The Doppler Effect and Malaysian Flight 370

Posted by PITHOCRATES - March 26th, 2014

Technology 101

A Swan Pushes Waves Together in front of it and Pulls Waves Apart behind it as it Paddles across Water

Throw a stone in the water and what do you see?  Little circular ripples in the water moving away from and centered on where the stone broke the surface of the water.  These are waves.  Energy.  They are more intense the closer they are to the point of disturbance.  And become less intense the further they are from the point of disturbance.  So you’ll see larger circular ripples in the water closer to where the stone hit the water.  And smaller circular ripples at increasing radii from the point of impact.

You’ll see these little circular waves, too, when something else disturbs the surface of the water.  Like a swan.  Or a duck.  As they paddle their feet they move forward in the water.  Pushing the water out ahead them.  If you look closely you’ll see ripples bunched up in front of them.  And ripples spaced further apart behind them.  This is because of their movement towards the previous ripple.

These waves ripple through the water at the same speed (assuming the swan or duck is paddling at a constant speed).  So each ripple will travel the same distance at the same speed from the paddling bird.  But as the bird moves forward each subsequent wave in that direction is starting its journey at a point further along in that direction.  So one wave may have gotten to a point (let’s call it Point A) in the water 3 inches ahead of where the bird created it.  Since creating that wave the bird continued to paddle.  And created another wave.  This one created only 2 inches from Point A.  And then the bird created another wave at only 1 inch from Point A.  So subsequent waves are ‘catching up’ to previous waves.  Thus bunching the waves up in front of the bird.  While the bird is pushing these waves closer together the bird is traveling away from the waves behind it.  Stretching those waves further apart from each other.

A Guitar makes Sound by Vibrating the Soundboard in the Body of the Guitar

If you’ve ever played a guitar or watched someone play the guitar you’ve probably noticed how the sound changes depending on where the player fingers the string on the fingerboard (or fretboard).  If the player presses down on the string closer to the body of the guitar the note sounds higher.  If the player presses down on the string further away from the body of the guitar the note sounds lower.  Why?  Frequency.

A guitar makes sound by vibrating the soundboard in the body of the guitar.  The faster it vibrates the higher pitch the sound.  The slower it vibrates the lower pitch the sound.  The string vibrates back and forth a number of times each second.  The more it moves back and forth in one second the higher the frequency and the higher the pitch.  The fewer times it does the lower the frequency and the lower the pitch.  Thinner strings vibrate faster than thicker strings.  Shorter strings vibrate faster than longer strings.  So a typical guitar has 6 strings of various thickness stretched from the soundboard across the fingerboard.

The vibrating soundboard creates sound waves that move through the air.  Similar to a rock breaking the surface of the water.  As a guitar player fingers different notes on the fingerboard the soundboard vibrates at different frequencies.  Making music.  If you’re attending a small concert where a soloist is playing, say, Spanish Dance No. 2: Oriental by Enrique Granados you would hear the same beautiful music wherever you were sitting in the room.  The sound waves would be radiating throughout the room like the ripples created when a rock breaks the surface of the water.  However, if the soloist was moving like a swan through the water it would be a different story.

Using the Doppler Effect they determined Malaysian Airlines Flight 370 traveled the Southern Route

Ever listen to the sounds of cars and trucks traveling down a highway?  Maybe while visiting your aunt and uncle who live on a highway out in the country?  Did you notice that they had a higher-pitched sound when they approached you than when after they had passed you by?  The next time something noisy passes you by listen.  Especially if they’re blowing their horn.  It’ll go from a higher-pitched sound to a lower-pitched sound just as it passes you.  Why?  Think of the waves a swan makes gliding through the water.  Bunching waves closer together in front of it.  And stretching them further apart behind it.  The same thing happens with sound waves.  Austrian physicist Christian Doppler noted this in 1842.  Something we now call the Doppler Effect.

