Unit 17: Sound
Sound as a pressure wave, decibels, beats between close frequencies, and the Doppler shift that bends a passing siren's pitch.
6 topics · 20 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.
Sound as a Pressure Wave
- How it worksHow does sound move through air as alternating regions of compressed and rarefied pressure?
- Common mistakeWhy isn't sound a transverse wave the way a wave on a string is?
- Going furtherWhat happens to sound's speed as it moves from air into a denser medium like water?
Your turn1 of 2
A student assumes sound waves must be transverse, since a wave on a guitar string is transverse. What is the actual nature of a sound wave in air?The Speed of Sound
- How it worksHow do the stiffness and density of a medium together set how fast sound travels through it?
- Worked exampleFind the time for thunder to arrive if lightning strikes 1 km away and sound travels at 343 m/s.
- Worked exampleFind the speed of sound in air at 20 degrees Celsius, using v = 331 + 0.6T in meters per second.
- Going furtherWhat happens to the speed of sound in air as the air's temperature rises?
Your turn1 of 2
Lightning strikes 3.63 km away, and sound travels at 343 m/s. How long does it take for the thunder to arrive, in seconds?Sound Intensity and Decibels
- How it worksHow does the logarithmic decibel scale compress an enormous range of sound intensities into manageable numbers?
- Worked exampleFind the sound intensity level in decibels of a sound with intensity 1e-6 W/m2, using a threshold intensity of 1e-12 W/m2.
- Common mistakeWhy doesn't doubling a sound's decibel level mean doubling how loud it actually sounds to your ears?
Your turn1 of 2
What is the sound intensity level, in decibels, of a sound with intensity 10−8 W/m2, using a threshold intensity of 1e-12 W/m2?Standing Waves in Air Columns
- How it worksHow do open and closed ends of a pipe force different patterns of nodes and antinodes onto a standing sound wave?
- Worked exampleFind the fundamental frequency of a 0.5 m pipe open at both ends, taking the speed of sound as 343 m/s.
- Common mistakeWhy doesn't a pipe closed at one end support the same set of harmonics as one open at both ends?
Your turn1 of 2
A pipe of length L is open at both ends, with the speed of sound equal to v. Write an expression, in terms of v and L, for its fundamental frequency.Beats
- How it worksHow does the superposition of two close frequencies create a beat frequency equal to their difference?
- Worked exampleFind the beat frequency produced by two tuning forks at 440 Hz and 444 Hz.
- Common mistakeWhy doesn't the beat frequency you hear equal the average of the two original frequencies?
Your turn1 of 2
Two tuning forks vibrate at 428 Hz and 474 Hz. What beat frequency do you hear, in hertz?The Doppler Effect
- How it worksHow does a moving sound source compress wavefronts ahead of it and stretch them out behind?
- Worked exampleFind the frequency heard by a stationary observer as a 500 Hz siren approaches at 30 m/s, with sound speed 343 m/s.
- Common mistakeWhy doesn't the Doppler effect mean the source itself is actually changing the frequency it emits?
- Ties togetherHow does the Doppler shift for sound waves reappear in the redshift astronomers use to measure a galaxy's motion?
Your turn1 of 2
A siren emits sound at frequency f and approaches a stationary observer at speed vs, with sound speed 343 m/s. Write an expression, in terms of f and vs, for the frequency the observer hears.