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Q: Harmonics: EMF to Audio ( Answered ,   1 Comment )
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 Subject: Harmonics: EMF to Audio Category: Science Asked by: jat-ga List Price: \$15.00 Posted: 30 Sep 2002 16:09 PDT Expires: 30 Oct 2002 15:09 PST Question ID: 70929
 ```In layman's terms, I'd like to know a little about harmonics and how it relates to light and sound. More specifically, take musical notes for a moment. If I hit a middle C, will "harmonics" cause some kind of resonance (that needs to be explained to me, as well) or "sympathetic" vibrations of the C-notes two to three octaves above? If so, then what about Ultrasound and frequencies in the EM spectrum? Is it possible that an ultrasonic frequency could cause resonance in much higher frequencies in the EM spectrum somewhere because of harmonics? I'm not sure I'm even asking the question properly, but hopefully you'll understand what I'm trying to ask!```
 ```Jenjerina provides some key definitions in the comment below. More specifics are perhaps best described in terms of some examples. Starting with a musical example, consider a musical instrument such as a violin. (Similar arguments can be made about many other instruments.) Suppose the player plays middle-C. There are indeed harmonics of this note with frequencies which are multiples of it. Twice the frequency (the "second" harmonic; conventional harmonic counting begins with the fundamental as number "one"!) is an octave higher (C again). Three times the frequency is an octave and a fifth higher (G). Four times the frequency is two octaves up. The fifth harmonic is two octaves and a fourth up (F). The sixth is an octave (2 times) above the third (G again). The seventh harmonic is the first one which is not a "perfect" interval (see https://answers.google.com/answers/main?cmd=threadview&id=16918). The eight harmonic is, of course, three octaves above the fundamental. How many of these harmonics are important? That depends on a number of factors. A harmonic is typically important if it reinforces something. That can be a natural "resonance" such as some feature of the body of the violin which tends to vibrate at that frequency. If there happens to be such a resonance it will be "reinforced"; i.e., there will tend to be energy transferred to that vibration mode. Such resonances contribute to the "timbre" or characteristic sound of a musical instrument. A reinforcement may also occur if two notes played at the same time have at least some matching harmonics. For example, a perfect fifth (two notes with a frequency ratio of 3/2) is a "pleasing" interval, because the second harmonic of the upper note matches the third harmonic of the lower note. In general, these effects tend to be more important if the match is closer and involves lower harmonics. In most cases, there is decreasing energy (amplitude) in higher harmonics quite aside from any decreasing sensitivity of the human ear as you go to higher frequencies. Note also that the fundamental sound generation mechanism of most instruments actually generates energy in harmonics as well as in the fundamental. Again, taking the violin example, a vibrating string is easily excited in harmonics as well as at the fundamental, because the string itself has resonances at the harmonics. Typically (but not always), the player tries to excite primarily the fundamental. Wind instruments derive their distinctive sounds in large part from particular patterns of harmonics which are excited simultaneously with the fundamental. Some instruments such as the viola d'amore actually have a special set of "sympathetic" strings which are not excited directly by plucking or bowing but are simply there to resonate when the main strings are played. The characteristic sound of a piano is influenced by the sympathetic vibration of strings other than the ones actively being struck as well. Harmonics are important in the analysis and management of other sorts of sound as well. Similar kinds of reinforcement may occur at harmonics of noise sources, for example. Your lawn mower could excite a resonance in your china cabinet. The cartoon image of an opera singer's high note causing a glass to break is due to the same sort of effect. (The effect is real if not nearly as common as the cartoon image might indicate.) Musical examples of harmonics and resonance effects may be among the easier examples to grasp, but the same principles apply to vibrations of any sort. The vibrating bridge example is a good one. Quartz crystal oscillators depend on certain resonances in precision-cut quartz crystals. Even atomic clocks depend on the same effect in the electrons of certain atoms. Many of the properties of molecules depend on the vibration properties of their constituent atoms. Light is also a form of vibration. Electromagnetic waves are light. Some