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The Doppler effect
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What happens to the pitch of a sound if the wavelength of its waves is increased?
Sound is compressions and rarefactions of a medium. Think of it as a vibration spreading out. It is spread through the air like waves. We can't see them, but we can hear them. Did you hear that? The sound changed when the ambulance drove by.
The pitch became lower after it passed. Of course, the sound is weaker when the ambulance is farther away. No. Not weaker, lower. I mean more bass and it had a higher pitch when it was approaching. Weird.
Wow, here's another one. Yes, they heard it right. We perceive sound differently when the distance between the sound source and the listener changes. Here's an ambulance. The siren sends out sound waves spreading in all directions. The sound waves move through the air air at about 340 meters per second but the ambulance is moving too. The sound waves in front of the ambulance come closer to each other.
Each wave is a bit shorter. A smaller wavelength means a higher frequency and a higher frequency means a higher pitch. When the ambulance has passed the listener the sound waves going back become farther from each other. The wavelength increases and the frequency is reduced. We get a lower pitched tone. The guy who figured out how the waves from moving sources behave was Christian Doppler.
Doppler was an Austrian mathematician and physicist who lived in the 19th century. This phenomenon was named the Doppler effect after him. Hey, how does it sound to the driver inside the ambulance? The driver doesn't know where we are standing listening and the sound changes right where we are. Exactly. You only notice the Doppler effect when the sound source is moving relative to you. It can be the sound source that moves or yourself.
If the distance increases, the pitch drops; and if the distance decreases, the pitch rises. But the driver of the ambulance is moving at the same speed and direction as the siren. The distance doesn't change, so the driver doesn't experience the Doppler effect. That was the Doppler effect's youngest brother, the sonic boom. It came from an airplane that was moving faster than sound - a super sonic plane. When a plane travels faster than sound, the sound waves that were supposed to spread in front of it end up behind it.
If you are standing here listening, you won't hear anything at all when the plane is right above your head, but what happens now? Now you hear sound from several sound waves at the same time. That makes for a lot of noise - a sonic boom. Just like the wake forming in the water behind a boat, a shock wave is built up behind a supersonic plane. It consists of an extreme compression and then an extreme rarefaction of the air. The sonic boom is rolling across the ground trailing behind the plane.
If the plane is flying at a low altitude, the sonic boom can be so powerful that it breaks windows and even buildings. This effect when several waves pass through the same place at the same time and influence each other is called interference. When the waves are synchronized so that a crest meets a crest and a trough meets a trough, the waves amplify each other. We call this a constructive interference. A sonic boom is a constructive interference of the shock waves that are created when something is traveling faster than the speed of sound. Sound is related to motion. If a sound source is moving relative to you, the sound will be different from one that comes from a stationary source. Ah!