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

5. 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

6. 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).

7. 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.

9. 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

10. Doppler Shift 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)