Unit 9: Sound, the Doppler Effect, and Resonance
Sound as a pressure wave squeezing through the air, and the two tricks — Doppler shift and resonance — that make it change pitch or suddenly get loud.
7 topics · 22 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
How Sound Travels
- How it worksHow do vibrating air molecules bunch into compressions and rarefactions that carry sound forward?
- Common mistakeWhy doesn't sound need a solid object to travel through, only some kind of matter?
- Going furtherWhat happens to the speed of sound when it travels through water instead of air?
Your turn1 of 2
A movie might show a dramatic explosion in outer space complete with a loud booming sound effect, yet in real space, an astronaut floating right next to that same explosion would hear nothing at all. Why can't an astronaut hear an explosion happening right next to them out in space?Pitch, Loudness, and the Decibel Scale
- How it worksHow does a sound wave's frequency determine its pitch while its amplitude determines its loudness?
- Worked exampleFind how the sound intensity changes when the decibel level increases from 60 dB to 70 dB.
- Common mistakeWhy isn't the decibel scale just a simple straight-line measure of loudness?
Your turn1 of 2
Find by what factor sound intensity changes when the decibel level increases by 43 dB.The Doppler Effect
- How it worksHow does a moving sound source squeeze its wavefronts together in front and stretch them out behind?
- Worked exampleFind whether the frequency heard is higher or lower for a listener as a train horn at 400 Hz approaches at 30 m/s.
- Common mistakeWhy doesn't the sound's actual pitch change at its source during a Doppler shift?
- Going furtherWhat happens to the pitch you hear from a moving source the moment it passes you and starts moving away?
Your turn1 of 2
Find the frequency heard by a stationary listener as a source emitting 1423 Hz approaches at 39 m/s, with sound traveling at 340 m/s.Resonance
- How it worksHow does pushing an object at its own natural frequency build up bigger and bigger vibrations?
- Common mistakeWhy doesn't pushing an object at just any frequency make it vibrate as wildly as pushing at its resonant frequency?
- Going furtherWhat happens to a wine glass when a singer's voice hits the glass's exact natural frequency?
Your turn1 of 2
Soldiers marching in perfectly synchronized step across a bridge once caused that bridge to violently sway and eventually collapse, even though each individual soldier's footstep exerted only a small, ordinary force on the structure. Why did soldiers marching in step once cause a bridge to collapse?Resonance in Musical Instruments
- How it worksHow does the length of an air column inside a tube decide which frequencies resonate inside it?
- Worked exampleFind the fundamental frequency of a 0.5 meter tube open at both ends with sound traveling at 340 m/s.
- Going furtherWhat happens to a wind instrument's pitch when a player shortens the length of its air column?
Your turn1 of 2
Find the fundamental frequency of a 3.51 meter tube, open at both ends, with sound traveling through it at 340 m/s.Beats
- How it worksHow do two sound waves of slightly different frequency drift in and out of phase to create a rhythmic beat?
- Worked exampleFind the beat frequency produced by two tuning forks vibrating at 440 Hz and 444 Hz.
- Common mistakeWhy isn't the beat frequency you hear the same as either of the two original sound frequencies?
Your turn1 of 2
Find the beat frequency produced by two tuning forks vibrating at 618 Hz and 596 Hz.The Doppler Effect for Light
- How it worksHow does a light source moving away stretch its wavelength toward the red end of the spectrum?
- Common mistakeWhy doesn't a redshifted galaxy mean its light has literally turned red to the eye?
- Going furtherWhat happens to the light from a star that is moving toward Earth instead of away from it?
Your turn1 of 2
Astronomers examine the specific colors of light coming from a distant galaxy and conclude that galaxy is racing away from Earth at high speed, without ever having sent a probe there or measured its motion directly in any other way. Why do astronomers say a distant galaxy is racing away from us just from the color of its light?