Good morning.
The Doppler Effect is named after Christian Doppler, a 19th century mathematician and
physicist who was able to mathematically describe what is now called the "Doppler Effect", and
it sounds like this...
[Car Hooting].
♪flipping physics♪
Perhaps you are already familiar with how the Doppler Effect sounds; however, I'm here
to make sure you understand why it sounds this way.
- Oh, great!
I always want to know why.
- Me too.
- Actually you know what?
I do too.
- Alright...
So let's start by recalling what this animation of a stationary sound source actually represents.
So, Bobby, remind me please.
- Well, uh -
- Actually, rather than using the car horn, let's switch to a constant 440 Hz frequency
being played on the speakers of the car stereo.
That is the 440 Hz concert pitch which we used last time.
- It's uh, harder to hear the Doppler Effect at 440 Hz rather than the car horn.
Can we just stick with the car horn?
- OK, we can go back to the car horn; however, the Doppler Effect is easier to understand
if we referr to a single frequency.
So let's keep referring to the demonstration as a single frequency but use the car horn demonstration.
[Car Hooting]
Sorry Bobby, we're back to you, you are telling us what this animation of a stationary sound
source actually represents.
- Right, um....
- Remember, sound is a longitudinal pressure wave.
- And those lines are crests.
- Which are also called wave fronts.
- Yeah, thanks.
So, sound is periodic changes in pressure in the medium which in this case is air.
Each of those lines represents a crest in the longitudinal pressure wave, that means
each crest or wave front is a location of high pressure.
That periodic pressure wave transfers sound energy through the air and the reason all
those circles are the same distance from one another is because the sound source is oscillating
in simple harmonic motion at a constant frequency.
- Do not forget that each circle actually represents a spherical pressure wave or
wave front going out in all directions from the sound source.
It is a circle instead of a sphere here because this is a two-dimensional space.
- Very nice everybody.
Now remember, this animation is for a stationary sound source.
Look at how the animation changes when the source of the sound is moving.
- Huh, I like that.
- Good, but I want to make sure that you are aware that each of those circles, which are actually spheres,
represent the sound coming from where the sound source was when it emitted the sound.
To make that clear, I have highlighted one of the wave crests so you can see that it
is a sphere that is increasing in size and the center of that sphere continues to be
in the location where the car was when the sound was emitted by the speakers of the car.
Sometimes that is a concept which gets lost in the overall understanding of this.
- Oh thanks!
- OK.
- Now let's add a human to our animation.
Notice the human or the observer is currently located such that the car, the source of the
sound, is coming towards the observer.
Bo, what do you notice about the distance between the spheres that are going towards the observer?
- On the side of the car where the guy is, the circles are closer to one another.
- And what does that mean?
- Well, the circles represent crests in the wave, and the distance between two crests
of a wave is the wavelength of the wave.
I guess that means that the wavelength of the sound wave which the guy hears when the
car is moving toward him is less than the wavelength of the original sound.
- Correct Bo.
The wavelength of the sound heard by the observer when the sound source is moving toward them
is smaller than the wavelength of the original sound.
And Billy, what does that mean for the frequency of the sound heard by the observer in this example?
- Okay, the observed wavelength is decreased and you are asking how that affects the observed frequency.
Well, we know the speed of a wave equals the frequency of a wave times its wavelength.
That means, the frequency and wavelength of a wave are inversely related, that means,
if the observed wavelength is decreased, the observed frequency must be increased.
In other words, when a sound source is moving towards an observer, the sound heard by the
observer has a smaller wavelength and larger frequency.
A larger frequency sounds like a higher pitch to us, so the sound here when the car is coming
towards us has a higher pitch.
- Absolutely correct Billy.
Bobby, could you please talk about what happens when the car is moving away from the observer.
- OK, this should be the inverse of what we just talked about.
Now the crests are farther away from one another so, the wavelength of the observed wave is larger.
Wavelength and frequency are inversely related to one another, so because the observed wavelength
is increased, the observed frequency is decreased.
That is why the frequency and pitch of the observed sound are lower when the car is moving away from us
- Very nice Bobby, but I want to specify that the frequency and wavelength
of the sound source remain unchanged.
To reiterate that point, please enjoy this split screen of two different viewpoints during
the demonstration.
On the left you see me in the car and on the right you see the event from the perspective
of the stationary observer. Before we heard what was happening from the perspective of
the stationary observer.
Now let's listen to what we hear from inside the car.
[Car Hooting]
The frequency and therefore the pitch you hear is constant.
- Sure, the person who is in the car is not moving relative to the source of the sound,
so the wavelength and frequency they observe are not affected by the motion of the car...
Thanks.
- But what if it's reversed and the car is stationary and the person is moving?
- Great question Bobby.
That actually causes the same Doppler Effect; if the sound source is stationary and the
observer is moving, as the observer moves toward the stationary sound source, the observed
wavelength is still decreased and therefore the observed frequency is still increased.
And the reverse is true as the observer moves away from the stationary sound source.
Thank you very much for learning with me today, I enjoyed learning with you.