PHYS16 – Lecture 36 & 37

Sound December 5 & 7, 2010

Outline for Sound

• Sound– Longitudinal Pressure Waves– Sound Velocity and Reflection– Resonance and Standing waves– Intensity and Sound Level– Interference and Beats– Doppler

Resonance and Standing Waves

Guitar Demonstration

Example Question

• A guitar string with a greater mass density has a/an:A) Greater velocityB) Higher frequencyC) Smaller wavelengthD) Unchanged wavelength

Example Question

• So what does the hole do?

Standing Waves with Sound

• Need two “ends” where sound reflects• Reflections constructively and destructively

interfere to setup up standing waves

closedor open both 1,2,3...n ,2

nL

closed oneopen one 1,2,3...n ,12

4

nL

Ruben’s Tube Demo

http://en.wikipedia.org/wiki/Rubens'_tube

Standing Waves with Sound

• Helmholtz Resonator

AVL

lfundamenta 2

A

As A increases, Wavelength decreases and Frequency increases!

http://www.diracdelta.co.uk/science/source/h/e/helmholtz%20resonator/image001.gif

Bottle Demo

http://www.physicsclassroom.com/class/sound/u11l4a3.gif

Intensity and Sound Level

Wave Intensity

• Intensity is the power per unit area in a wave

22

21 , AvA

tEP

API

http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/imgaco/isc2.gif

Sound Level

• Sound Level (β) - a relative intensity in decibels (dB)– 0 dB = smallest sound that can be heard– 50 dB = background noise– 70 dB = traffic– 110 dB = rock concert

2120

0

W/m10 ),log(10 III

Example Question

• If a Rock Concert is 110 dB, what is the intensity level in W/m2?

A) 0.1 W/m2

B) 10 W/m2

C) 1011 W /m2

D) 1098 W/m2

Example Question

• A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop?

A) 0.1 dBB) 1 dBC) 10 dBD) 100 dB

Interference and Beats

Sound Interference

• Sound waves can interfere destructively or constructively

edestructiv 2...1,0,n ,)21(veconstructi 2...1,0,n ,

nrnr

Dead Zones

• Places of destructive interference are called Dead Zones

• For Sound we usually don’t get dead zones because sounds are composed of many different frequencies

• How does having many frequencies in a sound eliminate dead zones?

Beats

• Beats occur when two sine waves (two notes) with very similar frequency interfere

t

t

t

21

21 )(21

fff

fff

Beat

avg

Y

Y

I

Example Question

• What is the beat frequency between 440 Hz and 442 Hz?

A) 441 HzB) 2 HzC) -2HzD) 1 Hz

Doppler Effect

Doppler Effect

• Frequency of sound changes when relative velocity changes– Frequencies on approach sound higher– Frequencies that recede sound lower

actualsoundobject

recede

actualsoundobject

approach

fvv

f

fvv

f

11

11

Example Question

• The frequency of an ambulance siren is 1200 Hz. The ambulance is going 60 mph (26.8 m/s). As the ambulance approaches your stopped car, you hear what frequency?

fapproach = 1300 Hz

Sound + Light = Optical Communication

http://www.scientificamerican.com/media/inline/F8CDD0E6-DFB3-6D63-DEE9270DAAFA852D_1.jpg

Main Points - Sound• Sound– a longitudinal pressure wave that

moves through a medium

• Velocity – 343 m/s in air• Reflection – occurs when velocity changes

• Resonance and Standing Waves – depends on open/closed ends

3Din except ,Tv

closedor open both 1,2,3...n ,2

nL

Main Points - Sound• Intensity and Sound Level

• Interference and Beats

• Doppler Effect

2120

0

W/m10 ),log(10 III

21 fffBeat

actualsoundobject

approach fvv

f

11