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    Frequencies of Disasters
    By Richard Mankiewicz | March 13th 2010 12:45 AM | 13 comments | Print | E-mail | Track Comments
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    I used to be lots of things, but all people see now is a red man. The universe has gifted me a rare autoimmune skin condition known as erythroderma...

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    How to explain resonance to a non-scientist? A few years back I heard a guest speaker on BBC Radio 4 trying to explain the resonance effects of pulsed microwave radiation on the brain in contrast to the thermal effects of the carrier frequency: sadly he failed miserably. What is it about resonance that makes it so hard to explain?

    I have taught it to A-level students and to undergraduate engineers. Electrical engineers, in particular, need to be thoroughly familiar with the phenomenon and yet, I could see that its significance eluded them. There are few, if any, good visible examples in real life. The Tacoma Narrows Bridge is one famous example, where strong winds set the bridge oscillating. Eventually it hit its resonant frequency and collapsed.



    The Millennium Bridge in London was quickly renamed the "Wobbly Bridge" as engineers had failed to take into account that pedestrians tended to walk in step. This social coherence created an input frequency acting on the bridge which then started to oscillate. To avoid a repeat of the Tacoma experience, it was quickly closed down until dampening systems were put in place.

    Everything has a natural frequency: complex objects with many components will also have different natural frequencies for each part. Bridges are large objects and have low natural frequencies that we can actually see - although the constructors would rather we didn't! But most of the time we experience frequencies that we can not actually see. Bang a drum and it will vibrate at its natural frequency. Tighten its drumhead and it will vibrate at a slightly higher frequency. We can hear these frequencies but we can't normally see them.

    However, here's an experiment that illustrates what's actually happening at the drumhead. Instead of striking the drum and letting it vibrate naturally, here the input is from an oscillator that makes the surface vibrate. The drumhead is a square metal plate and the input frequencies come from a speaker under the plate. Salt is sprinkled on the surface so that we can see how the plate vibrates and how it changes its behaviour as the input frequency is increased.



    These beautiful patterns are the ways in which the metal plate oscillates in response to the input frequencies from the speaker. But what happens if the input frequency starts to get close to the plate's natural frequency? This is easier to show if we switch materials again, this time from metal to glass. The video below has been slowed down so we can see how the glass reacts to an input frequency that is slowly increased.



    As the input frequency comes close to the resonant frequency of the glass the amplitude of the oscillations increases - that is, the glass is not just reacting to the input but also amplifies it up to the point at which it shakes so violently that it breaks. The problem with the bridges at the top is not that they were vibrating in reaction to an input but that those vibrations had a very high amplitude. But that is precisely what happens at resonant frequencies: an object will "over react" to an input when it is at or very near its own natural frequency. A low amplitude input thereby creates a high amplitude output if the right frequency is chosen.

    This resonance effect may be destructive or useful. Tuning into an analogue radio or TV signal works in the same way. Each radio station emits at a particular frequency, so in this case the input frequency is fixed and we alter the natural frequency of the receiver until it matches the station we want. Other stations that are being transmitted on other frequencies are effectively ignored because the receiver is now tuned to one station and its output is naturally amplified. It is even possible to hear a radio station through an ear-piece without extra amplification.

    Small inputs can thereby create large outputs. Whether such resonance phenomena are good or bad depends largely on whether they are useful or not, or whether intentional or accidental.

    Comments

    jtwitten
    Maybe start by defining the term? The examples you have provided do not highlight an extremely confusing aspect of the term "resonance" when used to communicate outside a specific field. Different fields use the term resonance to mean different things. Most of the variations are small, but they can effect how easily the concepts of one field (such as mechanics in your bridge examples) to another (such as chemistry or particle physics, which is more in line with your BBC Radio 4 interviewee).
    Aitch
    I can resonate with that, Josh!  ;-)

    Aitch
    Gerhard Adam
    Different fields use the term resonance to mean different things.
    I'm not aware of any different definitions being applied to resonance.  Even your Wikipedia entry indicates that this is a phenomena that occurs in a variety of circumstances but invariably affects vibrations or waves.
    Resonance phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and resonance of quantum wave functions.
    http://en.wikipedia.org/wiki/Resonance_frequency
    Mechanical examples tend to be the easiest to illustrate, however there is fundamentally no difference in the definition when it is applied in other circumstances.  In all cases, the point is that the frequency is in "sync" with the object and causes an increased vibratory effect because of it.  Whether it be the Tacoma Bridge or the hydrogen nuclei during an MRI scan, the effect is the same.  The hydrogen nuclei is capable of absorbing the energy because it is at the necessary frequency at which the proton can respond, just as the Tacoma Bridge was constructed in such a way as to respond to standing wave which was at the proper frequency for the bridge (given the size of the narrows). 

