Natural phenomena that show AM & FM modulation - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: January 07, 2022, 08:14:15 pm
content: ok, thinking caps onAs i got my head around the concept of F.M. modulation it suddenly came to me - a Slinky toy is a pretty good analog for that in terms of the amplitude and frequency of the modulating wave's effect on the carrier wave.Can anyone think of another physical 'demo' that would demonstrate A.M. modulation? or something that occurs in nature?Just a thought...

author: jpanhalt
author_rank: Super Contributor
timestamp: January 07, 2022, 08:47:21 pm
content: Ripples in a pond.

author: CaptDon
author_rank: Super Contributor
timestamp: January 09, 2022, 12:06:17 am
content: I never thought a slinky had a lot of 'FM' distribution. Nor even a lot of 'AM' either. I kind of think of it as the carrier wave albeit a very distorted waveform. I suppose it has some amplitude modulation of the carrier but I rather think the frequency distribution of the fundamental and harmonics is each a pretty tight group with not much 'shift'. Frequency shift of a steady carrier, well maybe certain bird calls or animal mating call sounds, although many are just one mono pitch. A 'chirp' by definition is a frequency shift which could also have a beginning and ending timbre or amplitude variation. The 'whistlers' and 'chirps' associated with aurora borealis have both pitch shift 'FM' and volume shift 'AM' during any one particular chirp or whistle. As if the ever changing visual patterns weren't cool enough already!!!

author: Quantumplate
author_rank: Regular Contributor
timestamp: January 09, 2022, 11:56:10 am
content: Trying to follow your train of thought! What's FM distribution? I googled it and got some perfume company...The only physical image I can think of that sort of represents AM mod. is a cartoon uvula screaming! If you think of the oval shape pre-scream as the carrier, and the added waves on top during the note. Silly, but an example for beginners I guess.

author: CaptDon
author_rank: Super Contributor
timestamp: January 10, 2022, 03:01:57 pm
content: I guess by FM distribution I meant DEVIATION. So FM is a carrier wave being shifted up and down 'the modulation' part. I never found the pitch of a slinky to be 'deviating' or 'modulating'. It was just sort of one continuous sound every part of the slinky resonant at the same pitch. So if it isn't shifting pitch it isn't FM'ing. It does however change in amplitude as it moves along.

author: Fred27
author_rank: Unknown
timestamp: January 10, 2022, 03:04:47 pm
content: I'm guessing that by FM you're thinking of when you stretch out a slinky and give it a quick shove along its length. Whilst that may look like a drawing of a frequency modulated signal, that's not what it is. It's a longditudinal wave travelling along the spring. (My apologies if that's not what you meant.)

author: GMJ
author_rank: Newbie
timestamp: January 10, 2022, 03:06:24 pm
content: I think he means visually? Slinky coil contracted is one frequency, spreads to represent a variation on that frequency, then returns to original frequency. No?

author: CaptDon
author_rank: Super Contributor
timestamp: January 10, 2022, 05:01:52 pm
content: GMJ, yes, I think your description of the visual effect is accurate!! That does to some degree look like an FM'ed wave on an oscilloscope. I guess we need a microphone and audio/ultrasonic range spectrum analyzer to look at the audio spectrum to observe any FM of the 'carrier'. I suspect the screen of the analyzer to look like a forest however with all the distortions of the carrier wave.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 10, 2022, 05:03:14 pm
content: Well in music, of course.AM: allegro, picolo, etcFM: tremolo--think of the violinist and cellist vibrating their hand on the fingerboard to create that nice effect.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2022, 05:46:28 pm
content: Doppler. Fading.Slinky sounds consist of altered frequencies if the slinky is in relative motion to the wave; the wave velocity depends on tension and pitch, so there will be an acoustic/longitudinal wave interaction.Note that just playing with a slinky, needn't involve these effects specifically: lots of noise sources are impulsive in nature (i.e., fumbling around with it, hitting parts on a surface, or some loops get overlapped then twang apart, etc.), and the medium itself is dispersive, that is, frequencies travel at different velocities. Partly because, due to how the helix is shaped, higher frequencies (waves with tighter curvature) see more stiffness, and therefore a higher velocity; partly because there are multiple wave modes (stretching, bending, twisting) in the material, which all move at different rates so an impulsive wave spreads out into ripples.Which is also what ripples on a pond (gravity waves) do: they are dispersive so an impulse separates into ripples of varying amplitude. (Try it, take a close look at the ripples next time you see some propagating!)Anyway, that's a possible example of Doppler effect, of a sort (the shaken slinky example). The more direct case is when the transmitter and receiver are in relative motion; or the wave medium itself is, which is also relevant here (not that you'll see many bodies of water where the flow is smooth enough to observe ripples in motion*). Which is a less familiar effect in the air, but should be a thing still.*That said, you can also observe some of the more dramatic (nonlinear) wave effects when rapid flow is seen. Where flow is faster than wave velocity, it's supersonic and waves cannot propagate up the flow. Consider the step around the base of a water stream hitting a flat surface (sink, say): near the stream, the velocity is high, then as it spreads out radially, flow decelerates, and then kind of... *fwoomp* it poofs out to a turbulent pileup of slower flow. In this region, gravity waves can propagate; of course waves all pile up near the shock front, making it very turbulent, but away from there, normal behavior can be seen.As for fading, this still tends to be very frequency dependent (due to interference effects), but generally it occurs with an array of reflectors moving slowly, their reflections sometimes overlapping a target area, sometimes avoiding it completely. The classic radio example is ionospheric fading in sky-wave SW propagation; ionospheric layers vary in conductivity, much as the aurora forms shimmering, undulating sheets -- the coincidence is no accident, of course! A similar effect occurs in a busy environment, at lower radio frequencies -- multipath. (If you listen to FM radio in the car much, you've probably noticed it sometimes fades out. Creep forward about a quarter wavelength -- 0.75m -- and it likely comes back.) Or weird acoustic effects over distance, like maybe you can hear fireworks or factory noises at certain distances (some miles) but not others, give or take weather conditions (layered air is refractive, and can also trap sound by atmospheric ducting).To be clear, many of these effects are frequency dependent -- due to interference. So you might not see them with visible light for example, where the scale is just so tiny (100s nm), or at best you see rainbow fringing. (Bubbles and oil slicks are indeed interference effects; these structures are thin enough, and consistently so, that you can make out the fringes, rather than it being a gray blur of all colors smeared together.) And whereas it looks like fading for a given radio channel, say, adjacent channels fade at different times/locations (with respect to receiver/antenna location that is).On a more abstract level, amplitude: easy, just vary how much power goes from transmitter to receiver somehow. Block line of sight, cause interference, add a filter, etc. Frequency: a lot harder, you can't do this with linear processes.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 10, 2022, 05:55:10 pm
content: Quote from: RandallMcRee on January 10, 2022, 05:03:14 pmI think that would be more vibrato than tremolo.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 08:23:35 pm
content: Most natural sources are AM, which is what makes FM so useful to avoid noise. The limiter used in the FM signal chain removes AM.

