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Chapter 15. Sound. 15.1 Properties of Sound. Sound - longitudinal wave. High pres - compressions Low pres – rarefactions Air molecules collide transmitting changes in air pressure. Frequency - number of oscillations / sec Vel of sound in air depends on temperature. - PowerPoint PPT Presentation
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Chapter 15
Sound
15.1 Properties of Sound
• Sound - longitudinal wave.
• High pres - compressions
• Low pres – rarefactions
• Air molecules collide transmitting changes in air pressure.
• Frequency - number of oscillations / sec
• Vel of sound in air depends on temperature.
• At 20º C, sound travels at 343 m/s.
• Sound can be reflected --echoes.
• Ex: sonar & echolocation
• Can be refracted.
• Can be diffracted & interfere to produce dead spots.
• Speed of sound is higher in liquids & solids than in gases.
• Travels 4x faster in water than in air.
• Travels about 11x as fast in steel
• Elasticity affects speed; not density.
• Ex. Prob 351 Prac Prob 352
Pitch & Loudness• Marin Mersenne & Galileo
connected pitch to frequency. • Pitch - frequency of wave. • Loudness depends on
amplitude of pressure changes.
• Sound level - measured in decibels (dB).
• 40 dB is 10 times larger than 20 dB.
• Loudness depends on sensitivity of our ears to sounds in different frequency ranges.
The Doppler Shift• Moves toward you, frequency is
higher.
• Moves away, frequency is lower.
• Doppler shift occurs in EM waves & mechanical.
• Ex: radar detectors, ultrasound,
• Astronomers use it to measure speed of galaxies
15.2 The Physics of Music
• Pythagoras used ratios to define his musical scale.
• If ratio is in whole numbers, pleasing sounds result.
• Octave - ratio of frequencies is 2:1
• Hermann Helmholtz & Lord Rayleigh studied how human voice as well as instruments produce sounds, & how human ear detects sounds.
• Sound - produced by vibrating objects.
• Human voice - vibrations of vocal cords.
• Brass instruments - lip of performer vibrates Fig 15-6 (a) 357
• Reed instruments - reed vibrates Fig 15-6 (b) 357
• Flute, organ, whistle - air is blown across an opening
• Stringed instruments - wire or string vibrates
• Electric guitars – use electronic devices to amplify vibrations
Resonance in Air Columns• Resonance increases amplitude of
vibration by repeatedly applying a small external force at natural frequency.
• Closed-pipe resonator - resonating tube with one end closed that resonates when its lengths are at an odd number of quarter wavelengths.
• Example: String instruments
• Standing wave has pressure nodes & antinodes. Fig 15-9 a 359
• Open-pipe resonator - both ends are open Fig 15-9 b 359
• Examples: saxophone and flute
• Standing wave in a pipe can be represented by sine wave. Fig 15-10 359
• Ex Prob 361 Prac Prob 363
Detection of Sound• Sound detectors convert sound energy
into other forms of energy. • The ear is sensitive to sounds with
frequencies between 1000 & 5000 Hz. Fig 15-14 363
• Three parts: outer - collects sound; penna middle - 3 tiny bones; stirrup, anvil, hammer inner - watery liquid; cochlea hair cells vibrate; sends to brain
• Loud sounds can permanently damage ears.
Sound Quality• Timbre - tone color • Beat - oscillation of wave amplitude;
frequency of beat is difference in frequencies of two waves.
• Dissonance - unpleasant sound • Consonance - pleasant sound • Pythagoras - consonance occurs
when wave frequencies have ratios that are small whole numbers.
• Fundamental - lowest resonant frequency
• Harmonics - wave of frequencies that are whole number multiples of fundamentals
• Noise consists of a large # of frequencies with no relationship.
• White noise - all frequencies present in equal amplitude; relaxing effect.
• Pythagoras used ratios to define his musical scale.
• If ratio is in whole numbers (1:2, 2:3 or 3:4), pleasing sounds result.
• Octave ratio of frequencies is 2:1
• Ex Prob 366 Prac Prob 367
Sources• http://www.physicsclassroom.com/mmedia/waves/lw.
html• http://science.howstuffworks.com/hearing1.htm• http://www.museumca.org/caves/onli_echo_ani.html• http://www.kettering.edu/~drussell/Demos/refract/refr
act.html• http://www.loa.espci.fr/~michael/en/acel/waveguide/w
aveguide.html• http://www.walter-fendt.de/ph11e/dopplereff.htm• Encarta• Physics: Principles and Problems, Glencoe.• http://www.walter-fendt.de/ph11e/beats.htm• http://maxwell.ucdavis.edu/~cole/phy9b/movies/
fundamental.mov
Sound Waves
Ear
Echo
Echolocation
Refraction
Diffraction
Red & Blue Shift
Pythagoras
Frequency