If a train is travelling down the track while blowing its horn it sounds the same aboard the train from the moment the engineer starts blowing it until he or she stops.  Just as the sound of a soloist playing Spanish Dance No. 2: Oriental sounds the same wherever you are in the room.  Because the distance between the source of the sound and the listener of the sound does not change.  But if you were standing stationary near the railroad track as the train traveled past you the frequency of the horn changes.  Because as it is approaching you it is pushing sound waves closer together.  Creating a higher frequency (or a higher-pitched sound).  As the train passes it is stretching those sound waves further apart.  Creating a lower frequency (or a lower-pitched sound).  This is the Doppler Effect.

When Malaysian Airlines Flight 370 shut off its transponder and ACARS (Aircraft Communications Addressing and Reporting System) stopped broadcasting the plane vanished.  But a satellite communicating with the airplane still ‘pinged’ the aircraft every hour of its remaining flight time.  And electronic handshake.  The satellite says, “Are you still there?”  And the plane responds, “Yes I am.”  No data was transmitted.  Only a sent and received signal.  Just a pulse of a constant frequency.  A ping.  But from those pings they could measure the time it took to send and receive those pings.  Which they could calculate distances between the satellite and the plane from.  Giving us the northern and southern possible routes as it traveled in an arc around the satellite.  But which way it went on that arc was a mystery.  Until they analyzed the frequencies of those pings.  And they detected a slight change in the frequencies.  Using the Doppler Effect they determined which side of the plane was bunching up the sound waves and what side of it was stretching them out.  And concluded the plane was traveling on the southern route.  Which is why all search efforts are now in the south Indian Ocean southeast from Australia.  Because, according to Christian Doppler, that’s the direction the plane flew.


Tags: , , , , , , , , , , , , ,

Sound Waves, Phonograph, Stylus, Piezoelectric & Magnetic Cartridges, Thermionic Emission, Vacuum Tube, PN-Junction, Transistor and Amplifier

Posted by PITHOCRATES - May 2nd, 2012

Technology 101

The First Phonographs used a Stylus attached to a Diaphragm to Vibrate the Air and a Horn for Amplification 

Sound is vibration.  Sound waves we hear are vibrations in the air.  A plucked guitar string vibrates.  It transfers that vibration to the soundboard on the guitar body.  The vibration of the soundboard vibrates the air inside the guitar body.  Amplifying it.  And shaping it.  Giving it a rich and resonant sound.  Creating music.  And we can reverse this process.  Taking these vibrations from the air.  And putting them into a piece of wax.  Via a vibrating needle.  Or stylus.  Cutting wavy grooves into wax.  And then we can even reverse this process.  By dragging a stylus through those same wavy groves.  Causing the stylus to vibrate.  And if we transfer those vibrations to the air we can hear those sound waves.  And listen to the music they make.

The first phonographs could reproduce sound.  But they didn’t sound very good.  The first phonographs were purely mechanical.  A stylus vibrated a diaphragm.  The diaphragm vibrated the air.  And a horn attached to that diaphragm was the only amplification.  Sort of like cupping your hands around your mouth when shouting.  Which reinforced and concentrated the sound waves.  Making them louder in the direction you were facing.  Which is how these early phonographs worked.  But the quality of the sound was terrible.  And played at only one volume.  Low.

Electric circuits changed the way we listen to music.  Because we could amplify those low volumes.  By changing the vibrations created from those wavy grooves into an electrical signal.  The first phonographs used a piezoelectric cartridge.  Which the stylus attached to.  The piezoelectric cartridge converted a mechanical pressure (the needle vibrating in the wavy groove) into electricity.  Later phonographs used a magnetic cartridge.  Which did the same thing only using a varying magnetic field.  The vibration of the needle moved a magnet or a coil through a magnetic field.  Thus inducing a current in a coil.  Then all you needed was an amplifier and a loudspeaker to make sweet music.

