Download Vocabulary List - Ms King Salz`s Physics Course

Sound
Longitudinal waves
Producing a Sound Wave
· Sound waves are longitudinal waves traveling
through a medium
· A tuning fork can be used as an example of
producing a sound wave
Using a Tuning Fork to
Produce a Sound Wave
·
·
·
·
A tuning fork will produce a pure musical
note
As the tines vibrate, they disturb the air
near them
As the tine swings to the right, it forces the
air molecules near it closer together
This produces a high density area in the air
· This is an area of compression
Using a Tuning Fork, cont.
· As the tine moves toward
the left, the air molecules
to the right of the tine
spread out
· This produces an area of
low density
· This area is called a
rarefaction
Using a Tuning Fork, final
· As the tuning fork continues to vibrate, a succession of
compressions and rarefactions spread out from the fork
· A sinusoidal curve can be used to represent the
longitudinal wave
· Crests correspond to compressions and troughs to rarefactions
Sound Waves and Sound
· Frequency determines pitch
· Amplitude determines volume
· A sound wave must be within a certain
frequency range and above a certain
minimum amplitude for us to hear it.
Categories of Sound Waves
· Audible waves
· Lay within the normal range of hearing of the human
ear
· Normally between 20 Hz to 20,000 Hz
· Notice: this range is based on FREQUENCY.
· Infrasonic waves
· Frequencies are below the audible range
· Earthquakes are an example
· Ultrasonic waves
· Frequencies are above the audible range
· Dog whistles are an example
Range of Human Hearing
· Humans, on average, can detect tones
from 20Hz to 20,000 Hz.
· This range gets smaller with age. As you
get older, you loose the ability to hear the
high end of the range.
· Different animals have different ranges
of hearing. Cool graphic
Decibel Scale
· Sound volume is commonly measured in
decibels.
· The decibel system is calibrated to
human hearing; thus, the lowest level of
sound (at a frequency of 1,000 Hz) that a
theoretical human can detect is the zero
set point for the scale. This is called the
“threshold of hearing.”
Threshold of hearing
· The average person can detect a 1000 Hz
sound at a minimum of 4 decibels, but
this changes with the frequency being
heard.
· This graph shows the minimum sound
volume the average person can detect
based on the frequency of sound
Threshold of Pain
· The loudest sound an average person can
tolerate is 120 – 130 decibels.
· The decibel system is logarithmic; this
means that each step on the scale is a
factor of 10x greater than the one before.
· So, 130 dB is not 130 times greater than
1 dB, but is 1 x 1013 times greater!
Hearing Safety
· Hearing damage can result from
exposure to loud sounds.
· Studies of rock musicians and animal
studies have suggested that exposure to
unsafe levels of sound can lead to
temporary or permanent hearing
impairment, such as
· Tinnitus (ringing in the ears) to hearing
loss.
Sound Effects
Changes in Frequency
Doppler Effect
· A Doppler effect is experienced whenever
there is relative motion between a source of
waves and an observer.
· When the source and the observer are moving
toward each other, the observer hears a higher
frequency
· When the source and the observer are moving away
from each other, the observer hears a lower
frequency
Doppler Effect, cont.
· Although the Doppler Effect is
commonly experienced with sound
waves, it is a phenomena common to all
waves
· Assumptions:
· The air is stationary
· All speed measurements are made relative to
the stationary medium
Doppler Effect, Case 1
(Observer Toward Source)
· An observer is moving
toward a stationary
source
· Due to his movement,
the observer detects
more wave fronts per
second
· The frequency heard is
higher than the one
produced
Doppler Effect, Case 1
(Observer Away from Source)
· An observer is
moving away from a
stationary source
· The observer detects
fewer wave fronts
per second
· The frequency
appears lower
Doppler Effect, Case 1 – a
moving observer
· When the observer is moving and the source
is stationary, the observed frequency is
±
· When moving away from the stationary source,
use –vo; when moving toward the stationary
source, use + vo;
IB parlance, moving observer
±
f’ = f [ (v ± uo) ÷
v]
· The observed
frequency is written
as f’
· The emitted
frequency is written
as f
· The observer’s
velocity is written as
uo
Doppler Effect, Case 2 - a
Moving Source
· As the source moves toward
the observer (A), the
wavelength appears shorter
and the frequency increases
· As the source moves away
from the observer (B), the
wavelength appears longer
and the frequency appears to
be lower
Doppler Effect Equation,
Moving Source
±
· Use –vs when the source is moving
toward the observer and +vs when the
source is moving away from the observer
In IB parlance,
±
f’ = f [v / (v
±us)]
· The observed
frequency is written
as f’
· The emitted
frequency is written
as f
· The observer’s
velocity is written as
us
Doppler Effect, General Case
· Both the source and the observer could be
moving
· When moving towards, vo is a postive value
and vs is a negative number, so that observed
Frequency appears greater than what is
generated
· When moving away, vo is negative and vs is
positive
Sound Effects II
Changes in amplitude
Loudness (volume)
· As you already know, changes in sound
volume are associated with changes in
amplitude.
· In order to understand how a sound can
change amplitude, we have to know a
little more about waves.
