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/refract.html http://www.loa.espci.fr/~michael/en/acel/waveguide/waveguide.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