Chapter 11: Intensity & Beats | Lecture Note

University:
California State University, Northridge
Course:
PHYS 100A | General Physics I
Academic year:

2023
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1
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KOLEJ MATRIKULASI SELANGOR physics\academic\1PN 7 Sound Waves 11.1 Intensity & Beats 1. Intensity – is the energy transported per unit time (sound power, P) across a unit area 𝐸 𝑃 𝐼= = , 𝑈𝑛𝑖𝑡: 𝑊 𝑚−2 𝑡𝐴 𝐴 2. The factors influence the  the distance from the value of sound intensity source of sound 1 are I 2 𝑟  the amplitude of the sound I  A2 I  3. 𝐴2 𝑟2 Beats - periodic variation in amplitude of sound at a given point due to superposition of two sound waves having slightly different frequencies 11.3 Doppler Effect 7. Doppler Effect – the apparent change in the pitch or frequency of a sound when there is relative motion between the source & the observer 8. Wavefront – a line or surface, in the path of a wave motion, on which the disturbances at every point have the same phase 9. 3 cases: (i) Frequency heard with an observer in motion 𝑣  𝑣𝑜 𝑓=( ) 𝑓𝑠 𝑣 (+)ve: observer moves toward the source (–)ve: observer moves away from the source (ii) Frequency heard with source in motion 𝑣 𝑓=( )𝑓 𝑣 ∓ 𝑣𝑠 𝑠 (–)ve: source moves toward the stationary observer (+)ve: source move away from stationary observer (iii) Source and observer are in motion 𝑣  𝑣𝑜 𝑓0 = ( )𝑓 𝑣 ∓ 𝑣𝑠 𝑠 The upper signs (+ v0 and – vs): Motion of one toward the other The lower signs (– v0 and + vs): Motion of one away from the other 10. The variation of apparent frequency and sound intensity when a source moves towards, passes through and moves away from the stationary observer. 4. The equation of the beat frequency is given by 1 1 𝑓𝑏 = |𝑓1 − 𝑓2 | = | − | 𝑇1 𝑇2 11.2 Application of Stationary Waves 5. Wave speed on the string 𝑇 𝑣=√ ,  = 𝑚 𝑙 = A 6. 𝑣 = 𝑓, 𝑓 = 𝑣  String Fundamental (1st harmonic) 1st overtone (2nd harmonic) 2nd overtone (3rd harmonic) In general Open pipe Closed pipe