You probably hear this every day, that weird phenomenon sounds makes when it whooshes you by quickly. In fact, the entire ‘whoosh’ effect – that ‘zzzzzzzzzzzhoooooooom!’ that seems all children are familiar with and use as a description for a passing car is a great hint that something is going on.

[youtube_video id=”swvQBsFcwRE”]

Of course, it only takes a few years and a drivers license to understand that a car that actually sounds like that while standing is not quite the car you would like to buy. Or rent. Or.. get into.

So why are we all using this sound to describe the moving car? What happens to the sound of a car when it is moving to get it to change so dramatically? I wonder.

No more.

No more wondering, that is. Cars will stick around. At least for a bit, until we start with the hovercrafts… and everyone knows they go “fffffffffffffffffffffzzzzzzzhhuuuuuum!” anyways.

The next experiment explains the phenomena called “The Doppler Effect“, where waves (not just sound waves, mind you. Doppler welcomes waves of all kinds, shapes and frequencies), seem different in movement.

A ‘Wavy’ Reminder:

Just so we’re all in the same level, here’s a reminder: We can describe waves with a few main characteristics: Amplitude, Wavelength and Frequency.

  • Amplitude defines the “strength” of the wave. In sound waves, it is the factor that determines how loud the sound is.
  • Wavelength is one full cycle the wave ‘completes’ from 0 to 180 (or, from ‘peak to peak’).
  • Frequency is determined by the amount of cycles (wavelengths) per second.Hight frequency will result in many cycles per second, and therefore a very small wavelength. Low frequency means few cycles per second and a long wavelength.

Formula for the Doppler Effect:

(credit: Wikipedia)

f is the original frequency in movement, and f’ is the “resulting” frequency (the one the mommy duckie actually hears).

V is the speed of the wave itself. Sound waves in air, for instance, have a speed of approximately 330 meters per second.

Vs is the speed of the moving object that is creating the sound – for that matter, the speed of my duckie. Solving that equation gives the resulting frequency (the “pitch”) that the stationary listener receives.

Preparations:

I didn’t have a lot of time to prepare, and I didn’t want to spend too much money on materials, so instead of working on properly connecting the buzzer to a battery, I just hooked up something very.. uhm.. amateurish. Anticipating the comments some of you will probably post, I must state, in advance, that I know it’s amateurish. I just don’t care. It worked.

Here’s the buzzer before:

The Buzzer

I bought it for $1.60 in one of the main electronics shops (you could probably find it for a lot cheaper in a non-franchise (or just outside of Manhattan).

For whoever’s interested, the original (non moving!) specified frequency for it is 2700+- 500Hz.

After I tweaked with it a bit and used paper clips (because that’s what I have here), it took the shape of this lovely piece of art (though an incredibly annoying one):

The Buzzer (finished)

That unconnected red line – when connected to the batteries – closes the circuit and activates the buzzer. I had to unhook it for fear of my sanity. Handle with care.

You can purchase an already-built buzzer. I am just too cheap.

Also, a friend of mine gave me the idea of playing a continuous single-note mp3 file (if you can find, or record one) on your favourite mp3 player and fling around the earphones. (Nice idea, genius, next time you spend 10 minutes building a stupid buzzer.)

Materials Used in this Experiment:

  • 2 Rubber Duckies with their Rubber Duckie Mommy.
  • Any kind of bucket, tub or bowl. Preferably clean water.
  • A buzzer or any type of annoying sound making device ($1.60 in Radio Shack). Do not use a baby.
  • Batteries for your buzzer (or whatever else makes it go bzzzz).
  • A Tennis ball.
  • A stocking (don’t take a nice one, the owner of that stocking will not like it).
  • A trusted (and trusting) friend.

So, What do you do? Simple. You turn the buzzer on, shove it into the tennis ball so that whatever happens it doesn’t break (and, also, if your hand gets slippery and it whooshes off to space, your friend’s face will have a nice round bump instead of a nasty battery-shaped one). You shove the now-very-annoying tennis ball into the stocking —

Note: This is the time to call your friend over to enjoy the wonders of this phenomenon. You, as the buzzer-slingshot performer, wont really notice any sound changes, but the person in front of you will, and your friend will thank you from the bottom of his heart.

Or his ear. Whichever will hurt less.

— start twirling the stocking around like a lasso above your head, or next to your body.

The buzzer moves quickly closer and farther from the person in front of you and the Doppler Effect kicks in. It might not sound as cool as a car ‘zhoooooooom’ing by but it certainly proves the point. Stop twirling the buzzer around and show your (hopefully appreciative) friend that the buzzer has a constant sound when it’s not brutally slingshot through the air.

Voila. You’ve created the Doppler Effect in your own house. Aren’t you proud?

Practical Applications

The Doppler Effect, as I stated before, does not discriminate on the type of waves it operates on. For that reason it has a lot of practical applications:

  • Police Radar: Yup! When police officers measure the speed of a passing car using their nifty-looking radar-gadget, that’s how they’re doing it. Well, they’re not actually calculating it themselves, of course, the radar is doing it for them – but it is using the Doppler Effect. The device is sending a wave with constant frequency towards a passing car and expects the reflection. Since the car is in movement, the reflection is bound to come back distorted from the Doppler Effect. Using the formula, it then calculates the exact speed of the passing car and notifies the police officer if a ticket is needed.
  • “Red Shift”: As we said, the Doppler Effect acts on waves of all kinds, including light. With light the effect is emphasized because different frequencies of light waves mean different visible colors. When astronomers look at the sky in search of new (and existing) galaxies, they measure the light frequencies from that galaxy. Galaxies that are shifted towards the “red” spectrum are lower frequency, and galaxies that are shifted to the “blue” spectrum are higher frequency. “Red Shift” is the term used on galaxies that move away from Earth, where their visible and invisible light frequencies are lower – shifted to the ‘red spectrum’. Measuring exact shifts can help astronomers understand how fast a stellar object is moving away (or towards) us.
  • Airplanes: Since airplanes are moving, the ground always receives a distorted transmission. For that reason, airplanes never use frequency-modulations (FM) for their transmissions (even though FM transmission is considered to be of higher quality) but rather modulations that are based on amplitude (AM). That way the ground can decipher the messages quickly regardless of the distortion and changes in speed.
  • Breaking the Sound Barrier:Moving airplanes get really fast. Really really fast, actually. If a plane moves faster than the spread of the sound waves, it is considered to “break” the sound barrier. A nice representation of it can be seen in this link from Kettering University.

Resources:

  • http://www.seed.slb.com/en/scictr/lab/doppler/index.htm
  • http://www.fearofphysics.com/cgi-bin/doppler.cgi?dir=a&vs=300&mode=wrap
  • Doppler Effect and Red Shift: http://www.youtube.com/watch?v=Man9ulEYSgk
  • http://www.space.com/scienceastronomy/redshift.html