light sources such as lasers tend to behave similarly to musical instruments in that they tend to naturally "vibrate" (emit light) at a characteristic frequency (color). The length of a laser "cavity" corresponds to the length of a violin string or the length of an organ pipe. Many such devices will also emit some light at harmonics of the fundamental frequency. Since the visible spectrum is only about an "octave," such harmonics tend not to be very important for the human perception of light. However, they can be very important in certain commercial lasers. There are special tricks that allow the doubling or tripling of the fundamental of certain lasers (i.e., most of the light energy is excited only in the second or third harmonic rather than the fundamental). For example, a Nd-YAG laser typically wants to emit infrared light at 1064 nm, but it can readily be excited at 532 nm (green) by frequency doubling tricks. An analogy to the timbre effects in musical instruments does exist for light in the form of the "color" of objects. Normal "white" light actually contains a broad spectrum of light (analogous to "white" noise--noise which does not have any distinct identifiable pitch). When such light is reflected off a surface, some colors are absorbed or reflected differently from others as the light interacts with the molecules of the surface. The light actually couples with electrons in the atoms. The frequency of visible light may be much higher than that of audible sound, but when you get to atomic scales, the vibration frequencies of individual atoms and electrons can actually match those of visible photons. A direct connection between "light" and "sound" is possible but of minor importance in most cases, simply because the frequency ranges of interest are usually so far apart that there is usually very little energy coupled between them. Extending into "ultrasound" and "radio" waves doesn't get you much closer in most applications. Most common couplings between light and sound are more indirect. Some processes generate both light and sound by different mechanisms (lightning and thunder, for example). Similarly light can generate sound if enough of it is absorbed into something to cause it to break or explode thereby generating vibrations which are emitted as sound. Obviously, the subject of vibrations, harmonics, resonances, light and sound covers a lot of territory. I hope these few paragraphs gave you a reasonable layman's introduction. If there are specific things which are of particular interest to you, please request a clarification I'd be happy to expand the discussion. Or if you really have a very specific problem in mind, perhaps you can explain it, and I can help you figure out what question you are really trying to ask. And if you need some links to further reading in particular areas, please ask for them as well. The above discussion was based on personal knowledge, but I can easily refer you to textbooks or web sites for more details on particular topics.``` Request for Answer Clarification by jat-ga on 03 Oct 2002 10:37 PDT ```Thanks. Your comments are helpful. But, to further clarify, I'm looking for a relationship between the higher, EMF frequencies and the lower frequencies we call "sound" (including ultra-sound). I notice that you don't normally see the "sound" frequencies listed on a spectrum chart of the other frequencies. I realize they are far apart as to their actual frequency ranges, but I'm still wondering if there still exists some kind of continuity between them. They are, after all, frequencies. An earlier piece of input I received made the distinction that soundwaves are different, since they are "compression frequencies", requiring a "medium". What I'm wondering, though, is something more like taking a violin string and plucking it in a vacuum. No sound is generated, but I assume it will vibrate with almost exactly the same "frequency" that it would when we hear the "compression waves" it generates when plucked in a normal atmospheric environment. So, if it vibrates in a vacuum, then how is it any different than, say, an antenna "vibrating" off a radio wave except for the fact that its vibration is much slower? If I'm thinking straight here, then it seems there would be a basis for maintaining that all frequencies are "related", whether they be sound or light, and therefore, harmonics can exist between sound frequencies and light frequencies. Hope this helps to clarify what I've got in mind...