AT&T Archives: Similiarities of Wave Behavior (Bonus Edition)

does the object in this photo look familiar to you possibly take you back to college physics class dr. John Shive host of this next film invented it at Bell Labs in the 1950s it's called the Shive wave generator Shive designed it as a teaching tool to help students understand wave motion concepts across disciplines from the electrical to the physical to the spectral the machine made its public debut in this film and was released along with the companion book it was part of a general effort on the part of 18 teased Bell Labs to provide novel educational tools for the next generation of scientist now it's a staple in physics classes around the world you can still buy one newly manufactured though it will set you back hundreds of dollars besides being an inventive engineer Shive was also a gifted lecturer and he deftly projects his love of physics in this 1959 film straight from 18 t archives and History Center here's similarities of wave behavior people who study waves extensively in the various fields of physics and engineering are impressed by the similarities which exist in the behavior of waves in various mechanical electrical acoustical and optical wave systems these similarities are due of course to the basic fact that waves of all kinds behave fundamentally alike these two rib like structures are mechanical wave demonstration machines with them I can show you many of the various wave properties which you may have studied experimentally or theoretically perhaps without ever having actually seen what the waves themselves are doing the waves which travel on this machine are really waves of twist or torsion which propagate along this central wire attached to the central wire are these cross arms which translate the torsional motion of the central wire to the up and down transverse motion of the ends of the cross arms which you see the entire structure is supported in a set of bearings which permit the central wire to twist freely this second wave machine is built similarly to the first it has cross arms which are considerably shorter than those on the first machine and as a consequence the speed of a wave along the second machine is about three times faster than the speed of a wave on this machine you'll see later on why I have these two machines first I want to show you the two different ways in which waves may be totally reflected observe the right now this machine has a free end that is this last cross arm is perfectly free to move without restraint if I start a wave at this end of the machine that wave travels down to the far end is reflected there and returns as a reflected replica of its original self but now let me terminate this machine in a different way I'll clamp this last cross arm to a heavy stand so that a wave arriving here can't produce any displacement of the cross-arm this time you see the wave is reflected upside down the reflection is still total as it was before but the wave is inverted upon reflection in a mechanical wave system then there are two ways in which the total reflection of waves may take place right-side up if the reflecting end of the medium is completely free and upside down if the reflecting and is completely restrained now if nature has any consistency and we like to think she does this alternative reflection possibility should exist for waves of all kinds and these other drawings here show that indeed it does take the case of an electrical transmission line for example you know that there are two ways in which electrical waves can be reflected depending on whether the far end of the transmission line is terminated by a short circuit or by an open circuit and similarly in the case of waves traveling down an acoustic tube there are two different reflection possibilities depending on whether the end of the tube is closed by a rigid termination or by the acoustic analogue of a free end note that in the mechanical situations a clamp is a constraint preventing the displacement of the end of the wave medium the electrical analog of this state of affairs is an open circuit termination on the end of a transmission light preventing the flow of current at such a termination and the acoustic analog is a rigid closure at the end of an acoustic to preventing the longitudinal vibration of the air in all of these cases the reflected waves come back upside down and in all of these other cases conversely which are analogous to the case of the free end of a reflecting wave medium the reflected waves come back right-side up you see nature really is consistent well now let's have a look at some of the other things we can do with our wave machine one of the most important topics in the general development of wave behavior is the principle of superposition when two waves traveling in opposite directions on the same medium as you see here in slow motion pass through each other the instantaneous amplitude of the resultant wave is the algebraic sum of the amplitudes of the two constituent waves this principle of superposition is equally valid when one of the two waves is positive and the other one is negative observe that in this case at the instant of exact coincidence there is a momentary cancellation of the amplitude and the medium appears to be undisturbed now let's have a look at the superposition behavior of continuous trains of periodic waves with this small motor and crank that can generate such a train of waves at this end of the machine and reflect them back upon themselves at this end all right in slow motion now here come the way after reflection the waves travel back up the Machine superposing on the oncoming waves which they meet to produce patterns of built-up amplitude at some places and complete cancellation at others the pattern that you see on this machine appears to be standing still doesn't it the various portions of the medium simply bob up and down in place without giving you the impression of motion in either direction along the machine observe that in this pattern there are places where the medium appears to be standing completely still all the time these dead spots or nodes are exactly half wavelength apart now I'm sure that you've all seen behavior of this kind before for example a vibrating string always vibrates naturally in one or more segments separated by such nodes now let me show you something else suppose I simply move this reflector a short distance up the line toward the generator just far enough so that the