Physics 101: Lecture 33 Sound

Physics 101: Lecture 33Sound • Today’s lecture will cover Textbook Sections16.5 - 16.10

From Last Time: Speed of a wave is a constant, i.e. it only depends on the medium through which it travels : v =  / T =  f Speed of a transverse wave traveling along a string:

Sound • Sound is a longitudinal wave, created by a vibrating object (e.g. , vocal cords, guitar string). • Sound needs a medium, it does not exist in vacuum. • Sound waves in air are traveling disturbances of varying air pressure: slightly less (rarefication) and higher than normal (condensation). • Each cycle of a sound wave consists of one rarefication and condensation. The frequency is the number of cycles per second that passes by a given location. • Human ear: frequency = 20 – 20,000 Hz • Loudness: amplitude of the pressure wave (air pressure over distance). The larger the amplitude the larger the sound.

Speed of Sound • Speed of a sound wave depends on the medium: Ideal gas (adiabatic compression and expansion of gas): v = (g k T/m)1/2 g=cp/cv Liquid (Bad=Bulk modulus, volume change under adiabatic conditions): v = (Bad/r)1/2 Solid Bars (Y=Young’s modulus): v = (Y/r)1/2

f0 f1 correct Concept Question A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the frequency of the sound wave inside and outside the balloon 1. f1 < f0 2. f1 = f0 3. f1 > f0

correct V1=965m/s V0=343m/s Concept Question A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the speed of the sound wave inside and outside the balloon 1. v1 < v0 2. v1 = v0 3. v1 > v0

l1 l0 correct  = v / f Concept Question A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the wavelength of the sound wave inside and outside the balloon 1. l1 < l0 2. l1 = l0 3. l1 > l0

Sound Intensity • Amount of energy transported by a sound wave per second is called power of the wave: Power = energy/time SI unit: [J/s= W] Sound intensity measures the sound power that passes Perpendicular trough a surface A per surface area: I = P/A SI unit: [W/m2] Decibels (dB) are used to compare two sound intensities: b = 10 dB Log (I/I0) I0= threshold of hearing

correct Concept Question Suppose you are standing a distance D away from a speaker that is radiating sound in a spherically uniform way. You walk away from the speaker until the loudness of the sound is reduced by a factor of two. About how far from the speaker are you now (neglecting any reflections from the ground)? 1. 10D 2. 4D 3. 3D 4. 2D

Speaker radiating power P I1 = P/(4pD12) D1 I2 = P/(4pD22) D2 Changing I by 10 will changeloudness by 2 (10 dB) We want I1/I2 = 10

Doppler Effect • The wavelength of a sound wave of a source moving with speed vs is shortened in front of the source by l’=l-vs T and lengthened behind the source by l’=l+vs T Thus, the frequency of the sound perceived by a stationary observer in front of the source is f0 = v/l’ = fs / (1-vs/v) => f0 > fs and behind the source f0 = v/l’ = fs / (1+vs/v) => f0 < fs v: speed of sound vs: speed of source fs: frequency emitted by the source f0: frequency heard by the observer

f f’ v vs vo correct Concept Question A: You are driving along the highway at 65 mph, and behind you a police car, also traveling at 65 mph, has its siren turned on. B: You and the police car have both pulled over to the side of the road, but the siren is still turned on. In which case does the frequency of the siren seem higher to you? 1. Case A 2. Case B 3. same

Physics 101: Lecture 33 Sound