``` Clarification of Answer by drdavid-ga on 03 Oct 2002 16:11 PDT ```As pointed out in the previous question you refer to: https://answers.google.com/answers/main?cmd=threadview&id=70246 there are in fact named radio bands corresponding to audio and ultrasonic frequencies. The word "frequency" refers to the repetition rate of any oscillation or repetitive event whether it is compression waves in air or in a wood block, transverse vibrations of a violin string, the oscillation of an electromagnetic wave, the swaying of a bridge, the swinging of a pendulum, or the number of people walking past the corner of 5th Street and 7th Avenue. So, yes, you can put all these frequencies on the same chart if you wish to. It is interesting to note, for example, that certain relatively less familiar forms of electromagnetic waves do, in fact, have the same frequencies as audible sound and ultrasound, but they still represent different kinds of vibration. And, yes, a violin string can still vibrate in a vacuum, since the characteristics of a string which allow it to vibrate do not depend on the presence of air. However, without the air, no vibrations (sound) can be transmitted, say, to a microphone mounted even a few inches away (assuming that it is isolated from any possible sound transmission through the mounting hardware--remember that sound can travel through solids as well as gases). Whether vibrations of one sort couple in any significant way to vibrations of another sort is not directly dependent on the frequency of the vibration, but rather on whether there is a coupling mechanism. If there is a coupling mechanism, and especially if there happen to be resonances in the secondary medium which match the driving frequency from the first medium, then important coupling can indeed occur. The Tacoma Narrows bridge failure is a classic example. There are also examples of similar failures when soldiers marched in step across some structure or many people at a disco were all moving synchronously. There are also ways that sound and light can couple, but they are relatively unusual situations, since the coupling mechanisms are relatively rare and the frequency ranges of interest are usually far apart. Even the ones that seem like good examples usually turn out to be not a direct coupling of the underlying vibration. For example, fluorescent lights tend to "hum," but not at the light frequency. They hum because there is a superimposed fluctuation of the light "intensity" driven by a vibrating electrical input. Such superpositions of frequencies--one higher-frequency vibration modulated at a much lower frequency (either the amplitude or the frequency can be modulated)--are actually very common. Radio and TV transmissions also do this by superimposing the information (for example, musical sound or speech) as a low frequency modulation of a "carrier" electromagnetic wave. I hope that helps clear up your confusion.```
 jat-ga rated this answer: ```Helpful. I hope to return to this subject sometime. Look forward to having you help me again...```
 ```Hi, I got stuck on answering you bit about the EMF spectrum so I'll put my research so far here: Key concepts: Resonance Every object has a unique natural frequency (also called harmonic frequency) of vibration. A periodic force occurring at the same frequency as the natural frequency of vibration of an object may cause the object to vibrate when the objects come into contact. This is called (mechanical) resonance. An example of this is a singer singing at the natural frequency of a wine glass. This causes the glass to resonate (that is vibrate at the resonant frequency which is the natural frequency) thus causing the bonds within the glass to deform beyond what it can handle and thus shatter. (It is not possible for one note to shatter a series of glasses as each piece of glass would have its own natural frequency.) Another example is a poorly designed bridge which has the same natural frequency as soldiers marching accross it or of the wind blowing through it. The Tacoma Narrows Bridge is one example of a bridge which has collapsed as a result of wind putting energy into the bridge at the natural frequency of the bridge. To see a movie of this bridge oscillating, check out http://sciencejoywagon.com/physicszone/lesson/09waves/resonan/resonanc.htm Fundamental Frequency When an object is forced to resonate, it vibrates in such a manner that standing wave patterns are developed in the object. For example, the strumming of a guitar string is an example of creating a standing wave. The lowest frequency which will produce a standing wave is called the fundamental frequency. Harmonics Harmonics or overtones are multiples of the fundamental frequency. They occur naturally when an object resonates. Harmonic waves occur at multiples of the fundamental frequency for example: let f = natural frequency = requency of resonance = harmonic frequency = 1st harmonic therefore harmonics occur at 2f = 2nd harmonic 3f = 3rd harmonic 4f = 4th harmonic 5f.... etc Very useful links: www.sasked.gov.sk.ca/docs/physics/u5c42phy.html http://www.biowaves.com/Physics/SoundPhysics.cfm http://electro.sau.edu/Homepage/SLResources.html http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/u11l4d.html Hope this helps! Jenjerina-ga```