Targets recognize the effects of the diffraction of sound waves

• Targets• recognize the effects of the

diffraction of sound waves• recognize the effects of the

interference of sound waves• perform calculations involving the

reflection of sound waves

• Do Now• put solutions to homework on board

• Homework• study

• when a sound wave encounters a change of medium, a portion of the energy is reflected

• the rest of the energy is transmitted

• the greater the dissimilarity of the mediums, the greater the amplitude of the reflected sound wave

• same principle as that observed with waves in the slinky and heavy coil

Sound Reflection

Angles of Incidence and Reflection

•angle of incidence = angle of reflection

45°

45°

angle of incidence

angle of reflection

Examples of Reflection

• standing waves on strings and in pipes

• echoes

• reverberations

• parabolic microphones

• room acoustics

• rotunda acoustics

• Greek theatre at Epidaurus

• sound pipes on playgrounds

Reflecting Surfaces

parabolic surfaces

rotunda effect anti-focusing surfaceshttp://hyperphysics.phy-astr.gsu.edu

Echo Sample Problem 1

• You stand at the top of a well and shout into the well. After 0.4 seconds you hear your echo. The speed of sound in the well is 343 m/s. How deep is the water in the well?

• 69 m

Echo Sample Problem 2

• You sing a middle C, 256 Hz, in the middle of a large canyon to see how it echos

a.What is the wavelength of sound wave if the speed of sound in air is 343 m/s?

b.If it takes 0.75 seconds for the wave to return from one side of the canyon and 0.81 seconds for the wave to return from the other side, how wide is the canyon?

Sound Diffraction

• the bending of waves around small obstacles and the spreading out of waves beyond small openings

• where small is relative to the wavelength

http://hyperphysics.phy-astr.gsu.edu

Examples of Diffraction

• hearing a classroom conversation in the hallway

• the difference of the sound of thunder depending on distance- high crack when it is close by, low rumble when it is far away

• sealing of loudspeaker cabinets

• location of speakers in a home theatre

Differences in diffraction• How do we know high frequency waves

do not diffract as well?

• Loudspeaker Example

http://hyperphysics.phy-astr.gsu.edu

More practice problems1. The speed of sound at 20°C is 343 m/s. If

you hear the crack of a gun 0.7 seconds after you see the gun shot, how far away is the shooter from you?

2. Two tuning forks are struck at the same time, one with a frequency of 256 Hz and one with a frequency of 280 Hz. What is the beat frequency produced?

3. What is the frequency of a sound with a wavelength of 30 cm if the speed of sound is 343 m/s?

More practice problems4. A sound with a frequency of 550 Hz has a

wavelength of 5 m in a pipe of iron. What is the speed of sound in this pipe?

5. If the speed of sound in air can be found by the equation v = 331 m/s + (0.6m/s)T. At what temperature does sound have a speed of 400 m/s?

6. A wave vibrates 135 times per second. What is the period of this wave?

7. The wavelength of a sound wave is 10 m and it has a speed of 343 m/s. What is the period of the wave?