Sound

1. Sound AP Physics B Chapter 12 Notes

2. Sound Characteristics • Sound is carried by longitudinal waves • The speed of sound varies with media • In air v = 331 + 0.6T where T is temperature in ˚C

3. Sound Characteristics • Loudness depends on intensity/amplitude • Pitch depends on frequency • Humans can hear 20Hz to 20,000Hz (audible range—old teachers are sometimes limited to 10kHz) • Ultrasonic is above 20kHz (ultrasound)--Infrasonic is below 20Hz

4. Sound Characteristics • Since sound is a longitudinal or pressure wave, it can be analyzed in terms of displacement of molecules and pressure—be careful because nodes and antinodes will be flipped!

5. Sources of Sound—Strings • The source of any sound is a vibrating object • The frequency of vibration dictates pitch

6. Sources of Sound—Strings • Recall: fn =nv/2L=nf1 and • Frequency is changed by shortening length or changing density and tension of strings

7. Vibrating String Example Problem Ex. 12-8 p. 331 A 0.32 m long violin string is tuned to play A above middle C at 440 Hz.(a) What is the wavelength of the fundamental string vibration (b) What are the frequency and wavelength of the sound wave? (c) Why is there a difference?

8. Sources of Sound—Wind Instruments • Wind instruments create sound through standing waves in a column of air

9. Sources of Sound—Wind Instruments • A tube open at both ends has displacement antinodes and pressure nodes at the ends (both ends open to the atmosphere). Same harmonics as for strings—integral.

10. Sources of Sound—Wind Instruments • A tube closed at one end has displacement nodes (and pressure antinode) at the closed end (air is not free to move)—organ pipe. Only odd harmonics

11. Wind Instruments—Example Problems Ex. 12-10 p. 334 A flute plays middle C (262Hz) as the fundamental f when all holes are covered. Approximately how long should the distance be from the mouthpiece to the end of the flute? Ex. 12-11 p. 333 What will the fundamental frequency and first three overtones for a 26cm long organ pipe at 20˚C if it is (a) open (b) closed?

12. Interference and Beats • Sound waves interfere the same way other waves do—constructively (at C) and destructively (at D) If BE equals ½, 1, 3/2, … λ, interference occurs

13. Interference and Beats • Waves can also interfere in time, causing beats—a phenomenon that causes a slow new wave who has fb = ∣f1 –f2∣-- f1and f2 must be close together http://hyperphysics.phy-astr.gsu.edu/hbase/sound/beat.html#c3

14. Doppler Effect • The Doppler effect is the perceived change in pitch of a sound from an object moving relative to the observer Higher Frequency Lower Frequency

15. Doppler Effect Consider the relative position of two waves emitted from a stationary source and a moving source—if we can figure change in λ, we can figure change in f

16. Doppler Effect • Using this difference in wavelength to find difference in f yields: Source moving toward observer Source moving away from observer

17. Doppler Effect • For an observer moving toward the source, λdoes not change but v does so: Observer moving toward source Observer moving away from source http://www.lon-capa.org/~mmp/applist/doppler/d.htm

18. Supersonic Speed—Sonic Booms • When the source moves faster than the speed of sound it “out runs” the sound wave http://www.youtube.com/watch?v=20UWwnfP5pA&feature=related