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PHYS_2014_Lecture_27

# PHYS_2014_Lecture_27 - Lecture 27 Sound Waves Wave...

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Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 1 Lecture 27: Sound Waves Wave Intensity 2 P P A 4 I r π = = Wave intensity is wave power over area.

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Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 2 2 1 1 2 2 2 P 4 P P 4 I r I r π π = = 4 π 2 1 4 r π 2 2 P r 2 1 2 2 2 1 I r I r = 1 2 2 2 1 2 and 4 4 P P I I r r π π = = For spherical waves, wave intensity decreases as 1/ r 2 :
Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 3 Speed of Sound Empirical expression for the speed of sound in air: P S v γ ρ = where P is the pressure (force per unit area), ρ is the density (mass per unit volume), and γ is 7/5 for diatomic molecules (e.g. N 2 and O 2 in air), 5/3 for monatomic gases (e.g. He, Ne, Ar), and 4/3 for triatomic gases (e.g. CO 2 ). Speed of sound at sea level and 20 ° C is ~344 m/s m 331.4 0.6T s S c v = + where T c is the temperature in degrees Celsius.

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Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 4 Decibel Scale Sound intensity level β in dB 0 10log dB I I β = where I is the intensity in W/m 2 and I 0 = 10 -12 W/m 2 . Note that this is a logarithmic scale. An increase in sound intensity of 10 dB corresponds to a 10 times increase in intensity in units of W/m 2 .
Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 5 Sound and Human Hearing threshold of pain threshold of hearing 1 10 -2 10 -4 10 -6 10 -8 10 -10 10 -12 Intensity (W/m 2 ) Humans can hear sound from 20 Hz to 20 kHz (at least when they are still young).

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Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 6 Frequency Range heard by our Furry Friends
Fall 2010 Oklahoma State University PHYS2014: Benton Lecture 27, Slide 7 At sea level at 20 ° C, air pressure is 1.03 × 10 5 N/m 2 and there are on average 2.51 × 10 25 air molecules per cubic meter. a) Find the speed of sound in air and in helium (He) with the same pressure and number of molecules per unit volume. b) For each gas, find the frequency of a sound wave having a wavelength of λ = 64 cm.

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PHYS_2014_Lecture_27 - Lecture 27 Sound Waves Wave...

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