Small Changes in the Control Grid Voltage of a Vacuum Tube make Larger Changes in the Plate Voltage

The first amplifiers used vacuum tubes.  Things that once filled our televisions and stereo systems.  Back in the old days.  Up until about the Seventies.  A vacuum tube operated on the principle of thermionic emission.  Which basically means if you heat a metal filament it will ‘boil off’ electrons.  The basic vacuum tube used for amplification consisted of a cathode and an anode.  Or filament and plate.  And a control grid in between.  Sealed in, of course, a vacuum.  Creating the triode.  The cathode (filament) and anode (plate) created an electric field when connected to a large power source.  The cathode is negative.  And the anode is positive.  When negatively charged electrons are ‘boiled off’ of the cathode the positive anode attracts them.  The greater the heat the greater the thermionic emission.  And the greater the current flow from cathode to anode.  Unless we change the electric field to inhibit the flow of current.  Which is the purpose of the control grid.

Small changes in the control grid voltage will make changes in the large current flowing from cathode to anode.  That is, the larger current replicates the smaller signal applied to the control grid.  This allows the triode to take the low voltage from a phonograph cartridge and amplify it to a higher voltage with enough power to drive a loudspeaker.  Which is similar to diaphragm and horn on the first phonographs.  Only the amplified electric signal moves a lot more air.  And better materials and construction create a better quality sound.  Amplifiers with vacuum tubes make beautiful music.  High-end audio equipment still uses them to this day.  Including almost all electric guitar amps.  So if they have the highest quality why don’t we use them elsewhere?  Because of thermionic emission.  And the heat required to ‘boil off’ those electrons.

Vacuum tubes worked well when plugged into line power.  Such as a radio in a house.  But they don’t work well on batteries.  Because it takes a lot of electric power to heat those filaments.  And you need pretty big batteries to get that kind of electric power.  Like a car battery.  But even a car battery didn’t let you listen to music for long when parked with the engine off.  Because those tubes drained that battery pretty fast.  So there were limitations in using vacuum tubes.  They draw a lot of power.  Produce a lot of heat.  And tend to be pieces of furniture in your house because of their physical size.

Small Changes in the Base Current of a Transistor is Replicated in the Larger Collector-Emitter Current

The transistor changed that.  Making music more portable.  Thanks to semiconductors.  Material with special electric properties.  Based on the amount of electrons in the atoms making up this material.  Atoms with extra electrons make material with a negative charge (N-material).  Atoms missing some electrons make material with a positive charge (P-material).  When you put these materials together the N and the P attract each other.  Electrons cross the junction and fill in the holes that were missing electrons.  And the ‘holes’ cross the junction and fill in the spaces where there were excess electrons.  (When an electron moved, say, from right to left it made a hole and filled a hole.  It made a hole where it once was.  And it filled a hole where it now is.  So it looks like the hole moved from left to right when the electron moved from right to left.)  Neutralizing the N-material and the P-material.  But creating a charged region around the junction.  And it’s this electron flow and hole flow that make these PN junctions work.  When you add a third material you get a transistor.  Made up of three parts (NPN or PNP).  Emitter, base, and collector.

To get the electrons and holes flowing you start applying voltages across the junctions.  A large current will flow from the collector to the emitter.  Similar to the current flow in a tube from cathode to anode.  And a small base current will change that current flow.  Just like the control grid in a vacuum tube.  Small changes in the base current will make similar changes in the larger collector-emitter current.  Just like in a vacuum tube, the larger current replicates the smaller signal applied to the ‘control’.  Or base.  This allows the transistor to take the low-level signal from a phonograph cartridge and amplify it to a higher level.  Just like a vacuum tube.  Only with a fraction of the electric power.  Because there are no filaments to heat. 

Low power consumption and the small physical size allowed much smaller amplifiers.  And amplifiers that everyday batteries could power.  Creating new ways to listen to music.  From the pocket-size transistor radio.  To the bigger stereo boombox.  To the iPod.  Where the basic principle of how we listen to music hasn’t changed.  Just how we vibrate the air that makes that music has.


Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,