Eleventh Grade · Science · week 26

Sound Waves and the Doppler Effect

Describe sound as a longitudinal wave, relate its speed to distance and time, and apply the Doppler effect to a moving source.

Sound is a longitudinal (compression) wave that travels through a physical medium -- air, water, or solids -- as alternating regions of higher and lower pressure; unlike light, it cannot travel through a vacuum at all, since there is no matter there to compress. The speed of sound in room-temperature air is about 343 m/s, far slower than light, which is why you see a lightning flash well before you hear its thunder. Pitch corresponds to frequency (higher frequency sounds higher-pitched) while loudness corresponds to amplitude (a bigger pressure swing sounds louder) -- two genuinely separate properties that are easy to blur together in casual speech.

The Doppler effect describes how a moving sound source shifts the pitch a listener hears: sound waves bunch up in front of an approaching source (shortening the effective wavelength, raising the observed frequency and pitch) and stretch out behind a receding source (lowering pitch). For a moving source and a stationary listener, f_observed = f_source * v / (v -+ v_source): use MINUS in the denominator for an approaching source (which raises the observed frequency) and PLUS for a receding source (which lowers it), where v is the speed of sound.

📋 For the teacher
Teach it (10–15 min):
  • The next time a real thunderstorm passes (safely, from indoors), count the seconds between the flash and the thunder together and compute the storm's distance using this week's formula -- a genuinely useful real-world skill from a single simple calculation.
  • Recall, or watch a video of, a car, train, or ambulance passing by and consciously listen for the pitch DROP right as it passes -- connect that lived experience directly to the two Doppler cases (approaching versus receding).
  • Make pitch and loudness feel like two independent dials: hum a note softly and then loudly (same pitch, different loudness), then hum a low note and a high note at the same loudness (same loudness, different pitch).
  • If any musical instrument is available, compare a shorter string or tube to a longer one and connect the resulting pitch difference back to the wavelength relationship from last week.
Talk about it:
  • Why does an approaching sound source cause a listener to hear a higher pitch?
  • What is the difference between pitch and loudness, and what wave property does each correspond to?
  • Why can you see lightning before you hear the thunder from the very same strike?

Mastery looks like: They relate sound's speed, distance, and travel time, and apply the Doppler effect formula to find the observed frequency of an approaching or receding source.

Common stumbles: Confusing pitch with loudness; using the wrong sign (plus instead of minus, or vice versa) in the Doppler formula for an approaching versus a receding source.

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