Why does the pitch of an airplane sound like it goes from high to low when it flies overhead?
In: Physics
Doppler effect. Sound waves spread out when they’re moving away from you so things sound lower and/or slower.
Sound is a mechanical wave, just like those in the water. As you know when you drop a stone to a standing water, waves are going in every direction at the same speed. But what if you would put a piece of plastic and drag it on the water? It also makes waves but ones in front are closer together than those in the back, same happens with sound when a plane travels across air, the diffrence is you cant SEE sound but you can hear it. Those changes came exactly from combining “normal” sound with speed of a plane and make waves closer or farther apart from one another and you hear it as a change in pitch.
That distortion is called Doppler Effect. By typing it in google you will find a lot of great resources about it because it’s really fun physics thing to show general public.
Doppler effect. As the waves are emitted from a different distance to the observer every time (since the source is moving) this creates an apparent change in wavelength to the observer, shorter as it’s moving towards you (high pitch) and longer as it moves away from you (lower pitch)
Basically the plane is making the same sound in every direction. If you think of the sound waves being emitted as pulses, the plane essentially closes the gap between sound pulses in front of it and distances itself from the pulses it makes behind it. The closer the sound pulses, the higher the pitch. The farther apart, the lower the pitch. If your relative position stays consistent, there won’t be a change in pitch. So when a plane passes overhead, you get the higher pitched sound as it gets closer and lower pitch immediately as it passes.
As others have said, that is the dopler effect, but it was not just airplanes.
The same thing happens with horns and sirens on trains and automobiles.
It even affects radio waves coming from fast moving objects. I don’t mean a car or truck with a CB radio. I mean, the affect is there, but cars and trucks moving on the highway can’t move fast enough for the dopler effect to be noticeable. But satellites and space vehicles in orbit _are_ moving fast enough for the dopler effect to be noticeable.
The satellite must be moving, relative to the observer. If it is, when the satellite is moving towards the observer the dopler effect will cause the satellite transmitter frequency to seem higher than it really is. When the satellite is moving away from the observer the satellite transmitter frequency will seem lower than it really is.
If I throw a ball at you while I’m running at you, the total speed of the ball is how hard I threw it + however fast I was running. If I throw a ball at you while I’m running away from you, the speed of the ball is how hard I threw it – however fast I’m running. As I run towards you, me throwing a ball makes the ball faster, and if I wait to throw the ball until after I run past you, it will make the ball slower.
Sound wants to do something similar, but there are two key differences. The sound is a wave, not a single object like a ball. Secondly, sound can’t speed up or slow down in air. (It can if the air gets thinner or colder, but for practical demonstrations we will say the air between you and the thing making the sound is all roughly the same). Since the sound can’t speed up, the sound wave itself actually squishes together and there is less distance between the peaks and valleys of the wave. This is known as an increase in frequency, which shifts the pitch higher. Inversely, when the thing making the sound is moving away, the sound wave is being stretched out and the pitch is lowered. This phenomenon is known as the Doppler effect.
Someone said “doppler effect”, and I think it requires just a little more ELI5 for it to make sense.
That pitch is a frequency. Frequency is how *frequent* something is, or how often it happens.
Now, whatever makes that engine noise is at a pretty high frequency. You’re hearing something loud happening thousands of times every minute. That is, it’s happening a lot. But also, as the plane flies past overhead it goes from moving *toward* you to moving *away*. That’s very important.
So you don’t hear each of those thousands of sounds a minute at the exact time they happen. The sound has to travel from the plane to your ears. If the plane stayed the same distance from you then it’d be simple. But it doesn’t. As it moves toward you, each sound happens a little closer to your ears than the last one did. So while they happened at the same frequency, because the distance changed so did the frequency you heard. *At your ears* the time between them is shorter, which means higher frequency.
And it’s the opposite after it passes overhead. Now as it moves away, each sound happens a little farther away than the last did. the frequency of that sound *at your ears* is lower because each sound has to travel even further, so the time between them *at your ears* is longer. Thus lower frequency.
Also cars and trains. And missiles, too, thought I’m fortunate enough to ever have one fired in my direction.
If you and I catch/throw a ball at the same speed and distance, its the same thing repeating; the same frequency. If I walk closer and closer as we throw the ball at the same speed, we increase how frequently (the frequency) we catch/throw it. If I’m moving away, the opposite.
Sound is just the frequency (how often) we sense the air waves move. If one of us was moving faster than we threw the ball, the game of catch wouldn’t even work, which would be like something moving faster than the speed of sound.
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