Download - 13 nature of-sound
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The Nature of Sound
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What is Sound?
In human experience:
That
which
we
hear.
In Physics:
Oscillation
of pressure transmitted
through a medium
at frequencies
between
20Hz and 20 kHz.
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Sound travels in
longitudinal waves —vibrating compressions and
rarefactions through air
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Forced Vibrations
setting up of vibrations in an object by a vibrating force
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Natural Frequency
The frequency at which an object tends to vibrate when hit, struck, plucked, strummed or somehow disturbed.
Depends on
– elasticity
– shape
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Harmonic frequencies
are multiples of the
fundamental natural
frequency.
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Resonance
occurs whenever successive impulses
are applied to a vibrating object
in rhythm with its natural frequency.
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Mechanical Resonance
Following the collapse several bridges led to notices being placed warning troops to break step when crossing the bridge.
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Shake-table crash testing
of a building model.
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Interference
combined effect of two or more overlapping waves
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Waves overlap crest-to-
crest, resulting
in a wave of increased amplitude.
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Waves overlap trough-to-
crest, resulting
in a wave of decreased amplitude.
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Destructive
interference is the principle
behind active
noise-cancelling
technology.
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Beats are periodic variations
in the loudness of sound
due to interference.
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Standing waves
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This standing wave is a sum of two waves travelling in opposite directions.
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Nodes of standing wave
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Reflection
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Sound waves refract when air
near the ground is warmer than
air above.
Refraction
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Interference
combined effect of two or more overlapping waves
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Beats are periodic variations
in the loudness of sound
due to interference.
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Doppler Effect
the change in frequency as measuredby an observer due to the motionof the source or listener.
Named after Austrian physicist
and mathematician, Christian
Johann Doppler
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Water Bug Doppler Effect
Top view of water waves made by a stationary bug jiggling (up and down) in still water.
Water waves made by a bug swimming in still water toward point B.
A & B receive different wave frequencies.
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Doppler Effect
Example of Doppler Effect:
Frequency of waves received by an observer increases as a sound source approaches. Wave frequency decreases as the source recedes.
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Stages of Wave Speeds
Bug swims at successively greater speeds. Overlapping at the edges occurs only when the bug swims faster than wave speed.
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Shock Waves and the Sonic Boom
Shock wavepattern of overlapping spheres that form a cone from objects
traveling faster than the speed of sound
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ShockwaveThe shockwave actually
consists of two cones.
– A high pressure cone with its apex at the bow
– A low pressure cone with its apex at the tail.
– A graph of the air pressure at ground level between the cone takes the shape of the letter N.
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Shockwave
The shock wave has not yet reached listener A, but it is now reaching listener B, and it has already reached listener C.
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Musical Sound
Graphical representations of noise and music.
(a) Noise has no clear repeatable pattern.
(b) Music has a frequency (repeatable wave), wavelength, and speed.
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Music and Standing Waves
Each harmonic of a guitar string is a standing wave.
Image shows the first four harmonics on the string.
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Variations in Tone
Images of a piano and clarinet playing note C
Each has the same frequency, but with different extra vibrations.
These differences produce tone.
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