travel time of a wave down and back is an exact multiple of the wave period under these conditions the new waves which are continually being sent out by the generator superpose upon the previously emitted and multiplied reflected waves in just the right phase to build up the amplitude to an abnormally large value new crest on previous crest and so on the system thereupon becomes resonant and the amplitude builds up to the noticeably larger value that you see here under these conditions the new energy which is being fed into the system at each cycle is exactly equal to the energy which is being dissipated per cycle through friction at this new amplitude now the initial adjustment which I made in the length of the machine to produce this resonant condition is called tuning I can just as well have produced resonance by leaving the line at its original length and tuning the frequency of the generator instead and I can destroy the resonant pattern I have here by changing the value of either of these parameters a little bit in either direction another way of looking upon a resonance system is to regard it as a reservoir for energy thus the escape wheel of a watch a child's swing a sounding organ pipe the resonant circuit of an electrical radio transmitter are all examples of resonant systems they exhibit abnormally large amplitudes of oscillation and they possess relatively large contents of energy which have been built up by superposition over many previous cycles now from here I want to go on to the subject of the impedance of the wave medium the generation of waves on a transmission line of any kind electrical optical or what-have-you involves two parameters and originating cause and a resulting effect in an electrical transmission line for example the cause of the waves is an AC voltage applied to the input end of the line while the result is an AC current flowing into the line the ratio of the cause to the effect is called the impedance you're probably most familiar with the term impedance through your studies in the field of electricity but this impedance concept is just as applicable to wave systems of other kinds as well on our mechanical wave machine there for instance the cause of the waves is the oscillating torque which I apply to the first cross arm and the result is the oscillating angular velocity imparted to the medium on that structure then the impedance the mechanical impedance is the ratio of the applied torque to the resulting angular velocity in terminating the other end of a transmission line we frequently seek to adjust the impedance of the terminating load to equal the impedance of the line itself when this is done all the wave energy traveling down the line is absorbed by the load and no reflection takes place the load is then said to be matched to the line now this matching condition can be demonstrated very nicely with the wave machine here by attaching to this last cross arm a mechanical impedance and adjusting the resistance the counter torque with which that impedance reacts back/on the motion of this cross arm such a mechanical impedance is afforded by this dashpot simply a tin can full of water with a little piston pumping up and down in it by sliding this dash part in or out along the last cross arm I can control the counter torque with which it resists the displacement of the medium and thus match the impedance of the dashpot to the impedance of the transmission line itself when this adjustment is properly made the dashpot absorbs all the wave energy that travels down the line and as you can see hardly any perceptible reflection takes place now instead of using single waves suppose we have a look at this behavior with a train of continuous waves here you see such a train of waves traveling down the machine from the generator to the load and disappearing into the dashpot now suppose I spoil this nice matching adjustment by moving the dashpot farther in toward the central wire the impedance of the load now no longer matches the impedance of the transmission line and a partial reflection of the wave energy occurs let's have a look at this partial reflection now with continuous waves instead we expect some kind of standing wave pattern to appear on this machine now don't we after all we have running waves going in this direction and returning waves coming in this direction of the same period on the same medium only this time the returning waves have the lesser amplitude because the reflection down here is only partial well I do see a standing wave pattern of sorts here are the nodes half a wavelength apart only notice now that the nodes are no longer standing still as they were in the case of 100% reflection suppose I have the camera speed up the picture for you so that you can see what the envelope of this standing wave pattern looks like now let me draw you a sketch of that standing way of envelope the amplitude at this point I'll call a sub maximum at this point a sub minimum now in wave theory the ratio of a max to a men has a particular significance it is called the standing wave ratio abbreviated SWR on the wave machine the standing wave ratio is about three to one now recall that when we had 100% reflection when the end of the machine was clamped we obtained a standing wave pattern with motionless nodes the standing wave ratio in that case was infinity on the other hand when I had a perfect impedance match there was no reflection at all and no standing wave pattern in that case a max and a men were the same and the standing wave ratio was unity now I think you can begin to see where I'm leading you the punch line is this in any practical case of reflection a measurement of the standing wave ratio on the medium just ahead of the termination enables one to calculate the percent reflection that is taking place there the complete expression is percent reflection is equal to the standing wave ratio minus one squared divided by the standing wave ratio plus 1 squared times 100 now just for fun let's measure the percent reflection that's taking place here with these identical scales I can measure the amplitude at a maximum and at a minimum of this partial standing wave pattern maximum five in about two if you and I agree the maximum amplitude we saw there was five units while the minimum amplitude was two units therefore the standing wave ratio is five halves substituting in the percent reflection formula we obtain percent reflection is equal to five halves minus one which is three halves squared