Interference of Waves
· Two traveling waves can meet and pass through each
other without being destroyed or even altered
· Waves obey the Superposition Principle
· If two or more traveling waves are moving through
a medium, the resulting wave is found by adding
together the displacements of the individual waves
point by point
· Actually only true for waves with small amplitudes
Constructive Interference
· Two waves, a and b,
have the same
frequency and
amplitude
· Are in phase
· The combined wave,
c, has the same
frequency and a
greater amplitude
Constructive Interference in a
String
· Two pulses are traveling in opposite directions
· The net displacement when they overlap is the sum of
the displacements of the pulses
· Note that the pulses are unchanged after the
interference
Destructive Interference
· Two waves, a and b,
have the same
amplitude and
frequency
· They are 180° out of
phase
· When they combine,
the waveforms cancel
Destructive Interference in a
String
· Two pulses are traveling in opposite directions
· The net displacement when they overlap is decreased
since the displacements of the pulses subtract
· Note that the pulses are unchanged after the
interference
I17.3, E17.1 andimation 2a
Interference of Sound Waves
· When constructive interference occurs, the
amplitude increases, and the sound gets
loud
· When destructive interference occurs, the
amplitude decreases, and the sound
diminishes
Constructive Interference
· Occurs when the
source velocity
exceeds the speed
of the wave itself
· The circles
represent the wave
fronts emitted by
the source
Constructive Interference
· At the edges, wave
crests or compressions
overlap, and so do wave
troughs or rarefactions
· The most familiar
example of this
phenomenon is a bow
wave from a ship
Shock Waves (Sonic Booms)
· In air, when a sound is made by something
that travels faster than the speed of sound,
the same thing happens
· Sound waves overlap in a cone shape.
Shock Waves, final
· Shock waves carry
energy concentrated on
the surface of the cone,
with correspondingly
great pressure variations
· A jet produces a shock
wave seen as a fog
Constructive and Destructive
Interference: Beats
· When two sound waves have
frequencies that are close but not the
same, sometimes the waves will
constructively interfere, and sometimes
they will destructively interfere.
· This results in a loud-soft-loud-soft
pattern
Beats
· Beats are alternations in loudness, due to interference
· Waves have slightly different frequencies and the time
between constructive and destructive interference
alternates
· The beat frequency equals the difference in frequency
between the two sources:
Constructive and Destructive
Interference: Standing Waves
· When a traveling wave reflects back on
itself, it creates traveling waves in both
directions
· The wave and its reflection interfere
according to the superposition principle
· With exactly the right frequency, the
wave will appear to stand still
· This is called a standing wave
Standing Waves on a String
Nodes
Anti-nodes
DO STANDING WAVE DEMO WITH
SNAKEY SPRING NOW!!
Natural Frequency
· Nearly every object will vibrate at a particular
frequency that is unique to that object.
· This frequency is called the “natural
frequency.”
· Tuning forks are made to have a specific
natural frequency.
· Show Natural Frequency Demonstrator now!!
An Example of Resonance
· Pendulum A is set in
motion
· The others begin to vibrate
due to the vibrations in
the flexible beam
· Pendulum C oscillates at
the greatest amplitude
since its length, and
therefore frequency,
matches that of A
Forced Vibrations
· A system with a driving force will force a
vibration at its frequency
· When the frequency of the driving force equals
the natural frequency of the system, the system
is said to be in resonance
· Do forced vibration demo w/tuning forks
NOW!
Other Examples of
Resonance
·
·
·
·
Child being pushed on a swing
Shattering glasses
Walls of Jericho
Upper deck of the Nimitz Freeway
collapse due to the Loma Prieta
earthquake
HARMONICS
For Honors Only
Standing Waves in Air
Columns
· If one end of the air column is closed, a
node must exist at this end since the
movement of the air is restricted
· If the end is open, the elements of the air
have complete freedom of movement and
an antinode exists
Tube Open at Both Ends
Resonance in Air Column
Open at Both Ends
· In a pipe open at both ends, the natural
frequency of vibration forms a series
whose harmonics are equal to integral
multiples of the fundamental frequency
Tube Closed at One End
Resonance in an Air Column
Closed at One End
· The closed end must be a node
· The open end is an antinode
· There are no even multiples of the
fundamental harmonic
Quality of Sound –
Tuning Fork
· Tuning fork
produces only the
fundamental
frequency
Quality of Sound –
Flute
· The same note
played on a flute
sounds differently
· The second harmonic
is very strong
· The fourth harmonic
is close in strength to
the first
Quality of Sound –
Clarinet
· The fifth harmonic is
very strong
· The first and fourth
harmonics are very
similar, with the
third being close to
them
Timbre
· In music, the characteristic sound of any
instrument is referred to as the quality of
sound, or the timbre, of the sound
· The quality depends on the mixture of
harmonics in the sound
Pitch
· Pitch is related mainly, although not completely, to the
frequency of the sound
· Pitch is not a physical property of the sound
· Frequency is the stimulus and pitch is the response
· It is a psychological reaction that allows humans to place the sound
on a scale
The Ear
· The outer ear consists of
the ear canal that
terminates at the
eardrum
· Just behind the eardrum
is the middle ear
· The bones in the middle
ear transmit sounds to
the inner ear
Frequency Response Curves
· Bottom curve is the
threshold of hearing
· Threshold of hearing is
strongly dependent on
frequency
· Easiest frequency to hear is
about 3300 Hz
· When the sound is loud
(top curve, threshold of
pain) all frequencies can be
heard equally well