author: TimFox
author_rank: Super Contributor
timestamp: January 10, 2022, 08:34:04 pm
content: A classic example of frequency modulation in sound is the Doppler effect, first demonstrated by C H D Buys Ballot in 1845 by translating musicians on a railroad car past the observer.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 10, 2022, 09:06:47 pm
content: Natural FM occurs in speech, birdsong and other animal sounds. A chirp is FM. A police siren is FM, Vibrato on a string instument creates FM. etc.....If you are interested in the maths of modulation and how it gives rise to the the harmonic content,this is a good explanationhttps://www.johndcook.com/blog/2016/02/17/analyzing-an-fm-signal/

author: LaserSteve
author_rank: Super Contributor
timestamp: January 11, 2022, 04:32:16 pm
content: Proton procession. Nuclear Magnetic Resonance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2022, 10:23:48 pm
content: BTW, one should make a distinction between sources, which are eminently modulatable by changing their characteristics; and media, which cannot modulate the frequency of sources under linear conditions, though frequency-dependent effects exist (which might be (mis?)interpreted as such), and nonlinear and time-varying systems can have FM effects.The first case is relatively trivial, as many parameters of a source can be varied; so I concentrated on the latter in my earlier comment.For that, often it's a length parameter, as with the fret of a guitar string, or length of a trumpet pipe, or effective length of a woodwind instrument; etc. Note that a tube resonator is also resonant at harmonics; harmonics can be selected through various means, from embouchure (how you hold your lips; more tension encourages higher frequencies), to opening/closing holes/stops/traps/etc. in the resonator that prohibit certain modes, or pull/shift the resonant frequency.Other sources are more impulsive, and the frequency is simply the repeat rate. Like a relaxation oscillator (a current charges a capacitor to a breakdown voltage, which resets it for the next cycle; etc.), or human vocal fry (the vocal chords are held closed under tension, until pressure builds up under them; pressure and tension vary the pitch). It's kind of not so much that it's a tone, as a train of impulses, which we perceive as a tone because it's fast enough.Resonators tend to be more stable in pitch, as dimensional parameters tend not to vary a whole lot; while relaxation oscillators are very nimble and, obviously, critically dependent on the power source.Note we can even have both. The human vocal tract is quite nuanced, with air through the vocal cords powering the system, and the throat, mouth and sinuses acting as resonators, filtering that sound to form vowel sounds / tones; consonants further modify flow paths and patterns, adding start/stop effects (and, perhaps, some frequency modulation there, as well as of amplitude), turbulent hissing (fricatives), etc. Or at higher frequencies, the vocal cords lock to the resonator mode, producing a "head voice" (or falsetto, but I'm honestly not quite sure of the distinction, if any?) where the resonator tunes the pitch in part, but it is excited by a (synchronized) impulsive source.Which also describes a lot of things, like guitars are plucked (impulsive source), but we perceive the pitch as the resonance of the strings (a filtering effect), etc.Tim

author: golden_labels
author_rank: Super Contributor
timestamp: January 12, 2022, 12:10:45 am
content: Neuron coding, assuming the rate coding model is right. In that scenario signals are passed using pulse-frequency modulation.