Sound

1
Sound

• Sound waves are longitudinal waves
• Molecules vibrate back and forth in direction of
  wave motion
• Regions with more molecules are compressed
• Regions with less molecules are rarified
• Sound waves can occur in any medium
• In fluids, waves are always longitudinal
• In solids, wave can also have transverse modes

2
Speed of Sound

• Speed of sound depends on how compressible the
  medium is, as well as how much inertia it has
• Compressibility depends on the Bulk Modulus
• Inertia depends on the density

3

• Speed also depends on temperature
• Comes from fact that density varies with
  temperature
• For air, speed is

4
Increasing the sound speed in air can be effected
by which of the following?

• decreasing the air temperature
• reducing the pressure of the air
• increasing the air temperature
• increasing the frequency of the sound
• increasing the amplitude of the sound wave

5
Suppose you're on a hot air balloon ride,
carrying a buzzer that emits a sound of frequency
f. If you accidentally drop the buzzer from the
balloon while it's rising at constant speed, what
can you conclude about the sound you hear as it
falls towards the ground?

• The frequency decreases and the intensity
  increases.
• The frequency decreases and the intensity
  decreases.
• The frequency increases and the intensity
  increases.
• The frequency remains the same but the intensity
  decreases.

6
Suppose you're on a hot air balloon ride,
carrying a buzzer that emits a sound of frequency
f. If you accidentally drop the buzzer from the
balloon while it's rising at constant speed, what
can you conclude about the sound you hear as it
falls towards the ground?

• The frequency decreases and the intensity
  increases.
• The frequency decreases and the intensity
  decreases.
• The frequency increases and the intensity
  increases.
• The frequency remains the same but the intensity
  decreases.

7
Energy and Intensity

• Define Intensity as the power per unit area

• Human hearing covers a wide range of intensities
• Use 1.00 Hz as reference frequency
• 1 x 10-12 W/m2 is typical threshold of hearing
• 1 W/m2 is typical threshold of pain

8
Decibels

• Due to large range of hearing, use logarithmic
  scale
• Define Bel as ratio of logarithm of intensities

• I0 1.0 x 10-12 W/m2
• Decibel is 10 Bels

9
Two sounds of the same intensity are added
together. The intensity of the resulting sound is
how many decibels greater than the intensity of
one of the original sounds? The resulting sound

• will have twice the decibel level as one of the
  original sounds.
• will have the same decibel level as one of the
  original sounds.
• will be two decibels higher than the original
  sound.
• will be three decibels higher than the original
  sound.

10
Two sounds of the same intensity are added
together. The intensity of the resulting sound is
how many decibels greater than the intensity of
one of the original sounds? The resulting sound

• will have twice the decibel level as one of the
  original sounds.
• will have the same decibel level as one of the
  original sounds.
• will be two decibels higher than the original
  sound.
• will be three decibels higher than the original
  sound.

11
Doppler Effect

• When the object and the receiver are traveling at
  different speeds, the frequency changes
• Change depends on whether the object or the
  receiver is moving, or both
• Assume the original frequency is f0

12
Stationary Source, Moving Receiver

• Let receiver be to right of source, moving to the
  right at a speed of vR.
• Let sound leave source with speed v.

13

• Receiver sees wavefronts moving past at a speed
  of v v - vR. Thus

• If receiver is towards object, replace (-) with
  ()