Chapter 15 Sound. 15.1 Properties of Sound Sound - longitudinal wave.Sound High pres - compressions Low pres – rarefactions Air molecules collide transmitting.

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