..." -- "resonance" entry, Britannica Online, 2005.

"at the resonance frequency" -- "resonance" entry, Newnes Dictionary of Electronics on xreferplus, 2005.

"The resonant frequency is the frequency at which ..." -- "resonance" entry, The New Penguin Dictionary of Science, on xreferplus, 2005.

"is said to be .. the resonant frequency" -- "resonance", Penguin Dictionary of Physics, 1982.

"resonant frequency" (headword) -- Chambers Science and Technology Dictionary, 1988.

"resonant frequency" (headword) -- OED2, with citations from 1925, 1934, 1964.

I think FV Hunt is being unnecessarily pedantic. Most authorities write "resonant", but a few write "resonance". Who cares? It's only a shorthand term for a real concept - it doesn't alter the concept itself. It reminds me of the famous pedant C.P. Scott, who complained that television would never catch on because it was a hybrid of Greek and Latin roots. Hunt has a point, and perhaps "resonance frequency" is more accurate than "resonant frequency", and if you agree with him then you can write the former, but it's not a good enough reason to go changing what others have written. --Heron 11:46, 21 August 2005 (UTC)

I agree that no one will be confused by the use of 'resonant frequency' instead of 'resonance frequency'. On the other hand, it is my opinion that the author(s) of an encyclopedic article should and would care about 'getting it right'. After all, you do agree that "Hunt has a point". While I question your description of the changing of the word resonant to resonance as a 'rewrite' of the article, I do see your point. Accordingly, I have added the link above to the 'External Links' section of the article. Alfred Centauri 13:14, 21 August 2005 (UTC)

Thank you for accommodating my objection, Alfred. I accept that the word "rewrite" was an exaggeration and, to be honest, I suppose it won't hurt if you do decide to change the phrase throughout the article. My only concern is that you don't start telling people that they are "wrong" for using "resonant", as I think the distinction is too pedantic to matter. --Heron 14:59, 21 August 2005 (UTC)

Agreed. I hope I haven't developed a reputation on Wikipedia for nitpicking for any reason other than to spur intelligent conversation. Alfred Centauri 16:47, 21 August 2005 (UTC)

[edit] Split into different articles

What do people think about having this article just explain the basic idea of resonance, then have seperate articles on Mechanical resonance, Electrical resonance and Acoustic resonance? There is certainly enough of a scope to warrant splitting this article into four different articles, and I think that making four different articles would more likely encourage those who know about one specific field to expand them. --Nathan (Talk) 03:49, 30 December 2005 (UTC)

Agreed. — Omegatron 23:59, 30 December 2005 (UTC)

Right, I'm gonna do it. It seems a very logical thing to do, and one other person agrees. I don't think there's a way of moving sections (to avoid destroying the history), so I'll just have to copy and paste. I'll leave notes in the edit summaries, though.--Nathan (Talk) 01:08, 31 December 2005 (UTC)

I've done the cutting and the pasting. Now those three articles need introductions, and some of the See Also items need to be cut and pasted as well. I'll do some of it another time if nobody beats me to it. *hint* --Nathan (Talk)

[edit] Rice resonance?

Is the link to the YouTube video actually an example of resonance and its effect on rice? It seems to me there is no reaction between the vibration of the sound and the natural vibrations of the rice but rather the rice is just creating a visual pattern of the areas of high and low vibration on the speaker surface. But maybe I'm just not clear enough on the concept.

[edit] disambiguation

It seems like this page could be a disambiguation page, rather than pointing to a separate disambig page. It doesn't seem like this page is overpowering enough (with respect to other resonance pages) to have control of the main title. Fresheneesz 02:20, 19 May 2006 (UTC)

Resonance is important as a general concept, so it seems like it deserves its own page. A page describing Q, energy transfer, etc, seems useful, to present the overall idea. I'm not sure we need to introduce quantum field theory to do that, I'd be happy with a mechanical pendulum.

[edit] Graphical equation question

(bkil 09:26, 12 July 2005 (UTC)) It might be useful to expand the equation
f=((T)/({rho}))^(1/2) / (2*L)=
=((T)/(m/L))^(1/2) / (2*L)
with these fractions
(+) =((T*L)/(m))^(1/2) / (2*L) =
(+) =((T*L)/(m))^(1/2) / (2*2*L*L)^(1/2) =
(+) =((T*L) / (4*m*L*L))^(1/2) =
(+) =((T) / (4*m*L))^(1/2)
ie.:

        ______
       /  T   |
f =   / -----
    \/  4 m L

to better visualize the relationship of the parameters, and the sentence following, that says:
Higher tension and shorter lengths increase the resonant frequency, and vice versa.

[edit] technical link

This needs to be more accessible. Telling the reader resonance in quantum field theory could be "this or that" without clarifying the distinction, or "see also [some esoteric topic]" isn't a good style. John Riemann Soong 00:47, 28 July 2006 (UTC)

I agree. The Quantum field theory section needs some serious work; it is practically incomprehensible as it is now. HEL 02:23, 15 October 2006 (UTC)

I rewrote the quantum field theory section and tried to make it less technical. If I've stepped on anyone's toes, I apologize; please let me know! The link to relativistic Breit-Wigner distribution now contains an (I hope) clear description that includes the propagator with its complex part. HEL 00:53, 20 October 2006 (UTC)