Transcript The Physics of Sound

The Physics of Sound
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Sound: a series of disturbances of molecules
within, and propagated through, an elastic
medium or…
Sound: is an alteration in the pressure,
particle displacement propagated in an elastic
medium
Elasticity: capacity of a medium to return to
its original position after being altered
Brownian motion: heat effects particle
velocity = + heat = + velocity
Sound Wave
Sine Wave
Compression\ Rarefaction
Degrees of Wave
Wave Length
Velocity
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Acceleration = increase in velocity
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Deceleration = decrease in velocity
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Increase density = increase V. (fastest
in solids, then liquids, then gases)
Degrees of Sine wave
Sound
Characteristics
Damping
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Free vibration - when no outside force is added to vibration
Lightly damping - when vibrations of a mass decay over time
Heavy damping - “ “
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rapidly
Critically “ “ “
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before 1 Hz.
Forced vibration - when outside force is added to vibration.
When removed = free vibration
Vibrator Length, Mass and
Stiffness
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If length decreases = frequency
increases
If length increases = frequency
decreases
If increase in mass = decrease in
frequency
If increase in stiffness = increase in
frequency
Vibration
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Periodic - a waveform that repeats over
time (i.e. speech)
Aperiodic - a waveform that doesn’t
repeat over time (noise)
Fourier Analysis
Frequency : Pitch :: Intensity :
Loudness
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Frequency is the number of complete oscillations of a vibrating
body per unit of time (in audiology we use Hertz Hz)
Pitch is the subjective impression of highness or lowness of a
sound. The psychological correlate of frequency.
Intensity is the amount of sound energy per unit of area (dB)
Loudness is the subjective impression of the power of a sound.
It is the psychological correlate of intensity (sones)
Resonance
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Resonance - the ability of a mass to vibrate at
a particular frequency with a minimum
application of external force. Also the ability
of a vibrator to put another vibrator into
vibration.
Resonant frequency - the frequency at which
a mass vibrates with the least amount of
external force; the natural frequency of
vibration of a mass
Fundamental
frequency/overtones/harmonics
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Fundamental frequency - the lowest
frequency of vibration in a complex wave
Overtones - any whole number multiple of
the fundamental frequency of a complex
wave
Harmonic - same as overtones except in
numbering system
(i.e. 1st overtone is = to 2nd harmonic)
Formant
Impedance
Opposition to the path of sound.
Denser the object the more the
impedance
Sound Measurement
Audiometer
 air conduction
 bone conduction
 speech (sound field,
free field and under
phones)
Sound level meter
 A scale
 B scale
 C scale
Intensity/Decibel
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It involves a ratio
It involves a logarithm
It is therefore non-linear
It is expressed in terms of various reference
levels which must be specified
It is a relative unit of measure
3 dB increase, if intensity levels are doubled
dB Levels
Force and Pressure
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Force - the impetus required to institute or alter the
velocity of a body. A Newton is also a measure of
force.
Pressure - force over an area of surface (i.e. Pascals
.0002 dynes per cm2 = the smallest pressure
variation for hearing.
Sound Pressure Level (SPL) .0002 dynes per
centimeter squared = 20 micropascals = 0 dB SPL
Thresholds
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The softest level an individual can hear
a sound 50% of the time.
Threshold of pain is about 140 dB (SPL)
Sensation Level (SL) = dB above
threshold
Power
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the rate at which work is done. Units of
measurement are: watts, or ergs
per/sec.
Audiometric Zero
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Standards
ASA 1951
ISO 1964
ANSI 1996