General Physics 2 Waves & Sound 1

Reflection & Transmission

when end is fixed, reflected wave in inverted

when end is free tomove, reflected wave is not inverted

General Physics 2 Waves & Sound 2

Reflection & Transmission

when a wave enters a heaviermaterial, part of the wave is reflectedand part is transmitted

General Physics 2 Waves & Sound 3

Interference; Principle of Superposition

DestructiveInterference

ConstructiveInterference

PartialDestructiveInterference

General Physics 2 Waves & Sound 4

Speed of Sound

• Different in different materials• v = 331 m/s

• in air at 00C and at 1 atm

• temperature dependence• v = (331 +0.60 T) m/s

General Physics 2 Waves & Sound 5

Decibels

• Intensity of sound is measured in decibels• range of 1012 between intensity of quietest

and loudest sounds we can hear• ear hears on logarithmic scale

• decibel - measure of intensity

General Physics 2 Waves & Sound 6

Intensity

General Physics 2 Waves & Sound 7

Human Ear

General Physics 2 Waves & Sound 8

Frequency Response of Ear

General Physics 2 Waves & Sound 9

Sources of sound

• String instruments• Strings are fixed at both

ends.• sound is amplified by

• sounding box• sounding board

• Figure out the equation for harmonic wavelengths in terms of L, where L is the length of the unstretched string.

General Physics 2 Waves & Sound 10

Pair Problem• A highway overpass was

observed to resonate as one full loop when a small earthquake shook the ground vertically at 4.0 Hz. The highway department put a support at the center of the overpass, anchoring it to the ground. What resonant frequency would you now expect for the overpass? Earthquakes rarely do significant shaking above 5 or 6 Hz. Did the modifications do any good?

Tacoma Narrows Bridge

• Watch movie

General Physics 2 Waves & Sound 11

General Physics 2 Waves & Sound 12

Sources of Sound: Wind Instruments

General Physics 2 Waves & Sound 13

Closed at one end

General Physics 2 Waves & Sound 14

Quality of Sound

General Physics 2 Waves & Sound 15

Interference & Beats

fbeats = fA - fB

General Physics 2 Waves & Sound 16

Doppler Effect

higherfrequency

lowerfrequency

General Physics 2 Waves & Sound 17

Think-Pair-Share

• A monitor is blowing a whistle in front of the child. At which position, A through E, will the child hear the highest frequency for the sound of the whistle? Explain your reasoning.

General Physics 2 Waves & Sound 18

Doppler Effect

• v = speed of wave• vo = speed of observer

• vs = speed of source

• f0 = emitted frequency

• f’ = observed frequency

• f’ > f0

• s and o approaching

• f’ < f0

• s and o receding

General Physics 2 Waves & Sound 19

Plug & Chug

• The predominant frequency of a certain fire engine’s siren is 1550 Hz when at rest. What frequency do you detect if you move with a speed of 30.0 m/s (a) toward the fire engine, and (b) away from it?

General Physics 2 Waves & Sound 20

Think-Pair-Share

• (a) Compare the shift in frequency if a 200-Hz source is moving toward you at 15 m/s, versus you moving toward it at 15 m/s. Are the two frequences exactly the same? Are they close?

• (b) Repeat the calculation for 150 m/s. What can you conclude about the asymmetry of the Doppler formula?

General Physics 2 Waves & Sound 21

Sonic Boom

If an object moves at the speed of sound as in (c),all the sound waves pile up at one point. The resultis an immense sound - a sonic boom!

General Physics 2 Waves & Sound 22

Ultrasound Imaging

color reflects strengthof echo

delay of echo indicatesdistance to boundary

General Physics 2 Waves & Sound 23

Think-Pair-Share

• If the velocity of blood flow in the aorta is normally about 0.32 m/s, what beat frequency would you expect if 5.50-MHz ultrasound waves were directed along the flow and reflected from the red blood cells? Assume that the waves travel with a speed of 1.54 x 103 m/s.

General Physics 2 Waves & Sound 24

Think-Pair-Share

• A single mosquito 5.0 m from a person makes a sound close to the threshold of human hearing (0 dB). What will be the sound level of 1000 such mosquitos?

General Physics 2 Waves & Sound 25

Interactive Physics

• PHET• phet.colorado.edu/sims/wave-on-a-string/

wave-on-a- string_en.html • Create a standing wave• Adjust the settings to match your calculations

for the harmonics