Glencoe Physics 2006

1
Chapter 15

• Glencoe Physics 2006
• Sound

2

• A. Properties and Detection of Sound
• 1. Sound Wave - a pressure oscillation that is
  transmitted in matter (medium)
• a. longitudinal wave - due to molecular movement
  in the medium
• b. in air the pressure oscillates around an
  average value, the mean air pressure
• c. velocity depends on mean air pressure and
  temperature of the air - 343 m/s at 1 atmosphere
  of pressure and 20oC.

3

• 2. Greater speed for sound in solids and liquids,
  but it cannot travel in a vacuum
• 3. Can be reflected, diffracted, and can
  interfere
• a. reflected waves are called echoes
• b. destructive interference causes dead
  spots

4

• 4. Problems
• a. A sound wave has a frequency of 261.1 Hz.
  What is the wavelength of this sound traveling in
  air at 343 m/s?
• b. The speed of sound in water is 1435 m/s. Find
  ? with a frequency of 261.1 Hz.
• c. When a frequency of 440 Hz is sent through
  steel, a wavelength of 11.66 m is measured. Find
  the velocity of sound in steel.

5

• d. What is the frequency of sound in air at 20oC
  having ? equal to the diameter of a 15 inch
  woofer? Of a 3.0 inch tweeter?
• e. Human ears can detect sounds from 20 Hz to 16
  kHz. Find the largest and smallest wavelengths
  we can detect. Use v 343 m/s

6

• 5. Detecting Sound
• a. Human Ear
• (1) Outer Ear
• (a) pinna, the fleshy visible portion collects
  the sound
• (b) auditory canal
• (c) eardrum - vibrates with sound wave
• (2) Middle Ear - three tiny bones in an air
  filled space
• (3) Inner Ear - watery liquid

7

• b. Pitch and Octave
• (1) Pitch
• (a) the frequency of the wave
• (b) also given by the name of a note on the
  musical scale
• (2) Octave - two notes with frequencies related
  by the ratio 21 are said to differ by an octave.
  Ratio, not size, is the key.

8

• c. Loudness is logarithmic for human ear -twice
  as loud equates to a 10dB gain
• (1) double the power intensity - 3 dB gain
• (2) increase power by factor of 10-10dB gain
• (3) increase by factor of 100 - 20dB gain
• (4) increase by factor of 1000 - 30 dB gain
• (5) intensity of the wave itself is proportional
  to the amplitude squared - energy is in the
  amplitude. Greater amplitude, greater loudness

9

• d. Intensity Level
• (1) Decibel is a measure of the relative loudness
  (intensity) of sound compared to the threshold of
  hearing for the average person.
• (a) Io sound intensity at the threshold
  of hearing (10-12 watts/m2)
• (b) ? 10 log (I/ Io), where Ipower/area
• (2) decibel is the unit of measure for sound
  level

10

• (a) threshold of pain is 1 watt/m2 (120dB)
• (b) Common values
• threshold of hearing 0 dB
• rustling leaves 10 dB
• whisper 30 dB
• mosquito 40 dB
• traffic 80 dB
• rock concert 120dB
• jet airplane 150dB

11

• 6. Doppler Shift - experienced whenever there is
  relative motion between source receiver
• a. When moving towards each other, frequency
  heard by observer is higher than frequency of
  sound actually emitted.
• b. When moving away from each other, frequency
  heard is lower.
• c. Common to all waves, not just sound.
• d. Formulas

12
(1) Sound source is fixed and the observer is
moving

vo speed of observer v speed of sound in
medium
vo
-vo
s
?
s
vo
vo
13
(2) Source is moving and the observer is fixed
- v s means the source is moving towards the
observer vs means the source is moving away
from the observer
? is smaller so f is greater
B hears decreased frequency
s
?
A hears increased frequency
ob B
ob A
? is larger
14

• B. The Physics of Music
• 1. Resonance
• a. All objects have a natural frequency of
  vibration, fo. Easily seen in a string
  instrument.
• b. A forced vibration is set up in an object when
  the object is driven by some periodic force.
  Note if the force is applied in a periodic
  manner, it has a frequency associated with it.

15

• c. The amplitude of the motion of the object
  being vibrated reaches a maximum when the
  frequency of the driving force is the same as the
  natural frequency of the object, fo.
• (1) In essence, a standing wave is produced.
• (2) moves with the same frequency
• (3) increased amplitude of vibration by
  repeatedly applying a small external force at the
  natural frequency of the object.

16
2. Closed pipe resonator - a resonating tube with
one end closed

• a. Wave from a tuning fork hits water and is
  reflected. If the reflected wave is at the same
  point in its oscillation as wave from the tuning
  fork, reinforces.
• b. In a standing wave, separation between two
  nodes is 1/2 ??

1/4 ?
17

• c. Pressure wave is reflected from the closed end
  w/o inversion.
• d. Pressure wave is reflected from the open end
  inverted.
• e. The open end of the pipe is a pressure node,
  which equates to a loudness maximum - pressure
  wave is 1/4 ? out of phase with the amplitude
  maximum

3/4 ?
18
3. Open pipe resonator - a resonating tube with
both ends open

• minimum length is 1/2 ?
• b. resonances are spaced by 1/2?
• c. sound is transmitted at both ends
• d. some sound is reflected at the open ends
• pressure nodes at each end, sound wave antinodes
  at each end

19

• 4. Quality of Sound
• a. Beat - oscillation in wave amplitude heard
  when two waves of different frequencies reach an
  observer
• (1) frequency of the beat is the difference in
  frequencies of the two waves
• (2) a pulsing variation in loudness is
  heard

20

• 5. Consonance and Dissonance
• a. two sounds with two different frequencies are
  played at the same time
• b. if combination is pleasant said to be
  consonance
• c. if combination is unpleasant said to be
  dissonance
• d. culturally based determination but Western
  cultures prefer frequencies with small,
  whole-numbered ratios

21

• 6. Human Voice - use the throat and mouth as
  resonators to impart information onto the carrier
  wave produced by the vocal cords. Changing the
  tongue position, nasal cavity all affect sound
• a. Push air from diaphragm pass the vocal cords.
  This causes them to vibrate at a natural
  frequency - the persons pitch
• b. modulate the frequency to carry information.

22

• 7. Harmonics - whole number multiples of an
  objects natural frequency.
• a. The natural frequency is sometimes called the
  first harmonic or fundamental frequency
• b. harmonics are multiples of the fundamental
  frequency