Sound Physics 123 01 14 19 Lecture IV 1 Sound Wave nature of sound Intensity of sound Standing sound waves String instruments Pipes Interference and beats Doppler effect 01 14 19 Lecture IV 2 Sound longitudinal wave in air 01 14 19 Lecture IV 3 Speed of sound Wave characteristics Wave length m Frequency f Hz pitch Wave velocity v f m s Wave speed property of material one to one correspondence of frequency and wave length in a given medium 01 14 19 Lecture IV v f 4 Intensity of sound Intensity of sound I 10 12 102 W m2 14 orders of magnitude Measure of loudness in Decibel in dB 10 log I I0 01 14 19 Lecture IV I0 5 Sensitivity of human ear Audible range really good speakers 20Hz 20 kHz 01 14 19 Lecture IV 6 Physics of a guitar Guitar strings sounding box resonator Strings force resonance in the sounding box Fundamental frequency Strings Tuning 01 14 19 Lecture IV 7 Physics of a guitar v Standing wave Fundamental frequency L 2 1 2L f1 v 1 f1 v 2L 01 14 19 Lecture IV string tension mass per unit length FT m l String theory Thicker string higher m l lower v lower frequency f Tuning Increase tension FT increase v increase frequency f Fingered string Decrease L decrease increase f 8 Wave velocity vs particle velocity f cyclic frequency k 2 wave vector D D0sin kx t Riding the wave kx t const kx t c x c k k t x0 vt Thus wave velocity v k f 2 f D D0sin kx t medium displacement at point x at time t Particle velocity vp dD dt D0cos kx t vmaxcos kx t vmax D0 01 14 19 Lecture IV 9 Physics of an organ Open and closed pipes resonators Boundary conditions imagine yourself in a crowded room Open end next to an open door Displacement freedom to move x max Pressure Atmospheric P P 0 Closed end pushed against a wall Displacement x 0 Pressure variation max P max 01 14 19 Lecture IV 10 Organ pipe f n nf1 x max P 0 01 14 19 Lecture IV 11 only odd harmonics f 2 n 1 2n 1 f1 Organ pipe x 0 P max 01 14 19 Lecture IV 12 Interference Two waves of the same frequency C Constructive interference A A 2A I 4I0 x 0 n dsin n D Destructive interference A A 0 I 0 x 2 n dsin n 1 2 01 14 19 Lecture IV 13 Beats Two waves of the similar frequencies f1 and f2 01 14 19 Lecture IV 14 Doppler effect sound source moving with velocity v s Distance between crests vsT l vs v 1vs v Frequency f f 1 vs v Moving towards you vs positive divide by a number 1 f f higher pitch Moving away from you vs negative divide by a number 1 f f lower pitch 01 14 19 Lecture IV 15 Demo data Open closed end pipe f 512 Hz v 343m s maybe less cold v f 67m 4l1 l1 4 0 17m l3 3 4 0 51m 01 14 19 Lecture IV 16 Intensity of waves Energy of oscillation E is proportional to amplitude squared A2 E A 2 Intensity I W m2 1 A r energy time power I area area Intensity I is proportional to amplitude squared A2 inversely proportional to r2 I A 01 14 19 2 Lecture IV 1 I 2 r 17
View Full Document
Unlocking...