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Chapter 15 - Sound

Chapter 15 - Sound. Sound wave is a longitudinal wave. Speed of Sound. The speed of sound depends on the medium and temperature Sound in air at 20°C (72°F) = 343 m/s Sound in air at 0°C (32°F) = 332 m/s Sound travels faster through solids and liquids than air. Why?

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Chapter 15 - Sound

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  1. Chapter 15 - Sound Sound wave is a longitudinal wave

  2. Speed of Sound • The speed of sound depends on the medium and temperature • Sound in air at 20°C (72°F) = 343 m/s • Sound in air at 0°C (32°F) = 332 m/s • Sound travels faster through solids and liquids than air. • Why? • There is more matter, which is why sound travels faster at higher temperature – more humidity more matter. Sound cannot travel through a vacuum. (FYI – p.351)

  3. Loudness • Loudness of sound depends on amplitude. • Loudness is measured by decibels (dB) • Every 20 dB = 10x greater amplitude

  4. Resonance • When the tube is at the right length then the reflected wave from the water will meet another vibration coming down from the tuning fork and they will interfere constructively. • Sound will amplify = Resonance

  5. Resonance in closed pipes • Resonance frequencies in a closed pipe must have a node and antinode • The shortest column of air that can have a node and antinode is ¼ λ long. • Additional resonance lengths are found at ½ λ intervals. (λ/4, 3λ/4, 5λ/4, 7λ/4, etc.) • If the length is any of these lengths then you will get resonance (constructive interference).

  6. Resonance in open pipes • Resonance frequencies in an open tube must have a node at both ends. • Shortest column of air = ½ λ and goes at ½ λ intervals (λ/2, λ, 3λ/2, 2λ, etc.) • If the length is any of these lengths then you will get resonance.

  7. Beats • Beats occur when 2 frequencies (very nearly identical) interfere and produce high and low sound levels. • The frequency of the beat = difference between the 2 frequencies

  8. DopplerShift • When the source is moving toward you the frequency increases thus pitch increases. When the source moves away the frequency decreases thus the pitch decreases. • The actual frequency does not change but when the source is moving toward you/or you toward the source the waves get crowded, λ decreases so frequency increases (in our ears). • When the source is moving away there is more space, λ increases so frequency decreases. • (fig. 15-5)

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