divided by five halves plus 1 which is 7 halves squared which gives us nine 49th or about 18 percent now note that I have just calculated the percent reflection on a mechanical wave system using an expression that was first developed in the field of AC electricity now this is a perfectly valid and logical thing for me to do since waves of all kinds behave alike now let me develop for you another idea waves are partly reflected not only at mismatched terminations but also at places where the impedance of the transmission medium changes abruptly suppose I connect these two wave machines and two end together they give me a single transmission line made up of two pieces having different impedances at this point where the two central wires are clamped together with this little clamp there is an abrupt discontinuity in the impedance of the transmission path a wave traveling slow along this medium will suddenly speed up when it crosses over into this one this behavior is similar to what happens to a beam of light when it emerges from glass into air now before I show you what I want to show you let me clip this dashpot down here in a matching position to prevent the unwanted reflections from this end it's this point here that we're really interested in alright here comes a wave and there he goes again do you see the reflected waves coming back from the midpoint here now that I've warned you what to look for this time you watch for that reflected wave coming back here he comes and once more there goes the main wave here comes the partly reflected wave you know our world simply abounds in examples of this kind electric waves light waves sound waves mechanical waves are partly reflected when they encounter impedance discontinuities usually in situations of this kind however continuous waves are involved after I connect up this motor we can see what happens when continuous waves meet an impedance discontinuity as we might expect a partial standing wave is produced if I wanted to determine how much energy is reflected there I could measure the maximum and minimum amplitudes of the envelope and calculate the percent reflection by the standing wave ratio method we used before that expression is equally good for calculating the energy loss at any kind of impedance mismatch or discontinuity often in our technology this partial reflection of wave energy by impedance discontinuities in the transmission path is economically wasteful and we seek to avoid it by inserting some kind of impedance matching device between the sections of the medium bar during the discontinuity here is an example of such a mechanical impedance matching device it is simply a short section of the wave medium exactly a quarter of a wavelength long and having a cross arm length intermediate between the cross arm lengths of the two machines it is called a quarter wave matching transformer I'll sandwich it in here and then show you how it operates to promote the transmission of wave energy across this discontinuity without reflection loss alright here come the waves and now you watch what happens there they go there is no perceptible partial standing wave on the input portion of this transmission system as there would be if reflections were coming back from the junction apparently then I have matched the impedances of these two machines this kind of transformer has a direct analog in optics the non reflecting coating on the lens of this camera is an optical quarter wave matching transformer it works in precisely the same way with light waves now unfortunately this type of transformer suffers two major drawbacks it operates only for continuous waves and it is effective only for a narrow range of frequencies including the particular one for which it was designed for applications in acoustics mechanics shortwave radio and the like where it is desired to accommodate much broader ranges of frequency we have a different kind of matching transformer to which this megaphone is a first approximation now the megaphone smooths over the impedance discontinuity between the air column in my throat and mouth and the free air in the space around me by means of a gradual taper the mechanical analog of a megaphone is a section of the wave medium whose cross arm length at the two hands matches the cross arm length of the two machines that I am trying to couple together with a gradual taper in-between a tapered section transformer such as this will match impedances over a relatively wide band of frequencies it is effective also for use with single waves and pulses watch what happens now when I send a number of pulses down this line here comes a short quick one and you see that hardly any perceptible reflection comes back here comes a somewhat longer one and again only a small amount of reflection returns you see the tapered section transformer is really a relatively broadband device here is another example of a tapered section transformer it is a piece of waveguide used to couple a section of rectangular cross section waveguide to another one of a smaller cross section in such a way as to prevent the reflective loss of radiation in electricity we employ transformers ranging all the way in size from this little fellow you can scarcely see clear up to the big transformers almost as big as houses that you find in hydroelectric stations even Nature herself is in there pitching inside the mammalian ear there are three tiny bones called the hammer the anvil and the stirrup their task is to provide an impedance linkage between the low impedance of the air in the outer ear and the high impedance of the liquid in the inner ear and in addition to all these transforming devices that are used with pulses and waves there are others that are employed with unidirectional motion for example in mechanic's all of the mechanical advantage machines such as gear trains and levers and pulleys and the like are really impedance transforming devices now in this film I hope I've been able to present you some different approaches and perhaps to lead you to a few new insights one thing that has always profoundly impressed me is this waves of all kinds behave alike and if you any process you choose you can learn the fundamentals of wave behavior either through the study of some discipline in which waves play an important role or even through the study of waves for themselves alone then you will always feel at home in any branch physics or engineering where the main show has to do with waves and how they behave you you

