Sound

1
Sound
2
What is it?

• Sound is a longitudinal wave that is produced by
  compression and rarefaction of matter.
• It requires a vibrating source that causes
  rhythmic variations in air pressure.

3
What happens?

• Vibrations cause air molecules to collide.
• This results in an oscillation in the pressure of
  a medium.

4
What happens?

• Sound can be reflected just like water waves.

5
What happens?

• Sound will not travel in a vacuum because there
  are no molecules to vibrate.
• The denser the medium, the better sound travels.

6
How fast does sound travel?

• 343 m/s at room temperature
• or
• 1150 ft/sec

7
Key Point

• The speed of sound varies with temperature.
• As the temperature of the medium varies, so does
  the kinetic energy and motion of the particles.

8
What is the rule?

• The speed of sound at 0º C. is 331.5 m/s.
• It increases 0.6 m/s for every degree increase in
  temperature.

9
Properties of Sound
10
Intensity and Loudness

• The intensity of sound is the time rate at which
  the sound energy flows through a unit area normal
  to the direction of propagation.

11
Loudness

• Loudness is related to the amplitude. The higher
  the amplitude, the more the energy, thus the
  louder the sound.

12
Equation for Intensity

• I P/A
• Intensity equals power per unit area.
• Units watts/m2

13
Relative Intensity

• This is the comparison of the intensity of a
  sound (I) to the intensity of the threshold of
  hearing (I0).
• Threshold of hearing is 10-12W/m2

14
Equation

• b 10 log (I/Io)
• Units dB (decibels)

15
Range of Human Hearing

• 20 to 20,000 Hz

16
Pitch and Frequency
17
Pitch

• Pitch is related to frequency.
• Each frequency represents a different pitch.
• Pitch is defined by the musical scale.

18
The Sources of Sound

• Strings
• Reeds
• Pipes
• Oscillators
• Lips
• Tympanic membranes

19
More Sources of Sound

• Vocal Chords
• Metal strips
• Bells
• Chimes

20
Intensity and Distance

• The intensity of a sound wave varies inversely
  with the square of the radius.
• I2/I1 r12/r22

21
The Doppler Effect
22
What is it?

• The Doppler Effect is a change it frequency
  associated with relative motion.
• This means motion of source or of the listener.

23
Well, what happens?

• When a sound source is moving, it wavelength is
  shortened.
• Therefore, its frequency goes up.

24
Equation

• For a moving source
• f f/(1-Vs/V)
• if object is approaching

25
Son of Equation

• and if the source is moving away
• f f/(1Vs/V)

26
Grandson of Equation

• but maybe the listener is moving
• f f(1 Vo/V)
• for moving awayand
• f f(1 Vo/V)
• for approaching

27
Standing Waves on a String
28
(No Transcript)
29
The Fundamental

• This is also called the first harmonic.
• It has two nodes and one antinode.
• It is the fundamental frequency.

30
The Second Harmonic

• It has three nodes and two antinodes.
• This is also called the first overtone.
• Its frequency is twice the fundamental frequency.

31
The Third Harmonic

• This is called the second overtone.
• It has four nodes and three antinodes.
• It is three times the fundamental frequency.

32
Waves in a Vibrating Column of Air
33
Open Pipe Resonators
34
Fundamental

• Fundamental frequency.
• Two antinodes and one node.
• l 2L

35
Second Harmonic

• First Overtone.
• Twice the fundamental frequency.
• l L
• Two nodes and three antinodes.

36
Third Harmonic

• l 2L/3
• Frequency is three times the fundamental.

37
Closed Pipe Resonators
38
Fundamental

• L l/4.
• One node and one antinode.

39
Third Harmonic

• Frequency is three times the fundamental.
• L 3l/4.
• First Overtone.

40
Fifth Harmonic

• Second Overtone.
• L 5l/4.
• Frequency is five times the fundamental.

41
Beats

• When two sound sources that have almost the same
  frequency are sounded together you hear a sound
  with a frequency that is the average of the two.

42
Son of Beats

• The variation in the amplitude between these two
  sounds is called a beat.
• The beat frequency is simply the difference
  between the two frequencies.