1. Ch. 13 - Sound The Nature of Sound Speed of Sound Human Hearing
Doppler Effect
Seeing With Sound

2. Production of Sound Waves
begins with a vibrating object
source of the disturbance
vocal chords
string on a violin, guitar, etc
tynes of a tuning fork
diaphragm of a radio speaker

3. Production of Sound Waves
air vibrates, causing changes in air pressure
high pressure = compressions
crests correspond to compressions
low pressure = rarefactions
results in a longitudinal wave
see Fig. 13-2

4. Frequency determines pitch highness/lowness of sound
higher frequency = higher pitch

5. Human Hearing ultrasonic waves infrasonic waves
human ear can hear between 20Hz and 20,000 Hz.
< 20Hz is infrasonic
> 20,000Hz is ultrasonic
higher f = shorter λ
small wavelengths can “see” much smaller objects
ultrasonic waves
infrasonic waves

6. Application:
Ultrasonic waves are used in medical testing because the wavelength is short enough to produce an image.
Most ultrasonic devices are around 10MHz. This allows for the emitting crystal to recapture the sound and produce the outline.
see Fig. 13-3
Echolocation works in the same manner.

7. “Sound Navigation Ranging”
Seeing with Sound
Ultrasonic waves - above 20,000 Hz
Medical Imaging
SONAR
“Sound Navigation Ranging”

8. Speed of Sound can travel through solids, liquids and gases
constant for a given medium
faster in solids
particles are closer together
can vibrate the next particle faster

9. Speed of Sound higher temperatures in gases also increase the speed
particles collide more often
not much difference in solids and liquids
particles are closer together

10. Motion of Sound vibrates out in a spherical wave called wave fronts
distance between wave fronts is wavelength
sound travels in three dimensions
rays can be drawn
show direction of wave motion
very far waves appear as a straight line
called a plane wave

11. Doppler Effect relative motion creates an observed change in frequency
if sound is on a moving object
also occurs in light
red shift
blue shift

12. Doppler Effect as object moves toward a stationary observer:
waves pass by more frequently than if standing still
frequency increases
higher pitch
as object moves away from a stationary observer:
waves pass by less frequently than if standing still
frequency decreases
lower pitch

13. Sound Intensity
the rate at which energy is transferred through a unit area of the plane wave
a.k.a. power per area of plane wave
intensity decreases as distance increases
spread over larger area

14. Sound Intensity
intensity and frequency both determine if sound is audible
frequency determines if “hearing” is possible
intensity determines the loudness (volume)
measured in decibels (dB)
see Table 13-2 on pg. 490

15. Human Hearing threshold of hearing
the softest sounds that can be heard by the average human ear
frequency of about 1000Hz
intensity of 1.0x10-12 W/m2
threshold of pain
intensity beyond 1.0W/m2
120
DECIBEL SCALE
110
100 80 70 40 18 10

16. Practice…
What is the intensity of the sound waves produced by a trumpet 3.2m away when the power output of the trumpet is 0.20W? Assume that the sound waves are spherical.

17. Forced Vibration and Resonance
a vibrating object is attached to another object
attached object experiences “forced vibrations”
sympathetic vibrations increase the sound
if at the object’s “natural frequency” it will vibrate much better
this is called resonance

18. Application:
The human ear is divided into three sections: outer, middle, and inner.
Hearing involves sound waves that travel down the ear canal in the outer ear and terminate at a thin flat piece of tissue called the eardrum.
The eardrum then vibrates and transfers these vibrations to the three small bones of the middle ear.
These bones in turn transmit the vibrations to the inner ear, which contains a snail shaped tube called the cochlea.
The small hairs and nerve fibers in the cochlea send impulses to the brain. The brain then interprets these waves.

19. converted to nerve impulses in cochlea
Human Hearing
sound wave
vibrates ear drum
amplified by bones
converted to nerve impulses in cochlea

20. Harmonics harmonics account for sound quality, or timbre
based on a standing wave, frequencies can be produced by musical instruments through strings or columns of air.
harmonics account for sound quality, or timbre
each instrument has its own mixture of harmonics at varying intensities
gives it its own unique sound
more harmonics, richer sound

21. Standing Waves when a wave is produced it reflects back
produces nodes and antinodes
frequency of the string determines
more/less nodes and antinodes
fundamental frequency
corresponds to the first standing wave
determines pitch

22. String Instruments (guitar, violin, etc.)
strings can vibrate in different ways
(different frequencies)
changes the number of nodes and antinodes
13th frequency is an octave above the 1st
n= harmonic number
l=length of the string

23. Columns of Air (flutes, piccolos, etc)
open column of air (flute, recorder, pipe organ)
see diagram pg. 496
n= harmonic number
l=length

24. Columns of Air (woodwinds, brass, etc)
closed column of air
(coke bottle, trumpet, clarinet)
see diagram pg. 497
n= harmonic number
l=length
only odd harmonics are present
n is odd

25. Beat interference between two waves of slightly different frequencies
interference pattern varies
alternation between loudness and softness
called beat
see pg. 502
# of beats per second corresponds to the difference between frequencies.
more beats  further off