Channel: AT&T Tech Channel

Views: 323,510

Keywords: AT&T, Tech, Channel, History, Archives, Bonus, Edition, Physics, Waves, JN, Shive, Bell, Labs, 1950s, Lecture, Education

Id: DovunOxlY1k

Length: 28min 2sec (1682 seconds)

Published: Tue Apr 03 2012

Reddit Comments

[u/JulesCC91]

Wow that was so cool to actually see! I've only ever seen animations of how waves work (and studied the math) but this physical model really makes it easy to grasp. I wish my professors used this when I was in school.

[u/[deleted]]

Yes that's a good video

[u/[deleted]]

Our professor sent this to us a few weeks ago and I normally have no desire to watch stuff he sends us, but this video was actually dope.

[u/Elderly_Ravioli]

This is an incredibly helpful video, wish I had watched it when I was taking transmission lines. Most impressive part to me though was when the guy just knew that 9/49 is about 18%

[u/delirium_filter]

Cool video but his EM wave analogy is backwards. A free end of the mechanical system is analogous to an open circuit and the reflected wave is positive on the transmission line. The clamped end is analogous to a short circuit (the end of the line is held at GND) and the reflected wave is negative.

[u/w2aew]

I agree - fantastic visualization. I like it better than my video on the topic of standing waves:

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https://www.youtube.com/watch?v=M1PgCOTDjvI

[u/lasqi]

For anyone that liked this, there's a YouTube channel Called Jeff Quintey that uploads restored technical and war documentaries from the 1930s onwards. There's some electrical content there that explains concepts in a similar style to this.

Edit: His Youtube account was closed, so his videos are on Vimeo.

[u/metallich]

This was wonderful, thanks for posting

[u/MSpiessH]

!RemindMe 400 days