General Physics(ppt) Ch 17

General Physics(ppt) Ch 17 - 17 Waves II A sound wave is...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
17 Waves II s A sound wave is defined roughly as any longitudinal wave . 1 s In this chapter we shall focus on sound waves that travel through the air and that are audible ( 聽得見的 ) to people. Wavefronts ( 波 前 ) are surfaces over which the oscillations due to the sound wave have the same value. Rays are directed lines perpendicular to the 波峰 波谷 波峰 波谷 波峰 波谷 2 wavefronts that indicate the direction of travel of the wavefronts. Far from the source, we approximate the wavefronts as planes , and the waves are said to be planar (plane wave).
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
s The speed of any mechanical wave, transverse or longitudinal, depends on both an inertial property ( 有性質 ) of the medium and an elastic property of the medium. For transverse wave, 3 s If the medium is air and the wave is longitudinal, we can guess that the inertial property is the volume density ρ of air. What shall we put for the elastic property ? s That is the bulk modulus B , defined as here Δ V/V is the fractional change in volume produced by a change in pressure Δ p. 4 s Thus, the speed of sound in a medium with bulk modulus B and density ρ is Formal Derivation of Eq. 17-3 Self-study
Background image of page 2
sound wav piston oO 疏 密 疏 密 疏 密 5 sound wave s The air elements oscillate parallel to the x axis , we could write their displacements in the form s(x,t) . s s(x,t) can be described as the sinusoidal form A the wave moves the ai pressure a any position x 6 s As the wave moves, the air pressure at any position x varies sinusoidally. To describe this variation we write Δ p<0 in corresponds to an expansion of the air, and Δ p>0 to a compression. Here Δ p m is the pressure amplitude , which is the maximum increase or decrease in pressure due to the wave. Δ p m is normally << the pressure p present when there is no wave.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
7 s The pressure amplitude Δ p m is related to the displacement amplitude s m in Eq. 17-13 by 8 Derivation of Eqs. 17-14 and 17-15 Self-study 90 o out of phase.
Background image of page 4
Pressure variation V V B p Δ - = Δ x A V Δ = s A V Δ = Δ s B s B p - = Δ - = Δ 9 x Δ s Δ x x Δ ( ) [ ] ( ) t kx ks t kx s x x s m m ω - - = - = sin cos [ ] ( ) t kx Bks p m - = Δ sin ( ) m m m s v Bks p ρω = = Δ amplitude k v v B / , 2 ρ = = s Two point sources S 1 and S 2 emit sound waves that are in phase and of identical wavelength λ . P 10 s If the waves traveled along paths with identical lengths to reach point P, they would be in phase there, which means that they would undergo fully constructive interference there. s However, if path L 2 traveled by the wave from S 2 is longer than path L 1 traveled by the wave from S 1 . The difference in path lengths means that the waves may not be in phase at point P. In other words, their phase difference at P depends on their path length difference Δ L=|L 2 -L 1 |.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
s Because a phase difference of 2 π rad corresponds to one wavelength. Thus, we can write the proportion
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 12/27/2011 for the course LSCI 103 taught by Professor K.y.liao during the Fall '11 term at National Cheng Kung University.

Page1 / 24

General Physics(ppt) Ch 17 - 17 Waves II A sound wave is...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online