Download Announcements Chapters 19-21: Waves and Sound Main Points

3/20/15 Announcements
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Today: Sound and waves
Wednesday: MT #2!
Friday: More Sound
Reading after Midterm: CH 21, focus on
differences between pitch, intensity, and
loudness
Main Points
•  Properties of waves (wavelength,
frequency, amplitude, speed)
•  Wave interference
•  Doppler Effect
•  Properties of sound (speed, pitch, etc.)
•  Musical sounds: pitch, loudness/intensity,
quality
Chapters 19-21:
Waves and Sound
•  Chapter 19: Wave Basics
•  Chapter 20: Sound Waves
•  Chapter 21: Music vs. Noise
Properties of Waves
•  Wavelength is the distance between two wave
peaks
•  Frequency is the number of times per second that
a wave vibrates up and down
wave speed = wavelength x frequency
More Wave Properties
•  Amplitude: Distance from midpoint to crest
of wave
Frequency & Period
Frequency = 1/Period
Period = 1/Frequency
They are inversely related, so they have
opposite behavior!
•  Period: Time to complete one vibration
1 3/20/15 Wavelength and Frequency
Examples:
Rank these waves
from small to large:
1.  Wavelength
2.  Frequency
3.  Period
wavelength x frequency = wave speed
The Doppler Effect
Doppler Effect
•  Definition: “The change in wavelength of
radiation due to relative radial motion
between the source and the observer.”
Real Life Example of Doppler Effect
The change in the pitch of a siren on a
police car, fire truck, or ambulance as it
zooms past (sound waves)
Doppler Effect
•  When something which is giving off light
moves towards or away from you, the
wavelength of the emitted light is changed or
shifted
V=0
Star
Light Wave
Astronomers deal with the Doppler
Effect of light waves
2 3/20/15 Doppler Effect
•  “Radial” means “along line of sight”
•  Doppler Effect happens only if the light
source is moving towards you or
away from you.
Conceptual check:
When sound waves from a police siren are
blueshifted, what property of the sound
wave is increasing?
A.  The amplitude
B.  The frequency
C.  The speed
D.  The wavelength
Types of Waves
•  Transverse Waves: like ocean waves or
waves created by shaking a string
•  Longitudinal Waves: compression waves,
such as sound
Wave Interference
Two types of interference: constructive and
destructive.
Announcements
•  Today: Finish Sound, maybe start new unit
E&M
•  Monday: Electricity
•  Reading: CH 22, focus on electric charges
and charging.
Standing Waves
•  When a wave is confined to a small space (such
as along a length of string), we can use
constructive interference to create a standing
wave.
Destructive:
Constructive:
•  This works for either transverse or longitudinal
waves!
3 3/20/15 Bow Waves and Shock Waves
•  Produced by objects moving at a greater
speed than the waves they are producing.
•  Bow waves: Boats moving faster than the
water waves
•  Shock waves: Planes or whips moving
faster than sound waves.
•  When the shock wave reaches you, you’ll
hear it!
Sound is just another wave!
•  Sound waves bounce off surfaces and can
be bent (or refracted)
•  Sound waves interfere with each other
•  Sound waves carry energy (although not
that much!)
Example: Frequency
Bats can hear squeaks of very high
frequency: up to 120,000 Hz! What is the
wavelength of these ultrasonic sound
waves?
Properties of sound waves:
•  Human range of hearing is from about
20-20,000 Hz
•  Frequency ! pitch
More on Sound Waves
•  Most sounds we hear travel through air,
but any elastic material can also transmit
sound waves!
•  Speed of sound in air: ~340 m/s
•  The speed of sound changes when
travelling through a different medium
(water, steel, etc.)
Music vs. Noise
•  Regular vs. irregular vibrations
•  Characterized by pitch, loudness, and
quality
wavelength x frequency = wave speed
4 3/20/15 Sound Intensity
•  Intensity depends on the amplitude of the
wave
•  The decibel scale (dB) is a logarithmic
scale (think like the Richter scale for
earthquake intensity)
•  Careful: Intensity is not the same as
loudness!
Harmonics
Harmonics: tones
with a frequency
that is a wholenumber multiple of
the fundamental
(lowest) frequency.
Quality of a Sound
•  Also called timbre or “color”
•  Quality allows you to distinguish between
sounds made by trumpet, piano, violin, etc.
•  Depends on the harmonics in the sound
Musical Sounds
Different instruments produce sounds with
different harmonics present at different
intensities.
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