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.

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