Download 06_Waves_Sound

Properties of Sound Waves
 Sound has many properties of waves:
 Transmit energy
 Travel around corners (diffraction)
 Requires a medium in which to travel:
Is sound a longitudinal or a transverse wave?
Reflection, Transmission & Transmission
 Reflection
 When waves “bounce
back” from a surface
 Transmission
 When waves pass
through a surface
 Diffraction
 Spreading of a wave
as it passes through a
pinhole or by an obstacle
Reflection: Echoes and Reverberation
 Reverberations are perceived when reflected sound waves
reach your ear in less than 0.1 s.
•Almost no delay is perceptible
between the original and reflected
wave. We hear only one very
long sound wave.
Diffraction: Waves from a Pinhole
 Amount of bend depends on the size of the opening relative
to the wavelength.
 Bending is most noticeable when the width is approx. same as
the wavelength
 Light and sound are types of waves.
 Why can we hear around corners, but not see around them?
Speed of Sound
 The speed of sound in air is temperature dependent:
v = 331 + 0.59 Tc
where
v is the speed of sound(m/s)
Tc is the temperature of the air (˚C)
 Sound travels fastest in solids, slower in liquids and
slowest in gases
Example
 The temperature was 4.0˚C one morning, as Sara
walked through the parking lot at the back of
Thomson. She shouted at a wall of the school, and
heard an echo. How far away was the wall?
Interference of Sound Waves
 If sound is a wave, then it must exhibit interference.
E.g. When the arms of a tuning fork vibrate, each arm
acts like a vibrating source. Sound waves radiate
outward from these sources.
Interference of Sound Waves: Beats
 When two waves with slightly different frequencies
interact (i.e. interfere), beats are produced.
Interference of Sound Waves: Beats
 The number of high intensity points that occur per
second is called the beat frequency.
 The beat frequency is the absolute value of the
difference of the frequencies of the component waves:
f beat = |f1 – f2|
where
f beat is the beat frequency (Hz)
f 1 is the frequency of one wave (Hz)
f2 is the frequency of the other wave (Hz)
Beat Frequency continued…
Example
 A tuning fork of unknown frequency is sounded at the
same time as one of frequency 440 Hz. As a result,
beats are heard. Over 15 s, 46 beats are produced.
What are the possible frequencies of the unknown
frequency tuning fork?
Refraction:
Bending of Waves
 The speed of a wave depends on what medium it’s in.
 Speed of sound in air: 331 m/s
 Speed of sound in water: 1482 m/s
 Refraction
 When a wave enters a different medium at an angle, the change in speed
causes the wave to bend.