Transcript Waves and Sound

Waves and Sound
AP Physics B
What is a wave
A ______ is a vibration or disturbance in space.
A _____________ is the
substance that all SOUND
WAVES travel through and
need to have in order to move.

Transverse wave-

Longitudinal wave –
Periodic waves

repetitive patterns occur as a result of simple
harmonic motion.
Think back to events that can create simple
harmonic motion…
Two types of Waves
The first type of wave is called Transverse.
Transverse Wave - A fixed point will move perpendicular with the wave motion.
Wave parts crest, trough, wavelength, amplitude, frequency,
period
Two types of Waves
The second type of wave is called Longitudinal.
Longitudinal Wave - A fixed point will move parallel with the wave motion
2 areas
Compression- an area of high molecular density and pressure
Rarefaction - an area of low molecular density and pressure
Wave components definitions:

Wavelength: λ,

Frequency : f,

Period: T = 1/f,

Speed of Propagation: v=f λ .

Amplitude (A) -
Wave Speed
You can find the speed of a wave by multiplying the wave’s
wavelength in meters by the frequency (cycles per second). Since a
“cycle” is not a standard unit this gives you meters/second.
Example
A harmonic wave is traveling along a rope. It is observed that the
oscillator that generates the wave completes 40.0 vibrations
in 30.0 s. Also, a given maximum travels 425 cm along a rope
in 10.0 s . What is the wavelength?
Although sound is a longitudinal wave, it
may be graphed as pressure-time and the
sinusoidal characteristic is evident:
Waves on a String


Speed is related to tension in the string:
__________________________________
Speed is related to mass per unit length of
the string: __________________________
____________________________
Fixed End Reflection
Consider a rope – one end free to move, the other end fixed. a pulse is
introduced at the free end (incident pulse) and travels toward the fixed end.
When the pulse reaches the fixed end, it reflects and is inverted.
Other notable characteristics of the reflected pulse include:

The speed of the reflected pulse is the ________ as the speed of the
incident pulse.

The wavelength of the reflected pulse is the __________ as the wavelength
of the incident pulse.

The amplitude of the reflected pulse is __________the amplitude of the
incident pulse. (in the real world. in physics land they would be the same)
Free End Reflection
Consider a rope – both ends are free. the reflected pulse is not
inverted.
Speed of waves on a string and speed of sound are
determined by the properties of the medium:



The transmitted pulse (in the more dense medium) is traveling __________
than the reflected pulse (in the less dense medium).
The transmitted pulse (in the more dense medium) has a _____________
wavelength than the reflected pulse (in the less dense medium).
The speed and the wavelength of the reflected pulse are the __________
as the speed and the wavelength of the incident pulse.



The transmitted pulse (in the less dense medium) is traveling ___________
than the reflected pulse (in the more dense medium).
The transmitted pulse (in the less dense medium) has a ______________
wavelength than the reflected pulse (in the more dense medium).
The speed and the wavelength of the reflected pulse are the __________
as the speed and the wavelength of the incident pulse.
The boundary behavior of waves in ropes can be
summarized by the following principles:

The wave speed is always greatest in the _________________.

The wavelength is always greatest in the _________________.

The frequency of a wave _______altered by crossing a
boundary.

The reflected pulse becomes ____________when a wave in a
less dense rope is heading towards a boundary with a more
dense rope.

The amplitude of the incident pulse is always _________ than
the amplitude of the reflected pulse. (some of the energy is
transferred to the boundary medium)
Consider a water bug floating on a
pond jigging up and down 2 times a
second…
B
A
Consider a water bug floating on a
pond jigging up and down 2 times a
second…
B
A
Consider a water bug floating on a
pond jigging up and down 2 times a
second…
B
A
Consider a water bug floating on a
pond jigging up and down 2 times a
second…
B
A
The Doppler Effect
1. ALWAYS establish a
coordinate system.
2. Make sure that the
positive direction is from
the source to the detector.
The Doppler Effect and Astronomy:

Electromagnetic radiation emitted by stars in a distant
galaxy would appear to be shifted downward in
frequency (__________________) if the star is rotating
in its cluster in a direction which is away from the Earth.
On the other hand, there is an upward shift in frequency
(________________) of such observed radiation if the
star is rotating in a direction that is towards the Earth.
Doppler Effect Animation
Superposition - Two waves in the same medium
interfere. the result is a new wave that is the sum of the
original waves

Constructive interference -
Superposition (cont.)

Destructive interference -

In phase –

Out of phase –
Standing Waves
A standing wave is
produced when a wave
that is traveling is
reflected back upon itself.
There are two main parts
to a standing wave:
 Antinodes –

Nodes –
Sound Waves
Sound Waves are a common type of standing wave as they
are caused by __________________.
One example of this involves shattering a
wine glass by hitting a musical note that is
on the same frequency as the natural
frequency of the glass. (Natural frequency
depends on the size, shape, and
composition of the object in question.)
Because the frequencies resonate, or are
in sync with one another, maximum energy
transfer is possible.
Video Clip
Sound Waves
The production of sound involves setting up a wave in air. To set up
a CONTINUOUS sound you will need to set a standing wave
pattern.
Three LARGE CLASSES of instruments
 Stringed - standing wave is set up in a tightly stretched string
 Percussion - standing wave is produced by the vibration of solid
objects
 Wind - standing wave is set up in a column of air that is either
OPEN or CLOSED
Closed Pipes
Have an antinode at one end and a node at the other. Each sound
you hear will occur when an antinode appears at the top of the
pipe. What is the SMALLEST length of pipe you can have to
hear a sound?
You get your first sound or encounter
your first antinode when the length of
the actual pipe is equal to a quarter of a
wavelength.
Closed Pipes - Harmonics
Harmonics are
MULTIPLES of the
fundamental frequency.
Closed Pipes - Harmonics
In a closed pipe you have an ANTINODE at the 3rd
harmonic position, therefore SOUND is
produced.
Open Pipes
OPEN PIPES- have an antinode on BOTH ends of the
tube. What is the SMALLEST length of pipe you
can have to hear a sound?
Open Pipes - Harmonics
Since harmonics are MULTIPLES of the
fundamental, the second harmonic of an
“open pipe” will be ONE WAVELENGTH.
The picture above is the SECOND harmonic or the FIRST OVERTONE.
Open pipes - Harmonics
Another half of a wavelength would ALSO produce an
antinode on BOTH ends. In fact, no matter how
many halves you add you will always have an
antinode on the ends
The picture above is the THIRD harmonic or the SECOND
OVERTONE.
Example
The speed of sound waves in air is found to be 340
m/s. Determine the fundamental frequency (1st
harmonic) of an open-end air column which has a
length of 67.5 cm.
Example
The windpipe of a typical whooping crane is about
1.525-m long. What is the lowest resonant
frequency of this pipe assuming it is a pipe closed
at one end? Assume a temperature of 37°C.
Polarization of Transverse Waves:


A light wave which is vibrating in more than one
plane is referred to as __________________. Light
emitted by the sun, by a lamp in the classroom, or
by a candle flame is _________________ light.
It is possible to transform unpolarized light into
____________________. Polarized light waves are
light waves in which the vibrations occur in a single
plane. The process of transforming unpolarized light
into polarized light is known as ____________.
Wave Intensity

There is an inverse-square relationship between wave intensity and
distance from source.

This holds true for both sound and light. The intensity is proportional
to the inverse of the square of the distance away from the source.
Intensity of a wave is power per unit area that passes
perpendicularly through a surface. I = P/A
