2 15.1 Properties of Sound Sound - longitudinal wave. High pres - compressionsLow pres – rarefactionsAir molecules collide transmitting changes in air pressure.
3 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.
4 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.
5 Travels 4x faster in water than in air. Travels about 11x as fast in steelElasticity affects speed; not density.Ex. Prob 351 Prac Prob 352
6 Pitch & LoudnessMarin Mersenne & Galileo connected pitch to frequency.Pitch - frequency of wave.Loudness depends on amplitude of pressure changes.
7 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.
8 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
9 15.2 The Physics of MusicPythagoras used ratios to define his musical scale.If ratio is in whole numbers, pleasing sounds result.Octave - ratio of frequencies is 2:1
10 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.
11 Brass instruments - lip of performer vibrates Fig 15-6 (a) 357 Reed instruments - reed vibrates Fig 15-6 (b) 357Flute, organ, whistle - air is blown across an openingStringed instruments - wire or string vibratesElectric guitars – use electronic devices to amplify vibrations
12 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
13 Standing wave has pressure nodes & antinodes. Fig 15-9 a 359 Open-pipe resonator - both ends are open Fig 15-9 b 359Examples: saxophone and fluteStanding wave in a pipe can be represented by sine wave. FigEx Prob 361 Prac Prob 363
14 Detection of SoundSound detectors convert sound energy into other forms of energy.The ear is sensitive to sounds with frequencies between 1000 & 5000 Hz. FigThree parts: outer - collects sound; penna middle - 3 tiny bones; stirrup, anvil, hammer inner - watery liquid; cochlea hair cells vibrate; sends to brainLoud sounds can permanently damage ears.
15 Sound Quality Timbre - tone color Beat - oscillation of wave amplitude; frequency of beat is difference in frequencies of two waves.Dissonance - unpleasant soundConsonance - pleasant soundPythagoras - consonance occurs when wave frequencies have ratios that are small whole numbers.
16 Fundamental - lowest resonant frequency Harmonics - wave of frequencies that are whole number multiples of fundamentalsNoise consists of a large # of frequencies with no relationship.White noise - all frequencies present in equal amplitude; relaxing effect.
17 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 2:1Ex Prob 366 Prac Prob 367
18 Sources http://www.physicsclassroom.com/mmedia/waves/lw.html EncartaPhysics: Principles and Problems, Glencoe.