Sec. 15.1
Properties of Sound
Ch. 15
Sound
 Objectives
– Demonstrate knowledge of the nature of
sound waves and the properties sound shares
with other waves.
– Solve problems relating the frequency,
wavelength, and velocity of sound.
– Relate the physical properties of sound waves
to the way we perceive sound.
– Define the Doppler shift and identify some of
its applications.
Sound Waves
 Are
pressure variations that are
transmitted through matter
 Air molecules colliding
 Longitudinal waves
 Speed depends on air temperature
Sound Waves
 Can’
Can’t
exist in a vacuum
 Echoes
– Reflected sound waves
– Sonar
– Warmer = less dense = slower
– Colder = more dense = faster
Loudness
 Depends
 We
on amplitude
hear based upon pressure differences
– Larger amplitudes create a higher pressure
differences
 Sound
level
– Decibels (dB)
– 0 dB (most faintly heard), 2 x 10-5 Pa
– Tenfold increase, 2 x 10-4 Pa is 20 dB
– 110 is painful (most concerts)
Pitch
 Difference
in frequency
– Middle C: 262 Hz
– E: 327 Hz
 Human
hearing
– 2020-16000 Hz
– SIM – Pitch/Beat Freq
– SIM – Pitch/Beating Freq
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Sec. 15.2
The Physics of Music
Doppler Shift
 Police
siren as it drives by SIM
 More sound waves reach you when the
vehicle is moving towards you
 Bats:
 Objectives
– Describe the origin of sound.
– Demonstrate an understanding of resonance,
especially as applied to air columns.
– Explain why there is a variation among
instruments and among voices using terms
– Insects flying away: lower frequency
– Insects flying towards: higher frequency
timbre, resonance, fundamental, and
harmonic.
– Determine why beats occur.
Sources of Sound
 Produced
by vibrating objects
– Vocal cords
– Brass instruments
– Reed instruments
– Stringed Instruments
– Others
Resonance
 Resonance
occurs when one object
vibrating at the same natural frequency of
a second object forces that second object
into vibrational motion.
 Can occur in an open or closed column of
air (tube)
 Length of air column determines the which
frequencies will resonanate
Open-end Resonance
Closed pipe resonance
One closed end (marimba, pipe organ, sea shell, etc.)


The loudest, first sound will be heard
when the tube is ¼ the wavelength
This is called the fundamental frequency
or 1st Harmonic of a closed tube
•Additional resonance lengths found at
half-wavelength intervals (0.75λ,
1.25λ, 1.75λ, etc.)
•These are called the 2nd Harmonic, 3rd
Harmonic, etc.
Woodwinds, brass

aka fundamental frequency

The loudest, first sound will be
heard when the tube is ½ the
wavelength
This is called the fundamental
frequency or 1st Harmonic of an
open tube
aka fundamental frequency
•Additional resonance lengths
found at half-wavelength intervals
(1.0λ, 1.5λ, 2.0λ, etc.)
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Closed vs. Open Tubes
 Closed
Open 
Sound Quality

Fundamental
– Lowest frequency

Q:
If you have an open tube and a closed tube, and each are the
same length, which will produce a higher pitch when the
fundamental frequency (aka 1st harmonic) resonates?
A:




Harmonics
– OddOdd-numbered multiples of the fundamental
frequencies

Dissonance
– Unpleasant set of pitches
Wavelength of f1 for the closed tube is 4L
Wavelength of f1 for the open tube is 2L
Wavelengths in open tubes half as long as in a closed ended tube
Leads to twice the frequency…
frequency… open tube will have high pitch

Musical intervals
 Octave
– Frequencies in a 1:2 ratio
– Ex: 1st notenote-440 Hz, octave higher would be
880 Hz
– Also corresponds to harmonics
Consonance
– Pleasant set of pitches
Beat Frequencies

What is a Beat?
– It is the oscillation of wave amplitude
– It is the result of alternative constructive and
destructive interference
– Beat frequency is the absolute value of the
difference between the two frequencies:
f beat  f1  f 2
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