Sound
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Sound Waves
Sound waves travel as
compression waves.
Another name for
compression waves is
longitudinal waves
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Factors affecting speed of sound
Temperature – Sound moves
through hot air faster than cold
air due to the increased speed of
the particles.
Type of medium – Sound moves
well through dense material and
material that returns quickly to
its original position.
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Factors affecting speed of sound
Thus, sound moves fastest in a solid
and slowest in a gas
Sound travels more quickly through
solids and liquids because the
individual molecules are closer
together than the molecules in gas
Sound waves CANNOT move
through a vacuum – they need a
medium!
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Bells in a bell jar…
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Decibels
The decibel (dB) is the unit
used to measure sound
intensity or loudness.
Loudness corresponds to
the amplitude of a wave.
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Decibels
Sounds greater than 120 dB
can cause pain in human ears.
Sounds with an amplitude
between 0 dB and 120 dB are
called audible.
Anything below 0 dB is
considered subaudible.
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Decibel drag races
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Frequency and Pitch
Pitch – how high or low a
sound is.
High pitch = high frequency
Low pitch = low frequency
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Frequency and Pitch
Humans can hear pitches that
have a frequency between 20
Hz and 20,000 Hz
Pitches above 20,000 Hz are
called ultrasonic.
Pitches below 20 Hz are called
subsonic.
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Phone Tone Generators
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Doppler Effect
The Doppler effect is a change in
the frequency or pitch of a sound
that is caused by either the
movement of the source or the
observer of the wave.
Example: The sound from an
ambulance siren changes as it
approaches the listener
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Doppler Effect
First observed in 1842 by
Christian Andreas Doppler
The Big Bang Theory - Doppler Costume
Video explaining the Physics of the Doppler Effect
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Resonance
Resonance is the vibration of
an object at its natural
frequency.
Example: Windows rattle
when the sound from a passing
truck matches the window’s
natural resonance.
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Resonance of different things
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Nodes and Anti-nodes
Node – A place
where two waves
meet and
destructively
interfere so that the
displacement is zero
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Nodes and Anti-nodes
Anti-node – the
point of largest
amplitude when
two waves
interfere
constructively
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Music – Natural Frequency
A natural frequency exists
without any driving source.
It is a natural frequency if its
waveform has nodes that match
up with the ends of the object.
The lowest frequency at which
this occurs is the fundamental, or
the 1st Harmonic.
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Music - Harmonics
Harmonics – a sound wave with a
pitch that is a multiple of the
natural frequency
Overtone – has a higher
frequency than the fundamental
Octave = ½ or double the
frequency of a sound; 8 notes on
the musical scale
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Music
Consonance – multiple waves
combining to form a pleasant sound
Dissonance – multiple waves
combining to form an unpleasant
sound
Acoustics – the control of noise and
the vibrations that cause noise
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Toccata Fugue in D Minor
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Music – open pipe resonators
Open pipe resonators – both
ends are open
Examples: brass instruments,
flutes, saxophones
It reflects an inverted wave
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Music - Closed pipe resonators
Closed pipe resonators –
have one end enclosed
Example: pan-flute,
blowing across a bottle top,
hanging pipes under
marimbas, xylophones
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