    In particular where it becomes relevant in EMF, is when a low energy EM field may find the resonant frequency around which cellular chemical functions take place and either accelerate them or trigger them to behave differently (i.e. changes in permeability).  In particular one such model is the Ion Cyclotron Resonance Model which "suggested an increase in transport of ions such as calcium, magnesium, potassium in the individual cells, if specific magnetic fields were applied."


    Mundus vult decipi
    jtwitten
    Resonance phenomena occur at all levels, but the details differ. Different fields assume different details. Those details are extremely important when one is talking about the expected results. Particles are not bridges or wine glasses.

    Besides, i can hardly be faulted for asking for a definition, rather than examples.
    rychardemanne
    Did you read the first sentence? Merely supplying a definition is close to useless when trying to explain a non-intuitive concept in a non-technical manner. Popular science books would be very slim indeed if that was their pedagogical technique - as well as being less than popular.

    In writing this I rejected a number of videos that struck me as poor illustrations. The standard introduction of one tuning fork setting off a second one just looks like the transfer of energy, somewhat like Newton's cradle. Even standard school laboratory equipment does not have a killer experiment that shows resonance. Unless, of course, there is one out there and I haven't found it. That's the great thing about videos of experiments that would be too expensive (and dangerous) to do in a classroom.
    jtwitten
    Well, my opinion may be worthless as a non-non-scientist, but frankly that attitude seems pretty insulting. If the choice is between providing a definition and an example, then you may be right. Why is it so bad to define the concept you will be talking about and then provide illustrative examples of that concept? Without establishing a set of core principles, it becomes difficult to generate real understanding of which aspects of an example are specific and which are general without exhaustive examples. Essentially, this throws away the combined experience of the scientific community.
    jtwitten
    As a side note, I read the first sentence, nay the first paragraph at least eight times. There is nothing in there about the utility of definitions, only an anecdote about a scientist doing a poor job of explaining a concept.
    jtwitten
    Here's an example of "resonance" being used in a way that it is conceptually quite distinct from the mechanical or electromagnetic context: orbital resonance. And, yes, astronomers will simply use the term "resonance" to describe this phenomenon.
    Gerhard Adam
    I don't see the distinction you're suggesting.  Resonance relates to some event frequency that becomes "sync'ed" up and produces an effect.  This is precisely what happens in orbital resonance where the frequency is a function of the number of orbits completed per unit time ("a regular, periodic gravitational influence on each other").  .
    ...a resonance ratio in this article should be interpreted as the ratio of number of orbits completed in the same time interval...
    While this certainly is physically different from the other examples, it is similar in kind.  Basically it consists of two or more bodies (objects) interacting in such a way that their interaction is greater together than they would be by simply summing the forces individually.
    Mundus vult decipi
    jtwitten
    That definition is so vague as to be useless. No wonder the public is confused. We are now working with calling any frequency based interaction a resonance. 

    Mechanical resonance is determined by the frequency at which the object's oscillations experience minimal damping. Essentially, the frequency at which energy can be put into the system most efficiently. Contrary to the common belief that you have reiterated, this is not a case of the system being greater than the sum of its parts as that would violate the second law of thermodynamics.

    Aitch
    Mechanical resonance is determined by the frequency at which the object's oscillations experience minimal damping. Essentially, the frequency at which energy can be put into the system most efficiently
    An example being a guitar string. It can be made to resonate by a radio or stereo playing the same note, whilst placed near the loudspeaker.
    It is possible to do the same experiment with a signal generator, and observe the effect of reducing or increasing the frequency, causing the string to oscillate with greater amplitude as the natural resonant frequency of the string, is passed through. Similar effects can be seen on the surface of a drink in a glass, placed on top of a loudspeaker, as the frequencies resonate to the wavelength of the surface of the liquid, or the glass itself.
    Aitch
    The simplest example I remember was a demo in a group of people I started swaying back and forth. I was close enough to someone else for them to be aware of it, and they swayed in sympathy, soon to be joined by several others, until eventually all 7 people were swaying to the same rhythm and the resonance effect of us all swaying in harmony resulted in the people on the outside stating that they felt as though they might fall over, only to feel pulled back the other way at the last moment It was quite silly, but fun, and brought smiles to peoples faces, and funnily enough led to a conversation about the differences between sympathy and empathy Do molecules vibrate sympathetically or empathetically? Aitch
    logicman
    I am reminded of Blaster Bates, who is famous for saying - of explosives - that sympathetic detonation isn't very sympathetic to the poor b***   ...

    Rycharde: If you want to actually feel  resonance you can stand on the wooden floor of a church while someone plays 'tocatta and fugue' with all the stops out.  Feel your own stomach dance to the music.

    Or - if you are feeling very brave - here's a a killer experiment that shows resonance:

    http://www.londoncircle.com/soundofdeaths.html