Talk:Resonance

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)

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)

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)