1. Sound

2. Objectives Understand the properties of sound waves.
Understand the concepts of pitch and loudness.
Know what factors affect the speed of sound.

3. Sound Waves Vibrating objects produce longitudinal sound waves.
Applet
tuning fork demo
compression: high
pressure
rarefaction: low

4. Human Hearing Human hearing range is 20 – 20000 hz.
infrasonic < 20 hz
ultrasonic > hz
pitch = frequency (rate of vibrations)
loudness: related to amplitude

5. Speed of Sound Sound travels through different media
at different speeds (but not through a vacuum).
Speed depends on…
density (solid > liquids > gases)
temperature (T↑, v↑)
v = ·T (in m/s, T in Celsius)
elasticity (tendency to return to original shape)

6. A jet makes a high
pressure ridge as
it passes the speed
of sound. This high
pressure can cause
water vapor to
condense, forming
a cloud.

7. Intensity and Loudness
Loudness is the human perception of sound intensity.
Sound intensity is related to the power.
intensity = power passing
through area (W/m2)
What is the intensity of a 15-W
speaker at 2.5 m? r
power (W)

8. Decibel Scale decibels (dB): a relative measure of sound intensity;
uses a logarithmic scale
# of dB : – – – – – etc.
intensity: X X X X
loudness: X X X X
quiet whisper ≈ 20 dB
vacuum cleaner ≈ 70 dB
lawn mower ≈ 90 dB
threshold of pain ≈ 120 dB
jet engine ≈ 150 dB

9. Decibel Calculations Io is threshold of human hearing = 1x10-12 W/m2
Io is threshold of human hearing = 1x10-12 W/m2
How many decibels is a W/m2 sound?
What is the intensity (in W/m2) of a 62 dB sound?

10. Resonance natural frequency: the frequency at
which an object naturally vibrates
(large = low, small = high)
forced vibration: a periodic
applied force that causes
something else to vibrate
example: swingset
resonance: an increase in vibrational amplitude; occurs if forced vibration frequency = natural frequency
examples: resonators, sound boxes, crystal glass,
microwave ovens, Tacoma Narrows bridge, sunscreen

11. Harmonics
fundamental frequency (f1) = longest wavelength standing wave that forms (1 antinode)
harmonic frequencies (f2, f3, etc.) are shorter, whole number multiples of the fundamental frequency (2A, 3A, etc.)
These harmonics combine into a single waveform.

12. Harmonics in a String Standing waves occur in stringed instruments.
f1: L = 1/2 · l
Harmonics
f2: L = 2/2 · l
fundamental frequency (f1)
2nd harmonic (f2)
3rd harmonic (f3)
4th harmonic (f4), etc.
f3: L = 3/2 · l
fn: L = n/2 · l

13. Stringed Instrument Problem
A banjo string has mass-to-length ratio of 5.8 x 10-3 kg/m.
What will be the 3rd harmonic frequency if the string is
65 cm long and it has a tension of 78 N?

14. Harmonics in Tube open-end tube n = 1, 2, 3, 4… Who is this?
Tube Harmonics
open-end tube
n = 1, 2, 3, 4…
Who is this?
closed-end tube
n = 1, 3, 5, 7…

15. Objectives
Understand the concept of the Doppler effect and how it causes the formation of bow waves and shock waves.
Understand the concept of beats.
Understand the concept of timbre.

16. Doppler Effect
Doppler effect: a change in perceived wave frequency caused by relative motion see it
approaching: freq ↑
receding: freq ↓
If source speed > wave speed
bow wave: 2-D (V-shape)
shock wave: 3-D (cone-shape)

17. Beats beats: a frequency caused by interference of two
waves of different frequency
Beat Frequency
fb = |f1 – f2|
Beats caused by interference
between transmitted microwaves
and Doppler shifted microwaves
cause beats—your speed is then
calculated by the “radar gun.”

18. Timbre timbre (tam-ber or tim-ber): the unique quality of sound
produced by a musical
instrument caused by multiple
harmonics
The same note is played,
but the clarinet has more
harmonic frequencies.