Producing sound waves Displacement 1.4 Sound Density Pressure Producing sound waves Speed of sound •Sound waves are longitudinal •Produced by compression and rarefaction of media (air) Energy and Intensity resulting in displacement in the direction of propagation. • The displacements result in oscillations in density and pressure. Spherical and Plane waves. Frequencies of sound wave Speed of sound Speed of sound in a fluid B infra-sonic Audible Sound v = ultra- sonic ρ ∆P B =− Bulk modulus 10 20,000 ∆V/V m Frequency (Hz) ρ= Density V Similarity to speed of a transverse wave on a string 30 0.015 Wavelength (m) in air elastic _property v = int ertial_property Speed of sound in air γP B v = v = ρ ρ γ is a constant that depends on the nature of the gas γ =7/5 for air Density is higher in water than in P - Pressure air. ρ -Density Why is the speed of sound higher in water than in air? Since P is proportional to the absolute temperature T by the ideal gas law. PV=nRT T v331= (m/s) 273 1 Energy and Intensity of sound waves o Find the speed of sound in air at 20 C. energy power P = time T area A v331= 273 273+ 20 v== 331 343m/ s 273 For calculations use v=340 m/s power P intensity I = = (units W/m2) area A Sound intensity level The ear is capable of distinguishing a wide range of sound intensities. The decibel is a measure of the sound intensity level ⎛⎞I What is the intensity β=10log⎜⎟ decibels of sound at a rock I ⎝⎠o concert? (W/m2) -12 2 Io = 10 W/m the threshold of hearing note- decibel is a logarithmic unit. covers a wide range of intensities. Spherical and plane waves Suppose you are standing near a loudspeaker that can A4r=π2 area of sphere is blasting away with 100 W of audio power. How far away from the speaker should you stand if you want to Power passes through larger areas. hear a sound level of 120 dB. ( assume that the sound is emitted uniformly in all directions.) P=I1A1 =I2A2 2 IAr211 ==2 IAr122 As sound spreads out uniformly from a point source the intensity decreases as 1/r2 2.