Interference of Sound Waves: Constructive and Destructive Effects - Prof. Jie Zou, Study notes of Physics

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Dr. Jie Zou

PHY 1071^

Chapter 4 (Hall)

Sound Propagation (Cont.)

Dr. Jie Zou

PHY 1071^

Outline^ „^ Interference^ „^

In phase „ Out of phase „ Constructive interference „ Destructive interference

Dr. Jie Zou

PHY 1071^

Interference of two soundwaves

„^ A pair of speakers (sources) puttingout identical sounds.^ „^ Two sound waves are

in phase

at A, C, or

D: Every time a crest from one speakerreaches A, so does a crest from the otherspeaker, and the two signals are said to bein phase at A. „ Two sound waves are

out of phase

at B

or E: Every time a crest from one speakerreaches B, so does a trough from the otherspeaker. One wave’s compression cancelsthe other’s rarefaction, and the net resultis no disturbance at all, and the twosignals are said to be out of phase at B. „ One would avoid ever driving two speakersin the same room with the same signal.

Dr. Jie Zou

PHY 1071^

5

Conditions for constructiveand destructive interference

„^ Constructive interference

: The

enhance intensity at A (or C or D) iscalled constructive interference.^ „^ Condition for constructiveinterference

: When there are one or more full wavelengths difference in distancebetween the two signals, i.e. L

-L^ = n 12

λ, n

= 0, ±1, ±2…. „ Destructive interference

: The

reduced (sometimes zero) intensity atB (or E) is called destructiveinterference.^ „^ Condition for destructive interference

When L

• L^ = (n + ½) 1 2

λ, n = 0, ±1, ±2,...

Dr. Jie Zou

PHY 1071^

Homework^ „^ Ch. 4 (Hall), P. 69,

Exercises

(#9 is a little more difficult).